Why do I have to use double glazing when they don’t?

Whether you’re a builder, architect or homeowner, this may be a question you have asked yourself during the course of your project.  I’ve heard it before and the other Friday when I was at the Pro Clima installation training day in Mount Gambier, in our post demo chit chat, one of the builders piped up. His question, and I’m paraphrasing, is below:

I’ve had to build with 120mm studs to get more insulation in my walls, double glazing, roof blanket… but the bloke next door has single glazing, 90mm studs etc… how??

It’s a great question and below are a few possible reasons I gave.

  1. Each house is simulated or assessed individually
  2. The house next door used a different energy efficiency compliance system under the NCC.
  3. The builder swapped out inferior windows or levels of insulation, ignoring what was specified
  4. The assessor who assessed next door had differing skill / integrity


Individual simulations

The thermal performance requirements, and resulting specifications of a home depend on many design features of your project.  Each house has its very own energy assessment and is assessed on its own merits.  For example, large houses with larger windows facing in different directions may require a different specification.  The main things that impact the energy efficiency specification are as follows:

  • Size of the building footprint
  • Orientation
  • Number of storeys
  • Size of windows

Other things matter too, for example wall construction, whether you have an elevated floor or a concrete slab, shading etc.  But typically in my experience, the above list are the biggest ones.

Did they use the same assessment method as you?

The most commonly known method for doing an energy assessment is a star rating and sometimes there is a misconception that this is the only way to obtain energy compliance.  There are in fact four ways to obtain energy efficiency compliance in Australia.  See below for more information. (Note: there are state variations)

  1. NatHERS star rating 
    1. Energy loads are simulated in a piece of software e.g. First Rate 5, AccuRate, Hero, Bers Pro
    2. A star rating is produced using heating and cooling loads
    3. Star Ratings are between 0 and 10 stars
    4. 6 stars is the current minimum, moving to 7 stars in NCC 2022 including a Whole of Home Assessment
  2. Verification against a Reference Building
    1. Energy loads are simulated in a piece of software
    2. The energy loads of the house you want to build are compared to the energy loads of a NCC-determined comparison / reference building.
    3. If the house you want to build uses less energy than the NCC-determined comparison /reference building, it can be built.
    4. This method is often used because it has greater flexibility to retain the design integrity and tends to provide a cheaper specification.
  3. DTS – Deemed to Satisfy
    1. Ticking off specific requirements of the code for each part of the building e.g. walls, roof, glazing
    2. This method is commonly used in-house by architects who have a standard specification and like to do their own assessments.  Understanding the glazing calculator is the trickiest part of doing it this way.
  4. Performance Solutions
    1. Other submissions that your certifier will accept that essentially show that your project emits less less carbon emissions than the building code allows

The intent is that these four methods produce buildings that perform thermally to a similar level, however in my experience, they produce four very different outcomes. 

Material Exchange

Builders going rogue on what is on the plans is not uncommon.  The reason a builder might choose a different window for example, is usually dependent on issues such as cost, supply issues, last minute design changes by the client or sometimes, simply what is easiest.  This happens all the time in the residential sector in particular and if the house isn’t likely to fall down or burn down, any other non-compliant issues go more or less unchecked. 

On a different, but related note, have you ever seen a “by owner after handover” specification? This is usually noted in relation to ceiling fans, floor coverings, or external window shading.  If these are noted on the plans, these items are modelled but may or may not be installed.  Each have an influence on the outcome of the assessment.

Energy Assessor skill / integrity

Just like in every other profession, there are differing levels of skill and honesty in energy assessors and it’s almost impossible to tell from your energy report unless you know what you’re looking for.  Most assessors do the right thing, and star ratings are audited from time to time, however energy assessments are complex.  The technical notes that govern how star ratings are modelled can be open to interpretation.  I can’t count the number of times I have pulled up and studied the tech note and been so grateful for the team of people around me who help me interpret.  There are a number of assessors out there who don’t have that support and therefore must push through on their own.  Verification Against a Reference Building is incredibly complex.  The glazing calculator can be very tricky.   a complex beast.


The energy efficiency section of the building code is a complex beast.  It is well acknowledged in the building industry that there can be a fair difference between was is designed, what is modelled and what is built (known as The Performance Gap – this is another lengthy discussion in itself).  So to summarise, if your neighbour is using single glazing and you have to use double glazing, it’s likely that the reason could be one of the following:

  • Their house is smaller and better orientated than yours
  • A different assessment method was used
  • The builder has used a product or material they shouldn’t
  • The skill level and/or integrity of your energy assessor differed from the skill level and/or integrity of their energy assessor

Having said all of this, it’s really important to understand that windows are the weakest point of your building envelope.  This is where you will lose and gain most of your heat and installing high-quality windows is the best thing you can do for your future thermal comfort.  As building code requirements increase, double glazing will become more and more common all over the country, not just in cold climates.

If you have further questions about energy efficiency, condensation management or air tightness, drop me an email or head to our website www.climasure.com.au and sign up to our mailing list.  We never spam, promise.

Hope to talk soon!


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