Good Design

Last Updated:
July 1, 2024
STEP 1: Good Design
This section is for Designers, Builders and those who want to know how to achieve a 7star+, Cost effective, Energy efficient house design.

Getting the Design right is Step No. 1.

But what makes for a GOOD design?

There are obviously a number of elements that make a design good; form and function being high on the list.  That we will leave to the designers out there. In this section when we talk about Good, it is in the context of thermal performance.

  • A design is good when it provides comfort.
  • A design is good when it is connects to your environment and fills your home with light and a quiet place of sanctuary.
  • A design is good when it does this efficiently & cheaply.

And lastly, a design is good, when it does all this while treading lightly on the planet.

And how is this achieved? The secret starts with Passive Solar Design.

Passive Solar Design & NatHERS

Passive Solar Design is just another just another word for Climatic Design.  And it is not hard. It is something people have always done since the earliest times - design their home to work with the climate. Using natural energy flows in the environment to provide comfortable indoor conditions, needing minimal extra energy requirements. You don't need to know formulas. Logic, and common-sense practices will take you as far as you need to go.

  • If you are in a cold climate, design to invite the sunlight in.
  • If you are in a hot climate, shade to keep it out.
  • If you need cooling in summer, determine the cool evening wind direction and create welcoming breeze paths through the house.
  • If the outside temperature fluctuates a lot, lock to the mass of the earth for stability of temperature inside.
  • And if the outside temperature is not always desirable, put in good insulation to keep the inside and outside environments separated.

Work with Nature and not against it, and your home will be cheap to heat and cool.

And won't need vast amounts of energy to maintain comfort.

This section will teach you the 5 Tools of Passive Solar Design that you need to know to design high performance 7 Star + homes, which is the first important step to creating low impact Zero Carbon Homes.

Star Ratings & Energy Saving

NatHERS stands for the National House Energy Rating Scheme, and is one of the pathways to achieve thermal compliance under the National Construction Code, using approved House Energy Rating software. In essence, a NatHERS star rating is simply a measure of how well a house is designed for its climate. There are 4 NatHERS accredited software, which all use the same CSIRO developed Chenath engine, so all give the same answer with the same input.

The softwares are:

Houses are rated by an accredited Thermal Performance Assessor, then ranked on a scale of 0 - 10.  With zero effectively being a tent, and 10 being a house that needs almost no extra mechanical heating or cooling to maintain comfortable temperatures.

The old 6 star requirement in the NCC was therefore like getting a 6 out of 10, or a C on a test. The new 7 star requirement is more like a B. That is, it is a challenge if you don't know what you are doing, but easy if you know how to use the 5 Rules of Passive Solar Design.

And how much energy do you save?

Well, it does depend a lot on your climate. Colder climates tend to use more energy on space conditioning than warmer ones.

But, on average, the pie graphs below show you the savings possible from Good Design, in reduced space conditioning needs. The first pie is the 'average' Aussie home energy use, and represents a bill of $2000+ per year, and an average CO2 emission approaching 6 tonnes per year from the fossil fuels burnt to provide the power for the services in each wedge of the pie. The WHITE section in each pie shows the indicative scale of energy savings for each star achieved.

As you can see, the new 7 Star criteria saves over a quarter of the pie, representing a saving of ~1 1/2 tonnes CO2 & $500 less in running costs a year, compared to the average home.

So how do you achieve the higher stars?  Let's get into the Rules…

Rule 1: Orientation

Orientate to North


This is the foundational question to ask when designing for a good thermal outcome.  It is also the foundational question for a good day-lighting outcome and Indoor/outdoor connection.  

But what does it mean to, "Orientate to North?"

The sun rises in the east and sets in the west, but because we are in the southern hemisphere it doesn't go directly overhead (unless you are above the Tropic of Capricorn), but stays in the north half of the sky.
In winter it passes lower in the sky and in summer higher in the sky.

We can take advantage of this in much of Australia, wherever winter heating is required, by putting living spaces, and as many other rooms as possible on the north side of the house.  Windows into these spaces, combined with well designed eaves to keep out the summer sun, allow those rooms to receive free heating all through winter.

The most important room to get right is the Living Space.  NatHERS assumes people will be in these areas during the day and well into the evening desiring 20-24 deg temperatures for comfort - so it pays to give the living pride of place to northern solar gain.

When designing on your block.  If some parts get winter sun and other parts get shade, try to locate the living to give it priority of any northern solar access available.  Sometimes this will take a creative response depending on the site constraints.

Homes can definitely pass 7 stars with poor solar access, but it makes it a whole lot easier, (and a whole lot more livable), if you can bathe the inside of the home in winter sun.

While it's hard to put a number on it, a home that makes good use of solar access to its living space could easily gain a 1/2 to 1+ star of advantage in its energy rating.
So here’s the rule of thumb to follow:
Position the Living to get northern solar access, and then do the same to as many other rooms as possible within the confines of the site and the brief. This will put you on the pathway not only to a high NatHERS score, but also to a wonderfully livable home.

Sun Path & Room Layout

Here's a video from one of our webinars, with a short section showing Passive Solar Design thought process and house layout that you can apply to your designs.

Rule 2: Windows & Eaves

Window Consideration is Vital


Windows are by far the weakest element of the building envelope. They are the No.1 reason buildings fail to get 6 Star - and will fail to get to the new 7 Star criteria. Sure, correctly placed windows can have a positive effect. Orientation counts! But when the sun’s not there, or the outside is too cold, or too hot, it’s the windows where heat is being gained or lost the fastest, which is why they are sometimes called, ‘thermal holes'.


A standard R2.5 Brick veneer wall will have a total built up Resistance Value (R-value) of R3 to R3.5, depending on whether they used reflective foil in the cavity. (Let's ignore thermal bridging of the framing for now…).
A standard single glazed Aluminium window has a U-value (conductance value) of around 6.7

An R-value is the opposite of a U-value (ie. R=1/U).  So, a single glazed aluminium window has an R-value of 1/6.7=R0.15

Therefore, a standard aluminium window is losing or gaining heat, when you don’t want it, 23 times as fast as the wall next to it!! (3.5÷0.15=23.3)


Top quality windows have a U-value of ~2.0, & therefore have an R-value of 0.5.  So, they are still 7x worse than the wall next to them!!!  (A big improvement, but still a hole).

MORAL: Don't have too much window.

How much window should you have? Read on...


Sub-Heading Navigation:

Where to place Window and How Much

Window Area and Local Climate


Solar Heat Gain Coefficient - SHGC's

Choosing a Window Manufacturer

What's the best frame type?

Eave sizing to Keep the Summer sun out

Where to place windows & how much?


As mentioned in Orientation above, the sun travels over the Northern part of the sky, so it makes sense in temperate & cool climates to take advantage of that free heat, by putting most windows on the North.
OK, but what about East & West? Do we get any benefit? Well, let's have a look at this interesting graph.

It’s a Melbourne example from the excellent Energy Smart Design Manual, but gives useful direction anywhere that winter heating is required in Australia.


Glass brings in sunlight.  But it is a very poor insulator. So at night, or on a cold cloudy day, it can lose more energy for you than it gains depending on direction. What this graph shows is that with single glazing only Northern windows give you more energy than they lose over 24hrs; with the dotted line being the breakeven point. So, with single glazing you really want to be judicious when placing windows in other directions.

But note the green band: If you choose high performance windows, both East and West can also be net positive energy contributors!


This time we see how much unwanted energy comes into the home through windows in summer with the dotted line this time being a 2-bar radiator operating 3 hours a day, which is what you don't want your windows to be in summer!
On unshaded glass (NO EAVES), North windows somehow still manage to have the second lowest heat ingress - behind direct South!


With sun shining on them all day, why is North gain so low?

It’s the ANGLE!

The more acute the angle, the more light gets reflected, and the less goes through. In summer the sun is much higher in the sky, so most of the sun's energy bounces off, instead of coming in. But for East in the morning and West in the evening, the sun is much lower, striking straight on to the glass, and most heat passes straight through - just when you don't want it!

And of course, if you do install a properly designed eave, north windows perform even better in summer!!

So, How Much Window and Where?

North windows are obviously great if you are in a climate where you want heat in winter.  But still a house can only hold so much heat, dependent on how much mass it has.  The rest gets radiated away from the structure.  And then when the sun goes down or it gets too hot, those north windows are no longer your friend.  There is a balance to be struck.  And here’s a good rule of thumb to put you on track.

Keep below a ratio of 22% Window-to-Floor area, (Eg. If you have 200m2 of house floor area, try not to go over 44m2 window area.) AND North should have the biggest proportion of those windows AND Most of the North windows should go to the Living Area.

NOTE: When calculating window area for the sake of this calculation, include frames. (ie. just use the height & width measurements on the window schedule.)

And what about other directions?

If you are in a climate that requires winter heating the next best direction is East. This is because the sun rises in the East, and when it’s cold it helps to start warming the house early.

West and south are generally considered problematic.  While West can be nice on a winter afternoon, it can have issues with summer overheating and can be tricky to screen from the setting summer sun, so should typically be minimised unless shading strategies or modelled window tinting are considered.

And the sun doesn’t shine from the south in winter, so if you are in a climate where you heat in winter, generally only place southern windows to frame views, or to create summer breeze paths.

While 22% is a well-trod rule of thumb, you can push upwards when you have more internal mass in the home to balance the solar gains.  If wanting to put on more window, always get a Thermal Performance Assessor on board at Design Stage


If you are in the Tropics winter heating won’t be a goal, so a north focus isn’t beneficial.  Instead place windows to pick up prevailing breezes, and minimise East and West windows which as discussed, are hard to shade out.  And remember your eaves.  Shade will be your friend!

The Archetypal tropical layout: Big eaves Nth & Sth, vertical shading trees east and west, and one room wide with window openings either side to maximise cooling breezes when they come.


Sub-Heading Navigation:

Where to place Window and How Much (Current)

Window Area and Local Climate


Solar Heat Gain Coefficient - SHGC's

Choosing a Window Manufacturer

What's the best frame type?

Eave sizing to Keep the Summer sun out

Window Area and Local Climate Considerations



As mentioned above, with Windows being the weakest element in the thermal building envelope, the rule of thumb is to aim below a 22% window area to floor area ratio when designing a home.

But now that thousands of energy ratings have been done all across Australia however, we can tighten the rules!

Presented below are some very insightful graphs from the NatHERS data portal:

Australia-wide <22% window-to-floor area is the most common ratio for higher performance

The graphs show the general relationship between window area & star ratings in each state.

The percentage figures given are the window to floor area ratio. The “Floor Area” is the total floor area, less the garage. The Window area includes the frame & is the area you get if you sum the window schedule.

Window area / Floor area = %

And what do the graphs say?

On average homes with less % windows perform better. And the <22% rule is pretty on point, as a general rule for Australia.

However, climate does vary things...

In the NT & (interestingly) the ACT, lower percentages are the norm for higher star ratings.

In TAS, VIC, & SA (the colder states), the percentage reduces till 7 stars, but then goes up again! This, likely, shows the benefit of passive solar design in these states, probably reflecting the inclusion of mass, solar access & performance glazing, that tend to be present in well designed, highly rated homes in these climates.

NSW & QLD show less pronounced benefit of window % reduction, probably reflecting a more benign climate.

And WA varies over such a wide climatic band, so it’s hard to interpret the one graph!

(WA readers, it may be better if you are designing up North, to use the NT data. If mid-state, use NSW data. And down South, the SA data.)

So, when designing, check your home window % against the relevant graph.

It won't guarantee the outcome. Where you place windows, & including the other rules of passive solar design is vital...

But it is a great first step to a good score.

And your thermal performance assessor can help guide window sizing from there, to reach those higher stars!


Sub-Heading Navigation:

Where to place Window and How Much

Window Area and Local Climate (Current)


Solar Heat Gain Coefficient - SHGC's

Choosing a Window Manufacturer

What's the best frame type?

Eave sizing to Keep the Summer sun out



In the diagram, the (Uw) value is an average measure of conductance for the whole window when you add up glass (Uc), and frame (Uf) conductance, and is measured in Watts (W). And because most of the time the outside temperature is not ideal, and you are trying to maintain a comfortable internal temperature, you ideally want a LOW conductance, or a Low (Uw) value. (Often just shortened to U-value)

U-value numbers in the Australian windows market range from ~1 to ~7. And this number represents how many Watts come through every m2 of window for every degree of temperature difference between inside and outside.

For instance:

If your home has 70m2 of glazing with aluminium frames and clear glass and a U value of 6.0W/m2 °C, on a winter’s night when it is 15°C colder outside compared with indoors, the heat loss through the windows would be: 6.0 × 15 × 70 = 6300W That is equivalent to the total heat output of a large room gas heater or a 6.5kW room air-conditioner running at full capacity!

If instead your windows were double glazed, low-e coated, argon filled, in PVC, Timber, or Thermally broken aluminium frames, and had a U value of 2.0W/m2 °C, it would only have 2.0 × 15 × 70 = 2100W or 1/3 the amount of heat loss!
So, U-values are IMPORTANT!

A lower U-value will give you better performance, Australia wide.

RULE OF THUMB FOR 7 STARS: Choose a U-value <3.  And the lower the better...

But, when you are comparing windows, watch out that you are judging apples to apples. In Australia our U-value ratings are done under AFRC (Australian Fenestration Rating Council) protocol, and can’t be compared to European or US values, which are lower in general.

Lastly, the U-value is actually the inverse of an R value.
So, if you wanted to find out what R value your windows are put a 1 over the top.
Eg. The single glazed aluminium window in the table above: 1/6.0 W.m2°C = R0.15
And the fancy double glazed window above: 1/2.0 = R0.5

So, even a high-performance window is still a lot poorer an insulator that the wall next to it!

Therefore, don’t go too far above the 22% Rule of Thumb, discussed in the section above, without a Thermal Performance Assessor to guide you. (Thermal Performance Assessors can be found here.)


Sub-Heading Navigation:

Where to place Window and How Much

Window Area and Local Climate

U-values (Current)

Solar Heat Gain Coefficient - SHGC's

Choosing a Window Manufacturer

What's the best frame type?

Eave sizing to Keep the Summer sun out

Solar Heat Gain Coefficient - SHGC's

SHGC measures how readily heat from direct sunlight flows through a whole window (glass and frame together). SHGC is expressed as a number between 0 and 1, with 1 theoretically being 100% transmission when the sun is straight on to the glass, and 0 being no light gets through. In reality the highest SHGC for glazed windows top out around 0.7

So what SHGC should you specify?

It depends on whether you are trying to maximise solar gain or not, and that depends on your climate.

Broadly, if we separate Australia’s 8 climate zones into 3 groups we get the following table:

In the far north, we are trying to cool, so a Low SHGC is preferable. A Thermal Performance Assessor can gain up to 1 star by optimising glazing. (Note, you also get big benefits for maximising openings in the tropics!)

In Australia’s middle we have a mixed climate, so are trying to strike a balance between heating in winter and cooling in summer. Depending on the house design, and how effective your shading is, NatHERS optimisation by a Thermal Performance Assessor can improve up to 0.5 stars.

In Cool & Cold climates we are primarily heating most of the year, so as long as you don’t go overboard with east & west windows, and design Nth eaves properly, we are trying to maximise the SHGC.

Hot Tip: As you may have noticed in the table, High U-value is a benefit in all climates. So generally maximise that first, then choose an appropriate SHCG.

Something to be aware of however is that with double-glazed, high-performance windows, a lot of the sunlight gets blocked by the usually chunkier frame, and then double glazing also cuts down transmission.  So, a good SHGC for a high-performance window is around 0.4 (ie. 40% sunlight comes through.)  As you go north, lower SHGC using window tints are worth exploring with an assessor, especially to any unshaded east and west windows.


Sub-Heading Navigation:

Where to place Window and How Much

Window Area and Local Climate


Solar Heat Gain Coefficient - SHGC's (Current)

Choosing a Window Manufacturer

What's the best frame type?

Eave sizing to Keep the Summer sun out

Choosing a Window Manufacturer

How do you choose a high-performing window? And where do you find them?

Here’s the Rules of Thumb so far:
  1. Keep below a ratio of 22% for window to floor area, considering your climate, with most of that to the north
  2. The lower the U-value across Australia the better (Anything less that 3 is pretty good)
  3. Assuming summer shading by eaves: In southern Australia, the higher the SHGC the better.  As you go North optimisation of SHGC by a Thermal Assessor is beneficial.

Ok, last step.  Where do we find windows?

The Windows Energy Rating Scheme (WERS) holds a database of all the windows that have been rated under the Australian Fenestration Rating Council (AFRC) protocols.  Literally thousands of windows from hundreds of companies across Australia.

That makes it a treasure trove to compare window performance from different companies!


1) Go to WERS Residential Search

2) You will see a database where you can select windows by various parameters.  

3) Fill in the U-value and SHGC you want in the search. (I typically choose Max U-value of 2.5, & SHGC of 0.4 [which is not bad for a high-performance window])

4) Put in any Filters you like (I usually put in the State, and the major window type I'm after)

5) Click Search

5) Click Uw to Rank table by U-value (lowest to highest)

6) Search down list for appropriate SHGC

7) Check Air Infiltration (Ai) is low (under 1 is good)

8) See which manufacturers are making windows with similar performance numbers.

9) Check out those manufacturers, and send out jobs to quote up, and compare price.

10) Choose the highest performance windows that suit the price point for your build.

And that’s it!  If you need to be more specific on opening styles or materials go for it.

In most areas outside the Tropics: Assuming you don't go overboard with area, have most windows on the north, shade out sun in summer, and choose windows with low u-values and higher SHGC's, your house will be on the way to good rating!


Sub-Heading Navigation:

Where to place Window and How Much

Window Area and Local Climate


Solar Heat Gain Coefficient - SHGC's

Choosing a Window Manufacturer (Current)

What's the best frame type?

Eave sizing to Keep the Summer sun out

What is the Best Frame Material for Window Performance?


Seeing spots?

Me too - & it’s GREAT!

It’s the new graphing function from the Windows Energy Rating Scheme called WERSLINK.


You can put in your required preferences for U-value, Solar Heat Gain Co-efficient (SHGC), material type & opening type, & then press search.

What you get back is dots.  

Each dot is a window.

And if you click the dot, you can find out who it is made by & other characteristics.



But it gets better.  

As we saw above, a low U-value is a big driver of high-performance housing Australia wide.


In the graphing filters, if you set the minimum U-value to 0 & the max to 7, you can see the entire Australian window market ranked by U-value!

If you then sort by frame type, you can see how the different window materials tend to perform.


So now we get to the good stuff...  


What type of window material performs the best?


Graph 1, would suggest this order (in general) from worst to best:

4) Aluminium

3) Thermally broken Aluminium

2) Timber / Composite

1) PVC


But let's zoom in to Graph 2.

What this shows, is that while PVC framed windows do tend to cluster below a U of 2 (which is great), Timber, Composite, & Thermally broken can all get there too.  


But which manufacturers are they & what is the glass specification?  

Just click the dot to find out!


Even straight aluminium can get down pretty low within-line framing & double glazing.


So, another great tool to aid window selection.

Check it out.


Sub-Heading Navigation:

Where to place Window and How Much

Window Area and Local Climate


Solar Heat Gain Coefficient - SHGC's

Choosing a Window Manufacturer

What's the best frame type? (Current)

Eave sizing to Keep the Summer sun out

Eave sizing & keeping the Summer sun out


Well, once again there is an easy guide to help you do this. This is a rule of thumb all builders (and especially designers!) should know. It's called the 45% rule and works for Northern Eaves in southern Australia up to northern NSW.

This is a formula to size an eave to let in winter sun, but keep out the summer sun, and it works like this:

  1. Measure the distance from the sill to the underside of where you want your eave and multiply by 45%.  This is how long your eave should be from the wall to the outside edge of your fascia, or if you have a gutter, the outside edge of that.
  2. This sets the eave length, but a look at the picture shows that if the window ran up to the underside of the eave, there would be an area always in shadow – even in winter.  So, in the ideal world where you can change the window, the next step is:
  3. Move the window head down from the underside of the eave 1/3 of the distance you just worked out.  (Alternatively if you can’t change the window size but can move the eave up on the wall, then re-adjust to achieve the same.)

The good news is you don’t need to be super precise to get most the benefit, so working to the closest eave sheet size is fine. Generally round down the sheet size in cooler climates, and round up the sheet size in warmer climates. If there’s multiple window sizes on a wall work to the biggest ones. If putting an eave over a single window run it past the horizontal edges by the width of the eave projection to give it shade when the sun is to the side. That’s it.  Easy right.  And a Thermal Performance Assessor can fine tune it from there…


Limitations: This only works for North Eaves.  East and West can only really be shaded by vertical blinds/awnings, &/or carefully placed or deciduous trees.

Tropical Considerations:
Shade is your friend, so the bigger the eave the better.  

Rule of thumb: Eave should be ½ the height of the wall.  

If you are above the Tropic of Capricorn, you will also be getting sun in from the south in summer, so make sure you have eaves on that side too.  Furthermore, try and avoid windows to east or west, as eaves don’t have a lot of effect in those directions, or alternatively plant shading trees.  

And, lastly, good passive ventilation gives a significant NatHERS performance gain in the tropics. See Rule 5 - Ventilation

The Archetypical Tropical layout: Big eaves Nth & Sth, vertical shading trees East & West, and one room wide with window openings either side to maximise cooling breezes when they come.


Sub-Heading Navigation:

Where to place Window and How Much

Window Area and Local Climate


Solar Heat Gain Coefficient - SHGC's

Choosing a Window Manufacturer

What's the best frame type?

Eave sizing to Keep the Summer sun out (Current)

Rule 3: Insulation & Tightness

Design for Effective Insulation

This section deals with the design criteria for insulation & a tight, low thermal bridging building fabric.  Which admittedly, while important, is the easy part.  For complementary details on the installation of the insulation, membranes and internal linings see Step 3 Performance Construction.


While the answer is more is usually better, it is also true that insulation has quickly diminishing returns as you add more R value.  So it becomes a question of cost effectiveness and space constraints.  You get a lot of bang for your buck for the first R1, less for the next R1, less for the one after that and so on…

It is also true that heat rises, so to stop it getting out of the house in cool conditions, the ceiling insulation tends to be the most important. This is also true in summer when high heat load hits the roof, and you want to stop heat coming down into the cool house below.  

The upshot of this is that higher insulation levels in the ceiling make sense.  Less so to the walls, and then less so to the floor. The picture below from the Energy Smart Housing Manual make this clear.  

(Note how much heat flows through windows, which is why it is very important to get them right!  See Rule 2: Windows & Eaves)

Hot Tip:
When considering cost, space constraints and location, the following insulation levels are a good place to start.
Ceiling: R4.0
Walls: R2.5  (works with common 90mm timber studs)
Raised subfloor: R2.5  (works with common 90mm timber floor joists)
For Tropical Climate Zones 1-3 where subfloor walls exist, start with R1.5 to subfloor walls, and none to subfloor, as this maximises cooling to subfloor.  Make sure all vents are maintained.

Next, when you get your Energy Rating done, ask your Thermal Performance Assessor this question:

- Do I have cavity space to fit more R of insulation?

- And what is the performance gain?

There will be a bit of climatic variability in the answers, which the rating can guide on.

Last Tip: If you have the space, it is nice to go R0.5 over the ideal to cover yourself from potential unintended thermal bridging!


Sub-Heading Navigation:

Design Out Compression

Design for Tightness

Tightness Layer 1 - the Wrap

Tightness Layer 2 - the plaster / internal lining

Design Out Thermal Bridging

Thermal Bridging 1 - Frame

Thermal Bridging 2 - Slab

Design Out Compression

The trick to good insulation is minimise gaps and compression.  While both of these are site install issues, there are a couple of common failure points in standard construction where design can make a really big difference to the outcome, and need to be considered:


We know that ceiling insulation MUST OVERLAP external insulated walls. Otherwise, you get a cold thermal bridge all around the perimeter of the house.

Where roofs meet the external wall in the attic, space can get very tight. Sometimes not much more than 120mm or so over the top-plate, depending on roof pitch and timber thickness. So usually, thick batts WON'T FIT unless PLANNED FOR in the structure.

For a real-world video example of the sort of problem it can cause, click image below.

If you are lucky, installers will use R2.5 to run along the edges, or rip batts in half to fit (less reliable). Otherwise, they will pull them back from the edges and you get an uninsulated perimeter.

Or more often, they try and squish them in, and the compressed batts will hit the roof, or membrane where there is one, and consequently become wet condensation traps.

Either way, your thermal perimeter is compromised.

What should you do?

Hot Tip: Consider thicker rafters. Or raised heal trusses. The R-values in the table give you an idea of the height to allow for. Or make sure your NatHERS assessor models R2.5 or R3 batts to the perimeter (depending on the height you have), and that this is then specified on the plans.

Otherwise, you might find your new R6/R7 insulated house pretty uncomfortable…


Box gutters are another common point of thermal failure.

The NatHERS rating however assumes there are no weak points in the insulation, otherwise known as ‘Thermal Bridges’.  It is important that designers help the builder by thinking through the structure prior to construction, looking for any points where the required insulation loft, or thickness, cannot be maintained.  (Builders should also do this prior to starting. See Step 3: Performance Construction).

Under a box gutter is a commonly compromised area, which will often either get missed completely during insulation, or have a batt squashed in place, which then works relatively ineffectually, and has the potential to pop plaster.  Depending on the space, it may be better insulated with 2 layers of high-density wall insulation, or a high-density foam board product. 

Either way, it needs to be worked out, rated up, and notated on plans prior to construction.


Another common point of failure in standard construction is the corner junction, as it creates a pocket in the framing that once papered and clad cannot be insulated from the inside. Internal to external wall stud junctions leave a similar cavity pocket on the outside.

While a builder, thinking ahead, could organise a bag of insulation on site, for carpenters to install prior to the wrap, it is easy to forget as it is out of construction order. Alternatively detailing a 3-stud corner (California Corner) on the plans, allows insulation to still be pushed home from the inside.  A solid corner also works but uses more timber.  Make sure you consider the cladding type and where you need supports to fix ends.

This small detail can make a big difference as there are a lot of external corner and internal/external junctions in a home!


Sub-Heading Navigation:

Design Out Compression (Current)

Design for Tightness

Tightness Layer 1 - the Wrap

Tightness Layer 2 - the plaster / internal lining

Design Out Thermal Bridging

Thermal Bridging 1 - Frame

Thermal Bridging 2 - Slab

Design for Tightness


Under NatHERS, new home tightness is assumed to be somewhere between 7-10 Airchanges per hour @ 50 Pascals of pressure (ie.<10ACH50).  The actual rate depends on windows chosen, downlights, extraction fans, vents, and the tightness of external doors, etc., entered by the Thermal Assessor.

In 2015 the CSIRO did some testing around Australia to check the tightness of new homes being built and found that while some homes were hitting the mark, most were not, with an average air leakage of 15.4ACH50.  (And an average of 25.5 for Perth!  COME ON PERTH!!)

Effectively this means that most new homes are not complying with their contractual obligations to build to the performance of the Thermal Assessment!

What does this mean for ratings?

NatHERS now has the ability to enter different tightness levels (possibly getting ready for future mandatory testing!), so we crunched some numbers using the Accurate software. We tested how a house that rates 7stars at 10ACH50, would rate at the different tightness measures in the table.

The results range from 7.8 stars for Passivhaus standard (showing the benefit of building tight), to 4.3 stars for the worst home in the study! 

So, the message is TIGHTNESS COUNTS.


(NOTE: It is important to be careful not to go too tight without adequate mechanical ventilation. The NCC gives direction under the Verification Pathway, that homes below 5ACH@50pa should be provided with extra mechanical ventilation to avoid potential health implications. While the strategies in this section will generally see homes built with standard construction falling between 5-10 ACH@50pa, with diligence it is possible to go considerably lower with standard construction. Potential issues with building tight and how to produce healthy internal environments are covered in Step 3: Performance Construction/Condensation & Indoor Air Quality.)


Sub-Heading Navigation:

Design Out Compression

Design for Tightness (Current)

Tightness Layer 1 - the Wrap

Tightness Layer 2 - the plaster / internal lining

Design Out Thermal Bridging

Thermal Bridging 1 - Frame

Thermal Bridging 2 - Slab

Tightness Layer 1 - the Wrap

There are 2 tightness layers to consider in standard construction. The first is the wrap.

Typical batt bulk insulation needs a still airspace to work.  If air is filtering through and around it, the house will not perform to its NatHERS rating.  Claddings cannot be relied upon to keep air infiltration at bay.  There are simply too many gaps and cracks in most cladding.  Our first main defence against breezes entering the home therefore is the Building Wrap.

Here’s a study from Bradford Insulation that puts some numbers to the benefits of reduced air infiltration from doing wraps right, to give you a guide of where to focus effort.

For the experiment, a 90mm timber stud wall was built on the face of a pressure chamber.  The test wall had two fixed windows, R2.0 Bradford Gold Batts, 10mm Gyprock, timber skirting & architraves, one power point and one light switch.  After an initial test with no wall wrap, the wall was wrapped with Bradford Thermoseal Wall Wrap.  Sensors monitored air pressure inside the chamber.  The test was repeated using Bradford Enviroseal Proctorwrap with very similar results.

So, what did they find? At a simulated 33km/h wind speed or 50 Pascals (Pa):

- a 20% reduction in air infiltration was achieved when the wall wrap was installed without any tape.

- a 58% reduction in air infiltration was achieved when the wall wrap was installed with overlaps and window frames sealed with tape.

- An 82% reduction in air infiltration was achieved when wall wrap was installed with overlaps, window frames sealed, and top plate & bottom plate all sealed with tape.

What this shows is just how important it is to do the taping and do it properly.  You are not just wasting your time!  And if you use a quality tape, it should last for the life of the house.

Thanks to the efforts of Jesse Clarke and the team at CSR for making the research available.

PS. Before sealing, make sure you have the right membrane for your climate zone and construction as per the NCC and manufacturers specs.

Specifying for a Tight Wrap:

If you want a tight wrap, specification, detailing and well noted plans go a long way to communicating how to get it done right, to the trades on site. Below are a couple of common points in standard construction where design detailing in the working drawings can make a really big difference to the outcome, and should be considered:


There are a lot of things to consider when choosing a wrap. We will explore this in Step 3: Performance Construction, when we introduce the Builders Thermal Performance Checklist and considerations at Lockup Stage. However, as can been seen from the Bradford test, the main consideration from a Tightness point of view, is not what wrap type you choose (membrane or reflective foil), but that the tape you choose seals long term, and won't delaminate. If the tape does peel off, your building will become leakier over time. For membrane wraps, look out for tapes that have been tested by an accelerated aging protocol to adhere for at least 100 years. You won't find this testing for reflective foil, so choose a quality foil tape with good adhesion. As a general rule, use the proprietary tape specified by the wrap manufacturer, unless you know it doesn't work.


As we saw above, taping the wall wrap at top and bottom can make a big difference in its effectiveness (an extra 24% compared to taping window and doors alone) and will significantly improve the tightness of your structure.

If taping to the top plate, make sure the tape you use will stick to timber long term.

Importantly, when taping to the bottom of the wall make sure it doesn't inhibit any flashings or drainage of the cavity and the ability carry any water past the bottom plate and away safely.


Don't leave it to the trades onsite to work out how windows, membranes, tapes, flashings, architraves and claddings all go together. That's a recipe for water leaks and air infiltration.

And even if the trades do get it right, it will have come with a lot of head scratching and wasted time on site when the team should be building and being more productive. This is a designer's job. Or where not provided on the plans, the builder's job, to work out the details prior to starting lockup. The above is an example detail, but every job will be different depending on window type, cladding type, whether reveals came with the windows, and aesthetics required.

Windows and doors are always weak points, and if there's going to be leaks and issues, that's almost always where they are found. So spend the time to work out the detail and put it on the plans. Imagine you are a drop of water, or the driving rain, or blowing wind. Will your detail keep all these out?

The time you take in working out the details will be more than made up in savings in reduced future rectification work. And of course, the house will be more energy efficient and comfortable for the clients as well.


Tape all penetrations. Ideally with proprietary patches, but a good quality tape does an adequate job

It is true to say that trades in our industry are not used to taping up penetrations, when they make them as part of carrying on their works. But it is really not a hard thing for them to do. And most trades will take it on with a little reminding.

Start by putting some notes on plans to require trades to tape any penetrations in the wrap. Ideally there should be a note in relevant places on Elevations, Electrical plans, and Notes pages at a minimum. Then make sure that you have the appropriate tapes and patches onsite ready to be used for penetrations, or better yet, put the requirement in the trades tender document.



Sub-Heading Navigation:

Design Out Compression

Design for Tightness

Tightness Layer 1 - the Wrap (Current)

Tightness Layer 2 - the plaster / internal lining

Design Out Thermal Bridging

Thermal Bridging 1 - Frame

Thermal Bridging 2 - Slab

Tightness Layer 2 – the plaster / internal lining

The second tightness layer in standard construction is the internal lining - which is usually plaster.

Unlike Passivhaus construction, standard construction in Australia will generally not tape and join external wall membranes to roof membranes, so there will always be the potential for unwanted air infiltration. However, we can further reduce infiltration by working to limit the penetrations on internal linings, which in standard construction is usually plaster. If plaster is tight and caulked, even if there are some leaks in the wrap layer, air will be unlikely to come in at problematic volumes if there is a limited exit pathway. And caulked painted plasterboard is an effective (and permeable) air barrier as long as junctions are caulked, and penetrations are minimised.

Specifying for a Tight Internal Lining

The following are a couple of common internal tightness failure points in standard construction where design can make a really big difference to the outcome, and need to be considered:


Downlights today are much better than they used to be.   They are now mostly LED’s.  And if you get the IC-4 type with the symbol in the pic, you no longer have to pull insulation back - making Swiss cheese of your thermal layer - but can run batts right over the top.  This is a good thing. 

But don’t go overboard with downlights just yet.  In standard construction, the plaster layer is the primary tightness layer for the house.  The more holes that you cut in this layer, the leakier the house will be.  While some penetrations are unavoidable, and a downlight or 2 is not the end of the world, the old practice of multiple grids of downlights is still a problem to be avoided. 

The picture here shows a thermal image of a downlight during a blower door test.  Dark, colder air from the roof space is leaking under negative pressure into the room.  This is what happens every time the wind blows.  The harder it blows the greater will be the internal to external pressure difference, and the more air will get transferred, making the house harder to heat and cool. 

So, where possible, specify lighting types where only a wire has to penetrate the plaster……and remember the job the plaster is trying to do for the thermal comfort of the house!



Did you know most new homes in Australia are not being built to the required tightness to achieve their energy rating?

And Cavity Sliders are one of the culprits!

Cavity Sliders represent a large hole in the plaster layer, which in standard construction, is also your tightness layer (along with the external membrane).  Whenever we allow holes in plaster, our structures become a bit more leaky. It is not uncommon to walk by a cavity sliding door when the wind is blowing outside and feel a breeze coming out!


Ideally all doors would be swing doors, and problem solved.  But sometimes there just isn't the space, & cavity sliding doors are needed.

So how to seal them?

We have found the best method is clad them with 3 or 4mm ply/mdf.


Here are the steps:

  1. Pre-install cavity sliders plumb in framing, allowing for the thickness of the ply, and where needed, shift the slider pocket across accordingly so only one side of the wall needs additional packing.  (Pack out studs where needed.)
  2. Cut the ply to size to cover both sides of the cavity pocket.
  3. Use 15mm screws at 300 centres to fix the ply to the rails.  (Screws must be shorter than the thickness of the rails)
  4. To finish the seal, use a good quality tape along the edge of the ply to seal between the ply and cavity pocket frame.  We use 72mm Ametalin Reinforced Insulation Ducting Tape (Silver tape) which adheres well and is airtight.


And you’re done!  You now have a sealed cavity unit ready for plastering!


Pretty easy to do. 

And your home will be tighter, more comfortable, and performing to it's rating!


Usually left uninsulated...

Needs to be insulated! and sealed tight!

One standard detail as above, put on the plans is all it takes. Good job for the apprentice.

‘Nough said really!


As well as the above, here are some tried and tested techniques for a tight internal plaster lining:

  •        Consider Squareset Cornice for a long-term seal at the wall ceiling junction.
  •        Consider Ezy Reveal type doors jambs and window reveals to stop leaks around the architraves
  •        Consider caulking the wall-floor junction with flexible sealant (to allow plaster expansion)
  •        And minimise powerpoints, vents, and openings in the plasterwork, especially on external walls.

As noted previously, if you apply some/most of these techniques you will be well under the required 10 airchanges at 50 pascals of pressure assumed by NatHERS. It is still important however to know your tightness and create a healthy build. So make sure you check out Step 3, Condensation & Indoor Air Quality.


Sub-Heading Navigation:

Design Out Compression

Design for Tightness

Tightness Layer 1 - the Wrap

Tightness Layer 2 - the plaster / internal lining (Current)

Design Out Thermal Bridging

Thermal Bridging 1 - Frame

Thermal Bridging 2 - Slab

Design Out Thermal Bridging

In this section we have so far:

  • Considered insulation levels
  • Checked for areas of compression, and
  • Provided a still air space for the insulation to work effectively (by specifying a tight wrap and well caulked internal lining)

This will put your builds on the right track to a great outcome.

Another important element to consider for an effectively insulated external shell, and one that is commonly neglected, is thermal bridging.

Thermal bridging occurs when any conductive material passes from the inside of the insulation to the outside, creating a heat flow bypass, and short circuit of the thermal layer.

Such flows are usually localised, but can have significant impact on the overall performance of the home, and can lead to significant discomfort in areas they occur, as well as presenting potential condensation problems (See Step 3: Performance Construction/Condensation & Indoor Air Quality)

The following are common areas of concern to consider when designing.

Specify for a low Bridging Shell

The following are areas of common thermal bridging failure in standard construction where design and helpful details on the plans can make a big difference to the outcome, and so should be considered:


For those interested, here's the link.


This does not mean that you can't achieve a good result, but like any product you have to understand what the limitations are for a successful outcome.

Steel is ~1000 times more conductive than pine, so when you put insulation between the studs, the heat still flows easily through the studwork around it. A recent report by the CSIRO, titled 'Thermal Bridging for Residential Building Energy Rating' (2020) found that if you take thermal bridging into account an Energy Rating for a steel framed home would de-rate by 0.4 to 1.2 stars!

It is also important to know that the standard R0.2, 10mm thermal tape currently applied to the outside of steel studs is not a panacea, only giving a small performance improvement, changing the average de-rating to 0.5 to 0.9 stars instead...

So, if you are building with steel, you should consider other strategies, like battening out to provide a physical air break between framing and cladding, and incorporating continuous insulation either to the external of the studwork (sometimes known as 'outsulation'), or to the internal side of the studwork instead (eg. insulated plasterboard sheets).

Some bridging does of course happen with timber studs, but because timber is a reasonably good insulator already, the derating effect is not as pronounced.

Also note that the 2022NCC and NatHERS Whole of Home both now account for steel framing de-rating and require extra undertakings for steel framed homes to bring them up to the level of timber framed homes.


Sub-Heading Navigation:

Design Out Compression

Design for Tightness

Tightness Layer 1 - the Wrap

Tightness Layer 2 - the plaster / internal lining

Design Out Thermal Bridging (Current)

Thermal Bridging 1 - Frame

Thermal Bridging 2 - Slab

Thermal Bridging 1 - Frame


Unlike Steel Stud framing, Structural Steel is still not included in thermal assessment. In reality however, it is very common to have steel beams that breach the studwork or touch linings either side of the wall, creating significant localised thermal bridging.

It is recommended that Designers and Builders check the Engineering Plans for any steel running from the inside of the insulation to the outside of the house.  This will often be the case for counterhung balconies, or structures supported off the house. Such external beams and metal structures will act as radiating fins, sucking heat out of the house in cold weather and radiating it to space. And doing exactly the opposite when it is hot and clients are trying to keep the house cool!

Talk to the engineer about going out from the house in timber instead, or using structural thermal break material, as in the picture above, that can create a thermal break to heat flow through the steel.

Likewise check for any steel posts or beams that take up the full width of wall or roof cavities and are likely to touch internal and external linings. For these it is recommended to pack out the wall a bit to create an air space break (10-20mm), or better yet, create space for some foam.

So, make sure you check your engineering. If you don't pick up bridging problems in design stage before the build begins, the house will not perform to it's thermal rating.





Who still gets their roof plumber to put on steel battens?

If you are using a building blanket you might want to think again…


As mentioned, steel is 1000 times more conductive than softwood.

And as can be seen in the picture, steel battens create a bridge circumventing the blanket.

Of course, there will usually also be batts between the rafters, which are not yet installed in the picture. But such bridging will certainly reduce the performance assumed by the star rating.


In summer, metal battens get hot, carrying in the heat of the sun beating down on the roof above.

But at night, in winter, they can get cold.

This in turn increases the potential for condensation on the steel roof battens under the blanket.

Another thing to try hard to avoid!


Hot Tip: The easy solution of course is to specify timber battens.


(Of course, if you don’t use a building blanket, and use a breathable membrane with counter battens over, then go for it! Use whichever battens you want...)





Ok, I know lintels are technically part of the wall construction. But we put them in over every window to support the loads down to the jamb studs on either side.

No window, no associated lintel. So they sort of go together.


Now, we have spoken previously about windows effectively being thermal holes on cold nights, or on hot days.

But have you ever considered that this thermal weakness gets compounded by the lintel over?


Let me explain...


In standard framing, lintels are usually 140x45pine, and get bigger as the openings get wider. 240mm+ is not an uncommon width.

The lintel section of wall above the window very rarely gets insulated as batts need to get ripped/rebated to fit in, so installers usually miss them.

AND they are under the misapprehension that 45mm thick pine is enough insulation in and of itself.



Softwood (pine) gives R0.1 for every 10mm of thickness. So 45mm give R0.45

The 45mm air space remaining gives another R0.16, for a total insulation of R0.61 in the cavity.

Compare that to R2.5 batts commonly specified in the wall, and you can see the problem.

(Note, it is even worse if you have structural steel lintels, which are reasonably common on wide glass openings.)


And let’s say you have 30 windows in the house with an average width of 1.5m and an average lintel of 140mm high.

That works out to 6.3m2 of poorly insulated wall!

And that area could be more if you have wide openings…


Windows are already thermal weak points.

Don’t make them even weaker by accepting uninsulated lintels!


Simply rebate out the insulation batt to fit over the face of the lintel as you go.
It increases the cavity insulation to R1.7.
Much better!


Yeah, it takes a little longer.

But I stress LITTLE.

And it lasts the life of the house, so worth doing.


We can often spend a lot on high performance windows.

Don’t ruin it with un-insulated lintels!


Sub-Heading Navigation:

Design Out Compression

Design for Tightness

Tightness Layer 1 - the Wrap

Tightness Layer 2 - the plaster / internal lining

Design Out Thermal Bridging

Thermal Bridging 1 - Frame (Current)

Thermal Bridging 2 - Slab

Thermal Bridging 2 - Slab


Concrete is a good conductor, and bypasses any insulation when directly connected to the ground.

This raises the question...SHOULD YOU INSULATE UNDER YOUR SLAB?

Well, it depends.

Conventional passive solar design usually has pictures of slab on ground, locking to the mass of the earth below, to help stabilise the house towards deep soil temperature (3m down).

But what if your deep soil temperature is too cold, or too hot?  Wouldn’t putting a highly conductive material like concrete on it, just exacerbate the problem?

Good question.

The answer is YES, but what are those temperatures?

In hot climates if your average soil temperature is above 19 degrees, locking to the ground will generally make it harder to cool down your house or take advantage of lower night temperatures. (And soil temperature generally raises a bit in summer exacerbating the problem).

In cool climates it will depend a bit more on your house design.  
A house designed to make good use of northern winter sunlight will be able to heat up a slab on soils not already at optimum temperatures to some degree in the cooler months, and then enjoy the coolness in summer.  But the general rule of thumb is that if the average soil temperature is less than 16 degrees it would be better to insulate under and keep that winter energy in the house a bit longer.

If you are in the goldilocks zone of 16 to 19 deg ground temperature, then couple directly.  A good design will give you comfortable winter performance, and nice cool floors in summer.

Of course, the best way to know and quantify the benefit of insulation is to do an energy rating at design stage.  Insulation costs money and resources after all!

Oh, and how do you know your deep soil temperature?

At 3m deep your soil temperature is the average of your climate.  
So, if you add all the max and min monthly temps over the year and divide by 12 you get it.

Then it generally goes up ~2 deg in summer and down ~2 deg in winter.

You can go to the BOM website below and click through to your local weather station to find it average temperatures for your climate.

Some decoupling options can be seen in the picture above.

Pic 1: Waffle slab.  While not 100% decoupled, makes a difference (around 1/2 star in cool climates) and is usually a cost-effective option.

Pic 2: Full insulation under.  Applicability will depend on soil report. Your engineer will need to advise.

NOTE: Insulate under slabs in Cool Climates where water tables are high!

A new CSIRO report, Looking Down to Look Ahead Slab Insulating and Heat Retention (2020), warns that soil moisture can have a big effect on how well a slab performs.

Basically, the wetter the soil, the more conductive it is, and the faster heat is lost to the ground.  The biggest determinant of soil moisture is water table depth.  Even at 5 meters deep you get a 24% increase in annual mean heat loss through the floor, as the water table keeps the soil above it somewhat moist.  At 3 meters depth that heat loss raises to 54%!  

Insulating under your slab helps a lot, and the CSIRO have given some guidance on how much to add depending on water table depth.

If you are in Victoria, there is also a link on the page to take you to the Vic Groundwater Portal that can help you determine water table depth in your area.   It is wise to check this all out before you build, rather than wait until your client comes to you later wondering why their house isn't performing as promised!



Concrete as you know is a great conductor, which is why it can be useful as a thermal reservoir in passive solar design. (See Rule 4: Thermal Mass)
But being a good conductor, it ideally needs to be on the inside of the insulation layer to avoid thermal bridging.
Common practice however often sees slab edges on display, and as you see in the second pic, on cool nights you get heat flowing out under the insulated external wall. And the opposite happens on hot summer days, and in more tropical areas.

Should we insulate it?

Of course. In principle yes.
You can definitely feel the difference if you walk on a polished or tiled concrete floor near the edge of a wall with exposed slab sides.
But we live in a resource constrained world where we need to make bang for buck decisions. There’s also embodied carbon in the insulation materials (usually petrochemical derived), so we also want to make sure we get the environmental balance right.

So. do we insulate or not?

Well, it depends on how much energy is being lost. Which depends on climate, and the freeboard height.

In NatHERS software you get some benefit for insulating the edge, typically about 0.2star. But NatHERS assumes minimum slab height, so also doesn’t give much guidance for high sided slabs. And as heat loss is directly proportional to exposed area the higher the slab edge the more heat it loses.

So, here’s the answer (at least a take on it):

Yes, slab edge insulation is a very good idea.
If you have already allowed for high performance windows,
And have efficient hot water & heat pump space conditioning sorted,
And have a PV system budgeted,
And money put aside for energy-efficient appliances,
And there’s still money left in the kitty…
…Then it’s probably the next item on the list.

But if you are in a more extreme climate, or have high slab sides, or want top shell performance, or are more Passivhaus inclined…
…Move it up the list!
Clients won't regret it.


Insulate between house slabs and paving slabs!



If you pour a concrete paving slab directly against the house slab, you effectively create the same situation as a high exposed slab edge. Through concrete-to-concrete connection, you make a large highly conductive surface area that will act as a radiating fin, drawing heat out of the house in winter and bringing it in during summer.

So, what to do?


Put some XPS foam in between.  In the pic you can see 30mm R1 XPS doing the job nicely, allowing house and paving to be placed in the one pour, while still being thermally decoupled.


If doing the paving as a secondary pour against a slab edge, the same foam can be used, or, if you need a thinner product, sticky backed Ableflex will also act as a workable break, though to a lesser degree.

And that’s it.

Hot Tip: It doesn't take much foam/Ableflex, and it doesn't take long, but it must be specified, or it won't happen onsite.

(NOTE: If your engineer has integrated the steelwork between house and paving slab, speak to them first before decoupling.)


Sub-Heading Navigation:

Design Out Compression

Design for Tightness

Tightness Layer 1 - the Wrap

Tightness Layer 2 - the plaster / internal lining

Design Out Thermal Bridging

Thermal Bridging 1 - Frame

Thermal Bridging 2 - Slab (Current)

Rule 4: Thermal Mass



Of the 5 elements of Passive Solar Design, (Orientation, Insulation, Fenestration, Thermal Mass, & Ventilation), thermal mass is the most nuanced and least straight forward.

Whether it gives operational performance benefits, and how much, depends on the design and the climate.

And it can often add cost.

Can often include a lot of embodied carbon (as mass products are often fired, or include a lot of cement.)

Can be difficult to thermally isolate (making thermal bridging more likely.)

And, when integrated with polystyrene, poses potential issues for recycling at end of life.

These potential downsides have led some to say Light & Tight is the way to go.

And it is.


But we need to be careful not to throw the baby out with the bathwater. It seems unwise to ignore a tool in the toolbox.

Thermal mass, done right, can…

  • Lower energy needs, naturally moderating external temperature swings
  • Give free heat in cooler conditions
  • Give free cool in warmer conditions
  • Give sound proofing
  • And a sense of solidity
  • Act as a hedge against unwanted air infiltration
  • And help regulate humidity

And of course, can often look great.

It can even be done with low embodied carbon.

So how should we use it?

When should we use it?

What are the pros?

What are the cons?

Read on. Then you can decide for yourself when it makes sense to use it on your projects...


Sub-Heading Navigation:

When to use Thermal Mass and How it Works?

Turbo Charging Thermal Mass

How Much Mass and Where?

Mass Superpower 1 - hedge against air leakage

Mass Superpower 2 - hygrothermal buffer

Mass Superpower 3 - Mass & Glass

Mass Superpower 4 - Mass as a Battery

When to Use it & How it Works?



Thermal mass is any heavyweight material (concrete, bricks, stone, etc.) on the inside of the insulative shell of the home and is often a feature in passive solar designed homes.  (Think polished/tiled concrete floors, or reverse brick/block walls, or rammed earth…)


But do you need high internal thermal mass to reach 7 stars?  Or to achieve Net Zero outcome?


Simple answer - No

But it can often be useful to make it easier to achieve. Or if you are wanting to push to the top of the star bands…

So, it begs the question, when should you consider high mass construction or use internal mass?

USE 1: 
If you are in a climate with high diurnal temperature variation, especially one that swings either side of comfort conditions for a significant part of the year, mass can be used to even out the temperature.


A house with high internal mass simply takes a longer time to heat up and a longer time to cool down, so tends to moderate the internal temperature compared to the external temperature swings.  

THE GENERAL RULE OF THUMB is that if your climate frequently has daily temperature ranges in excess of 7°, then thermal mass is likely to be your friend.  The bigger the temperature swings the more useful mass is.


The graphs, from the excellent Energy Smart Design Manual, show the point.

The black line shows a heavyweight home in summer and winter.  The dotted line is a lightweight home, and the green line the outdoor temperature. 

In summer mass moderates the daily temperature extremes. 

In winter mass moderates the night-time lows, as it takes longer to cool down compared to a lightweight home. 

(Note, in the winter graph, the house almost certainly has good solar gain during the day to help heat the mass.  If this can’t be achieved the winter benefit would be less obvious.)


This brings us to the next way that mass can be useful - as a thermal battery...


Sub-Heading Navigation:

When to use Thermal Mass and How it Works? (Current)

Turbo Charging Thermal Mass

How Much Mass and Where?

Mass Superpower 1 - hedge against air leakage

Mass Superpower 2 - hygrothermal buffer

Mass Superpower 3 - Mass & Glass

Mass Superpower 4 - Mass as a Battery

Turbo Charging Thermal Mass



A lightweight home can only hold so much energy.

As sunlight comes in, its energy can be thought of as a bit like water, pouring into a cup. As the home heats it’s temperature will go up and it will radiate more heat away more quickly.  Or if enough sunlight comes in, it will get uncomfortable, and the owners will open windows.  Either way, it’s capacity to absorb the free energy of the sun is limited.  The cup can only hold so much water.  

Adding thermal mass is like changing that cup to a bottle, or a bathtub, or a pool, depending on how much you put in.


This leads us to the second use for mass.

USE 2:
If you are in a climate where winter heating is required and have reasonable northern solar access, mass can be a thermal battery.

The following diagram shows how mass can be designed in to work for you as a thermal storage:

During the day in winter, if you have good northern sunlight coming in, a high mass home (in this case the concrete slab), increases the capacity to absorb that energy.  Once the sun goes down it will radiate stored heat back into the internal environment well into the night, keeping the temperatures higher than they otherwise would have been.


In summer the opposite occurs.

Even if you have used the 45deg rule to shade out the direct sunlight, and insulated the shell thoroughly, packets of heat will slowly make their way into the structure – it is impossible to stop completely.  However, in a high mass home, it takes a lot of packets of heat to change the temperature of the mass each degree, so the mass soaks up the heat infiltration and keeps the internal summer temperatures relatively low.  (Of course, if the heat wave goes on long enough, you will still need measures to cool the home.  See Your Home as a Thermal Battery, and Rule 5: Ventilation)

Ok, so we have seen that mass can often be useful, in both summer and winter.  But how much should you put in and where? Let's go over rules...


Sub-Heading Navigation:

When to use Thermal Mass and How it Works?

Turbo Charging Thermal Mass (Current)

How Much Mass and Where?

Mass Superpower 1 - hedge against air leakage

Mass Superpower 2 - hygrothermal buffer

Mass Superpower 3 - Mass & Glass

Mass Superpower 4 - Mass as a Battery

How Much Thermal Mass Should You Include and Where?

Junction House by Positive Footprints. Suspended bondek floor to upper living and solar access.


The answer, unfortunately, is very climate and design specific.

While an experienced designer may have good rule of thumb advice for a particular climate, the answer can only really be quantified by modelling from a Thermal Performance Assessor. Ideally this is done at Design Concept Stage while design and material choices are still fluid.

To calculate the optimal balance of thermal mass, to glazing, to house size, at a particular location, is frankly beyond most of us!

But it is relatively EASY in energy modelling software.

Where Mass is of benefit, the first rule is Area Counts.

And the most cost-effective way to spread thermal mass throughout a house is generally with a concrete slab.  It also locks somewhat to the mass of the earth, effectively increasing capacity further. So, if your site conditions make this a practical option, ask the assessor to model it.  This is usually not a difficult or time-consuming process. And ideally to model it with and without insulation under. 

You can then ask them to do the same with internal mass walls, or reverse brick veneer. 

Remember area counts, so choosing larger exposed walls will generally be better.

And which walls are best?

While walls and surfaces that receive direct northern sunlight are best, the heat will conduct through to other areas of the house. And light bounces around, also spreading the energy to surfaces not in direct sunlight.

Of walls that aren't in direct sunlight, external reverse masonry walls tend to perform a little bit better than internal masonry walls, as replacing the plaster with masonry increases the insulation levels a bit, and introduces a thermal lag for incoming heat through the external skin.

Such walls however tend to be more expensive than standard plaster, which is why modelling is useful to let you know if it's worth it from a performance gain point of view.

And if you do find thermal mass beneficial in your project, be it floors or walls, remember that a lot of thermally massive materials, either need to be fired, or contain cement, so can be high in embodied carbon. 

Where the operational energy gains are marginal, embodied carbon can be higher than operational carbon savings, especially as the grid decarbonises.  So, try and choose low embodied carbon massive materials where possible.  See Step 2 - Low Impact Materials
Recycled reverse brick veneer walls and a 60% cement replaced concrete floor, makes for a low embodied carbon, high mass home.


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When to use Thermal Mass and How it Works?

Turbo Charging Thermal Mass

How Much Mass and Where? (Current)

Mass Superpower 1 - hedge against air leakage

Mass Superpower 2 - hygrothermal buffer

Mass Superpower 3 - Mass & Glass

Mass Superpower 4 - Mass as a Battery

Mass as a Hedge against Air Leakage


SUPERPOWER 1: Mass is a hedge against inadvertent air leakage.

Bricks, concrete & other heavyweight building materials can hold a lot of heat compared to the same volume of air.

The following table gives you an idea of the how much energy (in kJ) different materials hold per m3, for each degree kelvin (k) of temperature.

Bricks for instance store 1360kJ of energy in each m3, for each degree it heats up.

Air by comparison at sea level holds just over 1kJ in each m3 of space, per degree of temperature.

That means bricks hold more than a thousand-fold the amount of energy in air, volume for volume.

Concrete is more like two thousand-fold.


It means that if you have mass in your home (concrete, rammed earth, etc), which has been heating all day as sunlight comes through windows, it gives you a lot of head-room against inadvertent air infiltration. Air that does leak in, can be re-heated without a noticeable change in temperature of the structure.

This is not to say that we shouldn’t be trying to get our structures tight!

If the leak is big enough, and the air cold (or hot) enough, the capacity of the mass will eventually drain for a noticeable temperature change. (Or for very big leaks, it hits you before the mass has had a chance to re-heat it. Brrrr!)

This capacity to re-heat incidental infiltration can be especially useful for homes that aren’t going super tight with HVAC, & which therefore require some passive infiltration through the fabric for indoor air quality when windows are shut. Mass can reheat this air infiltration without discomfort for the occupant.

And even for Passive Homes with heat recovery ventilation, this feature still gives a benefit. HRVs continuously bring in air, & while such systems are around 90% efficient, this means they are still 10% inefficient...

A bit of mass, combined with north windows, may just help make up the difference.

All else being equal, a higher mass home has a lot more capacity to re-heat or re-cool any unwanted air infiltration, vis-à-vis the same tightness lightweight home.

But that is only the first SuperPower. Mass has 3 more...


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When to use Thermal Mass and How it Works?

Turbo Charging Thermal Mass

How Much Mass and Where?

Mass Superpower 1 - hedge against air leakage (Current)

Mass Superpower 2 - hygrothermal buffer

Mass Superpower 3 - Mass & Glass

Mass Superpower 4 - Mass as a Battery

Mass as Humidity Control



SUPERPOWER2: Mass can help control internal humidity.

Nearly all materials in the home have some ability to absorb moisture.

But high mass materials (Bricks, blocks, established concrete, rammed earth, mudbrick, etc.) have a much greater capacity to absorb and release moisture to the internal air, than thinner lightweight building products.

This ability can help to regulate internal humidity levels.

When humidity is high in the internal air, the massive elements in the home can absorb and moderate the peaks.

When humidity levels are low in the internal air, the moisture will come out again, and help raise internal air moisture levels.

The diagram from the excellent ABCB Condensation in Buildings Handbook shows the relative ability of high mass walls vs. lightweight walls to absorb humidity. This ability of heavyweight walls to absorb and release moisture in the air is known as Hygrothermal Buffering.

(And don’t make the mistake of saying “Hydro” thermal if you want to maintain the street cred!)

Hot Tip: Unfired mass building materials are even better at this ability!

Mudbrick, and Adobe earth products are champions at this! Just make sure you use the recommended breathable natural paints…

Earth building products also tend to be 250-300mm thick, so introduce A LOT of thermal mass into the structure. And, as an aside, they are great at stopping sound transmission, (arguably another SuperPower!) so are a great choice as internal mass walls between living and bedroom areas…

Mud brick wall with lime-based paint provides hygrothermal buffering AND excellent sound insulation between living and bedroom areas.

Oh, and why do we want to be able moderate humidity extremes?

Humidity levels above 70% can lead to mould growth.

Humidity levels below 40% can feel uncomfortable.

Mass can help form part of a strategy for getting humidity levels into the Goldilocks zone, where internal moisture is JUST RIGHT…

For more on Humidy and Condensation Control see Step 3, Condensation & Indoor Air Quality)


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When to use Thermal Mass and How it Works?

Turbo Charging Thermal Mass

How Much Mass and Where?

Mass Superpower 1 - hedge against air leakage

Mass Superpower 2 - hygrothermal buffer (Current)

Mass Superpower 3 - Mass & Glass

Mass Superpower 4 - Mass as a Battery

Mass & Glass



There is of course the combined third option of TIGHT & MASS & GLASS.

We have looked at Window Area vs Performance in Rule 2, (and even shown this graph), but it is worth delving a bit deeper.


Normally there is a correlation between smaller overall window area and performance.

This is because windows are effectively thermal holes when conditions are more extreme outside (like at night for instance.)

In Cooler Climates however you can buck this trend to a certain extent by the inclusion of appropriate mass, combined with appropriately placed windows.


Here’s the rules in Cool to Mixed climates:

IF you have good solar access,
you have most of your windows facing North (south in the Northern hemisphere)
you don’t go overboard with glass on the other directions (keeping close to daylighting requirements in the building code)
adding MASS & GLASS (to the north) will likely improve performance.


You can see this effect represented in the Tassie graph of Window Area vs Star Rating Performance above.  Almost certainly the window area rise at 8 stars is accompanied by a proportionate increase in mass.

There’s also a similar trend in the other southern states, but it becomes less pronounced as you move north.  (See Rule 2, Window Area and Local Climate Considerations.)


So, if you have reasonable solar access, ask your Energy Assessors to test this effect in an Optimisation Report, by putting in a bit of mass and seeing what would happen if you made northern windows a bit bigger.


You may just find your house has a hidden Extra Gear…


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When to use Thermal Mass and How it Works?

Turbo Charging Thermal Mass

How Much Mass and Where?

Mass Superpower 1 - hedge against air leakage

Mass Superpower 2 - hygrothermal buffer

Mass Superpower 3 - Mass & Glass (Current)

Mass Superpower 4 - Mass as a Battery

Mass as a Battery

Downsize Up(grade) House by Positive Footprints


Traditionally thermal mass required correct application of window placement to really make good use of it and charge it during the day in winter.

It also required good application of eaves shading to keep the sun out and benefit from the coolness of mass during summer.

But now, with cost effective Photovoltaics (PV), we can put this effect on steroids!

Let’s say it’s winter. Some sunlight is coming in through well placed windows and heating the slab or internal brick walls. Great!

But will it be enough to keep the house warm all day?

Well, it depends on the day. Often you could do with a bit more…

But if you have PV on the roof, you will have that bit more!

And then some.

Don’t send that extra power out to the grid for a piddling return. Use your house like a battery and put it into your mass!

Turn on the A/c and charge your mass with heat.

Then, when the sun goes down, turn the heaters off and bathe in the radiance of the warm surfaces, keeping you toasty throughout the night.

In summer it’s the opposite. You will be making plenty of power on those hot days.

So put on the aircon and charge the mass with cool.

When the sun goes down, turn off the aircon, and the mass will keep you comfortable inside on those sticky nights.

This is a lovely application of old technology meets new. A real match made in heaven. A superpower if you like...

And something lightweight homes can’t make full use of.

Passive is the foundation, but Active is here.

With a bit of mass, your house can be a battery.

Use it!

PS. Got a BIG PV system? Also heat your hot water during the day.

Still have power to spare…get an electric car & an e-bike.

The future is here, and it’s Active!


Sub-Heading Navigation:

When to use Thermal Mass and How it Works?

Turbo Charging Thermal Mass

How Much Mass and Where?

Mass Superpower 1 - hedge against air leakage

Mass Superpower 2 - hygrothermal buffer

Mass Superpower 3 - Mass & Glass

Mass Superpower 4 - Mass as a Battery (Current)

Rule 5: Ventilation

Identify Cooling Summer Breezes & Align Ventilation Pathways for Free Summer Cooling


If you want some free summer cooling, design to welcome cooling summer breezes.  As part of the concept design process, it is important to know from whence your summer breezes blow, so that you can make sure your floorplan incorporates the openings in such a way that these potentially cooling breezes are encouraged to blow through the structure. Local and historical knowledge of the area in summer is preferable, but failing that the Bureau of Metereology has created wind roses for its weather stations across Australia.

To find your local summer breezes go to . Use the map function and select 'climate statistics', and type in the name of the town/suburb, then select the closest weather station on the map. Open the 'Monthly Climate Stats' for the weather station selected. Download the 3pm summer wind rose (or use Jan), and use it to mark up on your block which way the prevailing breeze blows. Your floorplan and window layout should then, as much as possible, align to allow the prevailing breezes through the structure.

The basic rule of thumb is that the exit window should be 20% bigger than the entrance window, and the flow should be as straight as possible without intricate passageways to negotiate.

Casement windows can be very useful, as not only do they open 100%, but they can also be placed on the sides of the house, hinged towards the prevailing breeze to scoop it in as it goes past.

Moving air can also give an approximate 3 degrees of cooling effect as it goes across the skin evaporating off the moisture. So even if the breeze is the same air temperature outside and in, having good breeze paths through the house will be beneficial, and feel cooler. This is why ceiling fans are also so useful.

It is also worth checking if there are any summer nighttime prevailing breezes, as opening windows and bringing in the night air will passively cool the house over the course of a summer's night, ready to close windows the next morning, in preparation for the next hot day. This ability to purge the heat is particularly useful for houses with higher thermal mass.


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Passive Cooling

Passive Cooling

Another consideration is stratification. 

Simply put, Heat rises.

You can make use of this phenomenon for some free nighttime cooling, even when no wind is blowing. 

In a double storey or single with popped clerestory area, a window (with a motor), can be placed at the highest point.  The warmest air will naturally make its way there and exit the house when open.  This tends to create a negative pressure on the ground floor.  If you open a window or two at ground level, cool night air will be drawn in.  This cool air will flow over the surfaces of the house, absorbing heat out of the structure.  It will then get warmer, expand and become more bouyant, and move up to the highlight window and exit the house. 

This creates low pressure, which in turn draws in more cool air at ground level.  And the process repeats.  Over the course of a nighttime, this airflow can cool the structure, ready for the next hot day.

Easy and cheap to do.

Elegant and poetic.

So, consider window placement. 

When conditions are right, nature and physics, are your friend...

Check out the excellent sustainability bible that is the Your Home manual here.


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Passive Cooling (Current)

NatHERS as a Design Tool

DO – 2 – RATINGS!!


Hear me out…


At the end of the design process all new homes following the NatHERS pathway under the National Construction Code, need to meet the minimum house energy rating (to be raised to 7 stars in Oct 2023, and adopted by most states over the coming 18 months), in order to get their Building Permit.

So Designers send plans off to a Thermal Performance Assessor (TPA) to get a rating done.

However, by this late stage, there has been Townplanning, Engineering, Working drawings, meetings with Council, & months (if not years) of time & effort.

And for all intents & purposes the design is set in stone...

So for the TPA, if the plans doesn’t rate up, all they can do is increase insulation to cavity maximums.  And if that doesn’t get the house over the line, they can double glaze windows until they reach a pass mark.  Plans are then stamped & handed back.


This is a recipe for MINIMUM COMPLIANCE,

& really shackles the power of the software as a DESIGN TOOL!

To leverage this power,

DO A RATING AT CONCEPT DESIGN STAGE, while the design is still fluid & can be changed!


The software is very powerful in that it can predict temperatures in the different rooms of the house throughout a simulated year, using local climate data. 

The TPA can then play with different design scenarios, insulation levels, building materials, window & eave sizes, etc., to optimise performance for each room of the house, & produce a Thermal Optimisation Report.

This information can then be fed back into the design... 

And how much difference does it make?
Depends on the initial design.
For most designs usually 1 star or more.
Even for designs steeped in Passive Solar principles, there is usually at least ½ star of improvement options to be found!

Of course, this does mean 2 ratings, as there will be slight changes to be reassessed prior to Building Permit.

But as we move to 7 stars the early knowledge that you are on the right track will save HEARTACHE & EXTRA COST down the track.

And for the Owner?

A second report fee will save thousands over the life of the house,

& make every moment more comfortable...

It’s not a hard sell...

NOTE: If you don't already have a Thermal Performance Assessor, Design Matters National is the peak body for Designers and Thermal Performance Assessors, and has a handy practitioner lookup HERE.

Cost Effective Tips to Acing 7★



It’s a fair question, in this climate of rising house prices.


There have been many guesses in various media reports.






And guess what?

They are ALL right!


It ‘could’ cost $40,000 if you are trying to improve an existing 6 star home plan, that only just got to 6 stars with double glazing, & now requires triple glazing & stud thickening for extra insulation, to achieve 7 stars.

Such an approach is however the most expensive way to achieve 7 Stars – by thinking of it only as a product substitution problem.

On the other hand, it can also cost nothing in many cases - if the designer is well versed in how to design for the local climate, so that the house doesn’t need much energy for heating and cooling in the first place.

This is a point that somehow seems to be overlooked in most media articles.

So let me ‘yell’ it now in all-caps…



And by GOOD DESIGN I mean using the 5 Passive Solar Principles…

…all of which are conveniently laid out and explained in detail above.


While we usually promote these principles as a way of achieving high star designs on the way to Net Zero outcomes, the same principles CAN ALSO BE USED for those seeking low-cost compliance.

BUT, if you want the lowest cost compliance, (or lowest cost high performance), you also need to know where the where the easy points lie.


And that’s what this section is about.

It’s about the little1-percenters.

The little adjustments that can be made to plans, that by themselves may only give a minimal improvement, but in aggregate can give that extra half a star you need to get a plan over the line, or achieve 7 stars without reaching for expensive products.


Nice information to have when the budget is tight!

And the tips start flowing now…


Sub-Heading Navigation:

Orientation - What if you're in Shade?

Windows - You need the most expensive Right?

Insulation - How much do you need for 7★?

Mass - How much do you need for 7★?

Ventilation - What you should know

Last Penny Savers Tips to Acing 7★

Orientation - What if you're in Shade?



It’s usually the first line in a tirade about how expensive 7 stars will be.


Firstly, it is VERY RARE to find a block with no northern solar access.  The sun moves across the sky, & changes apparent altitude both during the day & over the seasons. Things that may shade in deep winter often don’t from shoulder season onwards.  
And even on shady sites, there are usually parts of every block that get sun at various times of the day even in winter, especially as you move higher up the walls and to roof level.


SO, in the first instance, if you are in a Cool to Mixed climates, seek out what Nth solar access you have and maximise it. Good solar access will usually gain you at least a star in NatHERS.  
Poor solar access MAXIMISED will usually gain you ½ star.  


(And if you don’t know how to maximise STEP 1: Orientation above.)


BUT, what if you DO have a block COMPLETELY in Shade?

Is that a fail?



Remember we only need to achieve 7 stars, not 10!


In such a situation, you follow more of an insulation strategy with careful window consideration.  We will look at this next.

There are however still some extra points to pick up in Rule 1: Orientation

1. If Nth is shaded in Cool/Mixed climates, then orientate E, then W, & lastly Sth.

East tends to be better than west, as it starts the home heating early, and isn’t so problematic with afternoon summer sun shading.

2. Buffer your living space with other rooms

The main kitchen/living area is typically responsible for the lion’s share of the energy use.  So use bedrooms, utilities & garages to buffer the Kit/Liv from the colder Sth (& West) side of the home.  Consider those rooms almost as extra insulation for the living zone.

3. Minimise wall to floor area

Another way to minimise heat loss is to minimise wall area.  A compact design will be more efficient than a longer, thinner, or articulated design. Less external surface means a better score, & lower cost!


Next, let's look at some tips for Windows…


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Orientation - What if you're in Shade? (Current)

Windows - You need the most expensive Right?

Insulation - How much do you need for 7★?

Mass - How much do you need for 7★?

Ventilation - What you should know

Last Penny Savers Tips to Acing 7★

Windows - You need the most expensive, Right?



Usually not... if you have a good design.

Don’t get me wrong.

High performance windows are GREAT if they fit in the budget.

They also have a number of benefits from durability to sound attenuation.

And, WILL enable all but the most inefficient designs across the 7-star line…

But with Good Design,
in most climates,
on most blocks,
you CAN get to 7 with single glazing…
…IF you design for climate and don’t go overboard with window sizing.

We’ve spoken before about the main window rules:

- Keep below 22% window-to-floor area.

- And place most windows to Nth, then East, then West, & lastly Sth.

Doing this will already keep you on track for a moderate costing window schedule.

But HERE ARE SOME OTHER WINDOW TIPS to help you get to 7 stars cost effectively:

1. Specify actual window models on plans rather than just a Generic (e.g. “Double-glazed aluminium windows”)

The generic windows in NatHERS, while giving a lot of freedom to choose from a variety of windows during the build, tend on average to be of lower performance than actual models available in the marketplace. So, choosing an actual model (E.g. Rylock AA Series SuperLiteWindow - DbleGlazed - with In-Line Reveal RYL-332-313), will usually gain you more NatHERS points than the generic equivalent.

2. In Cool/Mixed climates, move Nth windows out of any shading where possible.

Remember, windows (even high-performance windows) are the weakest element in the thermal shell of the house. So, make sure you put them where they are doing some good getting energy in.

3. Where funds are tight, if you need to double glaze, start with the Living zones.

In NatHERS, living zones are assumed conditioned at a higher minimum temp compared to bedrooms, (where users are usually under covers.) Windows are also commonly bigger in Living spaces. For both these reasons you get a bigger bang-for-buck in energy saving when specifying higher performance windows to living spaces compared to bedrooms.

Now let's look at Insulation Tips…


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Orientation - What if you're in Shade?

Windows - You need the most expensive Right? (Current)

Insulation - How much do you need for 7★?

Mass - How much do you need for 7★?

Ventilation - What you should know

Last Penny Savers Tips to Acing 7★

Insulation - How much do you need for 7★?


While it is true that is more is usually better, (& double stud walls are awesome 😊), it is also true that insulation has quickly diminishing returns as you add more R value. So it becomes a question of cost effectiveness & space constraints. You get a lot of bang for your buck for the first R1, less for the next R1, less for the one after that & so on…

It is also true that heat rises, so to stop it getting out of the house in cool conditions, the ceiling insulation tends to be the most important. And in summer when high heat load hits the roof, thicker insulation limits heat coming down into the cool house below.

Given this, the rule of thumb to put homes on course for 7+ is to start with:

Ceilings R4
Walls R2.5
Subfloors R2
… & ask the Thermal Performance Assessor to optimise for climate from there.

There are however a few low-cost extra tips to consider with insulation…

1. In Tropical Climate Zones 1-3 where cooling needs prevail, if the house has a walled subfloor, try R1.5 to those subfloor walls but none to subfloor itself. This maximises heat loss from the house to the subfloor which is kept cooler from the outside due to the wall insulation. (Make sure all vents are maintained.)

2. Confirm attic cavity height at external wall edges is enough to fit insulation. IF NOT, ASSESSORS ARE REQUIRED TO DOWNGRADE to R3 to the perimeter of the roof.
Eg. The densest R4 batt is 140mm thick. Plus 20mm ventilation gap requirement. Plus 30mm drape (if having foil backed blanket) = 190mm min. height at attic edge.

3. Chasing points? Look for low emissivity (e) membranes. A membrane with e=0.05 abutting an airspace gives ~+R0.5 These tend to be foils so check NCC permeability Class requirements for your climate zone & consider waterproofing. Particularly effective for roofs in hot climates (eg. Foil backed blanket)

4. Specify on the plans:
– IC rated downlights
– Extraction fans with backdraft dampers, &
– Weatherstripping & draft excluders to external doors, including garage.
This allows assessors to confidently choose the minimum air leakage options in the software.

Read on for Mass tips...


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Orientation - What if you're in Shade?

Windows - You need the most expensive Right?

Insulation - How much do you need for 7★? (Current)

Mass - How much do you need for 7★?

Ventilation - What you should know

Last Penny Savers Tips to Acing 7★

Mass - How much do you need for 7★?


Simple answer – YES.


If you ignore mass, or don’t understand when to use it, you might be making 7+ Stars harder than it needs to be.

Firstly, all homes use mass, even lightweight homes.

Did you know single storey lightweight homes usually perform better than double storey lightweight homes of the same size?


Because single storey homes have more surface area adjacent to the huge mass of the ground, which stabilizes their temperature compared to a double storey house of the same volume, all else being equal.

You can generally increase a lightweight home’s performance, & gain some extra points, if you close in the subfloor to minimum ventilation requirements – making subfloor conditions even more constant.

And change floors to a slab, and in most climates the performance will rise again, as the connection with the ground is improved once more. ½ a star gain is not unusual (though in cooler climates with colder soils slabs will generally benefit from insulation under to slow the pace of conduction to ground…)


There is only one way to really KNOW how much benefit you will get from using mass strategies…and that is to ask an energy assessor to model mass options at DESIGN CONCEPT STAGE.

Having said that, here are some good tips to know about mass when trying to improve the thermal performance of a design:

1. Because ground floors are typically more stable, you usually get a higher NatHERS score putting living areas on the ground floor

2. If you are in a climate with daily temperature swings typically >7°, or are in cooler climates AND have good solar access to heat the mass in winter…, then mass is very likely beneficial

3. Making mass a dark colour in cool to mixed climates increases winter performance

4. Floor coverings, typically reduce the benefit of a slab floor, in relation to their density

5. A house with more mass can typically have more window area (esp. to the Nth)

These are just some tips.

To properly understand mass, see Rule 4: Thermal Mass


Sub-Heading Navigation:

Orientation - What if you're in Shade?

Windows - You need the most expensive Right?

Insulation - How much do you need for 7★?

Mass - How much do you need for 7★? (Current)

Ventilation - What you should know

Last Penny Savers Tips to Acing 7★

Ventilation - What you should know


When indoor temperatures get too hot, before airconditioning is triggered, NatHERS applies some IF/THEN strategies:

IF it's cooler outside,

THEN the owner will open windows if there are sashes to be opened.

(The effectiveness then depends on the outdoor vs. indoor temp difference & the size of openings)

IF it's not cooler outside,

THEN the owner will turn on any ceiling fans.

(While fans don’t actually change air temperature, they can give up to 3° cooling effect depending on size, as the generated breeze evaporates moisture from the skin.)

IF the indoor temperature is still above comfort,

THEN the software calculates how much energy is required to be removed by air-conditioning, to bring temperatures back down to comfort levels.

While natural ventilation strategies are useful for all climates whenever it gets hot, the further Nth in Australia you go, the more cooling needs predominate & the more cooling strategies effect energy savings & ratings.

So, here’s some tips:

1. Window sash selection effects openable area. Openable area affects ventilation. Choose sash types that maximise openings. See picture above.

2. Ceiling fans to all habitable rooms can be worth up to 1.5★ in Darwin, & 1★ in Brisbane. Bigger fans & multiple fans provide more ventilation (up to a point), & score better.

3. In double stories, the NCC specifies window restrictors to limit sash openings to 125mm max, for sashes below 1700mm, to prevent child falls. While very important this can ruin ratings where heat loads are an issue. (Common in hotter climates). Instead, choose the safety screen option, or include a fully openable sash above 1700mm.

4. Roof ventilation also significantly lowers attic heat loads in hot climates. (See NCC ventilator area & venting requirements.)

NOTE: Effects 1-3 will benefit homes in cooler climates during summer too! The ★ benefits are just less because overheating hours are less.


on a hot summer night, clients won’t care what rating you got, if you haven’t implemented good ventilation strategies!


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Orientation - What if you're in Shade?

Windows - You need the most expensive Right?

Insulation - How much do you need for 7★?

Mass - How much do you need for 7★?

Ventilation - What you should know (Current)

Last Penny Savers Tips to Acing 7★

Last Penny Savers Tips to Acing 7★


1. Choose colours suitable to the climate.

The general rule is:

In cooler climates, darker colours improve the rating.

In warmer climates, lighter colours.

There’s some nuance to consider though…

Roof & Walls:

The more insulation you have in roof & walls, the less the cladding colour has effect. So changing colour may not give the benefit one might expect.

Also note, colour changes can exacerbate temps in extreme weather, so when checking the benefit of a change with an assessor, ask them to also check the effect on peak loads before confirming the change.


Interestingly, window frames, because they are less insulative than roofs & walls, often do give a handy little bump in the rating, by changing colour.

And as you move Nth to hotter climates tinting glazing (lowering SHGC) becomes increasingly beneficial. SHGC optimisation by an assessor can gain up to a ⭐️ in the tropics.

Mass Floors:

Where mass works for a home, making that mass dark will increase its effectiveness. Darkening the polished concrete or tile colour on a slab floor, can often give a useful increase in the rating.

2. Minimise internal hallways where possible

This one takes a bit of explanation, see below slide.



3. Get energy assessors on your team at DESIGN CONCEPT STAGE to do an Optimisation Report.

I’ve written about this before, but the thermal benefit to the home of doing this while the design is still fluid, can’t be understated. 1⭐️+ improvements are not uncommon.


4. USE PASSIVE SOLAR PRINCIPLES AS YOU DESIGN, or choose a designer who does.

At this point in the building industry, everyone needs to understand the basics, but designers should be experts. And it’s easy! Just read this page...

Remember, it is hard to unstuff a bad design, & limp it over the line.

But 7 stars is just the beginning when designing through the lens of Passive Solar Design…


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Orientation - What if you're in Shade?

Windows - You need the most expensive Right?

Insulation - How much do you need for 7★?

Mass - How much do you need for 7★?

Ventilation - What you should know

Last Penny Savers Tips to Acing 7★ (Current)

SBA TOOL: How to ACE 7 Stars & Beyond. Cheat Sheet

Click on the image below to download our cheat sheet on How To Ace 7+ Stars.  It goes hand in hand with the information presented in this STEP 1: Good Design, and in the Webinar below.

Use it as a Climatic Design Checklist at Design Concept Stage, to check if your design is on track for 7 Star+ performance!


Designing for Thermal Performance

The Net Zero Homes builders' course is a partnership between Design Matters National and the SA Gov, and was created by the author of this website (Jeremy Spencer - Positive Footprints), so covers much the same ground.

If you like listening rather than reading, this course will be for you. The good news is that all 5 modules are completely FREE at the moment under a funding grant to professionals in the industy.

Highly recommended - register while funding lasts at

Though we suggest watching the videos in order, Module 1, deals with the topics discussed in this step most closely.

The Secrets to acing 7 Star+ House Design & the coming NCC Whole of House requirement

This webinar by Builders Declare also covers Passive Solar design with a question period at the end.


  • Are all your house designs, and all your builds, 7 star+ ?
  • Do you know what the proposed Whole of House changes to the NCC heading your way are all about?
  • Do you know where the industry is heading over the next 10 years?
  • And did you know NatHERS software is just a tool for quantifying Passive Solar Design - and do you know what the 5 Passive Design Rules are?

If the answer is "No" to any of these questions you need to come to this webinar!

Jeremy Spencer, long time Thermal Performance Assessor and Sustainable Builder from Positive Footprints will go through:

- The coming 7 Star Whole of House code and what it means to all players in the industry.
- The 5 easy Principles that will make you an expert in Passive Solar Design (at least almost so!)
- Rules of thumb to cost effectively Ace 7 stars +


Whether you get involved in house design, or just build the plans that come across your table, this is vital information for everyone in the built environment who wants their business to flourish over the coming decade.  For designers and architects this webinar will help you avoid the embarrassment of plans not rating up during assessment.  And for builders, this webinar will give you the knowledge to look over a proposed set of plans and know whether or not there are likely to be issues in meeting the rating.  You don't want to spend time and effort on cost appraisals and advice for plans that won't make the mark...

The following are further useful links for Good Design:

Your Home Manual  (See Passive Design section)


Sustainability Victoria Energy Smart Housing Manual  (Cool/Temperate Passive Design Focus)


CoolMob (Tropical Climate Passive Design Strategies)

Wind Roses / & Climate data


Window Energy Rating Scheme (WERS) Database


Window instructional video (explaining SHGC, Low E, & window performance)


To find a Thermal Performance Assessor for Thermal Improvement at design stage (& final Compliance )

Member Directory - Find a Thermal Performance Assessor (TPA) (


To find a Sustainable Designer - select "Environmental Design" under the Expertise Tab on the Design Matters National member lookup.

Member Directory - Find a Designer (

Information on NatHERS

Case Studies of Sustainably Designed Homes

General Discussions Good Design