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Am I overthinking my energy rating?

Q+A: Am I overthinking the energy rating for my new home?

We are building a 7.3 provisionally star rated home in Duncraig, Perth.

I have been heavily invested on making this home thermally comfortable as I’ve never been so cold as I have in Australian homes in winter!

With many rooms achieving 9.9  or 10 star ratings and the open plan living achieves  7.7 stars, there are three rooms which achieve only 4.4 stars due to high heating demands.

My question is: if I go for a ’darker’ medium roof colour ie. 0.58 solar absorbance over 0.47, will this help with the reduce the heating demands?

I’m also thinking I would prefer  lower heating demands as it’s so much more expensive to heat a home than to cool it. 

We are also installing solar panels which will then provide electricity for the cooling on those intense Perth summer days.

I am also considering double glazing on the south side along with the high-performance glazing we have already stipulated to achieve the above star rating.

However with many of those rooms achieving 9.9 is it worth it? They are south facing rooms.

Am I over thinking this?!

You’re not overthinking your energy rating! I wish there were more people out there like you 😊

It’s great that you are aiming for a better than minimum energy rating for your new home.

Interesting that you note that you are more affected by the cold. Many people don’t realise there are actually more days when it is too cold rather than too hot in Perth.

It sounds like your energy assessor is using BERS Pro to conduct your energy rating, as this is the only energy rating software I know of that reports star ratings for individual rooms. This isn’t officially part of the NatHERS protocol, and is a standalone feature implemented by the software developer of BERS Pro to try and provide some guidance as to how each zone complies. I’m not a fan of this, as I believe BERS Pro uses averaged heating and cooling loads to calculate these star ratings, and so it doesn’t truly reflect how each individual room will perform (but I could be mistaken about this). Other energy rating softwares such as FirstRate 5, AccuRate and HERO will report the actual calculated heating and cooling loads of each zone. If you can get your energy assessor to provide these heating and cooling load values for each room, this will give us a much better idea as to how each room is performing.

If you select a darker colour for your roof, yes, it will help to reduce the heating load, by increasing the solar radiation that is absorbed by the roof in winter. BUT… you will also see an increase in the cooling load over summer ie. your home will now use more energy to stay cool in summer. You will often find that the “improvement” in reduced heating load over winter is negated by a comparable increase in the summer cooling load. And while there are more cold days than hot days in Perth over a typical year, when it’s hot in Perth it is VERY hot. So while the star rating may appear to improve or stay the same by specifying a darker roof, the actual thermal comfort in summer becomes proportionately worse compared to a small improvement in winter.

In terms of the relative cost for heating versus cooling your home, it very much depends on how you are planning to heat/cool your home. Split-system inverter reverse cycle air conditioning is one of the most energy efficient (and cost-efficient) forms of heating AND cooling available.

Also be aware that darker coloured roofs reduce the efficiency of solar PV because it causes the panels to overheat. With a darker coloured roof, unfortunately you can get up to a reduction of 25% in your solar PV performance.

As for where you might upgrade to double glazing for some windows or rooms, this might be something worth considering. While the numbers might suggest the room is comfortable overall, being adjacent to a lower quality window on a cold winter’s night will still be uncomfortable. This Fifth Estate article is worth a read.

Alternatively, you could install quality window coverings with a closed or recessed pelmet to help reduce heat loss from these windows. The type of window covering doesn’t get included in a certified energy rating, as we are required to assume every window has a crappy roller/holland blind. But the window covering can make a big difference to the thermal performance of a room and be more cost-effective than upgrading the window glazing or frame. Some energy rating tools like AccuRate Sustainability allow you to run a simulation in “non-rating mode”, where you can change the window treatments just to see how the heating and cooling loads will change, even though this won’t get included in the final star rating.

Got a question? Send it to me here.



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