People often ask me why I don’t use the NatHERS (star rating) software for my reports and advice. The simple answer is that I prefer to use an energy conservation approach to a solar gain approach. What’s the difference you ask?
An energy conservation approach requires a robust building envelope capable of reducing the required heating and cooling load by retaining desired conditions indoors and keeping the outside conditions outside.
The solar gain method, that NatHERS employs, promotes the sun to heat the house via radiant heat transfer through the windows, (and conductive heat transfer through the window frames, walls, and roof…..which I find to be quite stupid).
I often ask my clients, “to what extent are you relying on your windows to heat your house?” The reply is usually, “I’m not.” I never ask my clients, “to what extent are you relying on your external walls to heat your house?” But this is what NatHERS software, and now the new NCC2022 DtS provisions is trying to do.
Example of required DtS wall insulation (2.4m wall height – Climate zone 6):
Light wall colour with no eave = R2.0
Light wall colour with >450 eave = not permitted
Dark wall colour with no eave = R1.5
All walls are non-compliant if a carport or verandah is installed.
According to the currently proposed methodology, dark colours and minimal shading is encouraged to increase heat flow through the fabric. At the same time, insulation is installed to slow down the heat flow through the fabric. Two competing strategies are at play here.
Call me crazy but I like to minimise things like conductive heat flow and thermal bridging through the building fabric. Until now, I’ve been able to do this within the DtS provisions.
According to all proposed pathways of our residential energy code, black walls, no eaves, and a black roof is now the path forward. Previously I have only needed to provide a performance Solution for a better glazing calculation. Now I will also need to provide a Performance Solution for better walls and roofs.
Lastly, I’m not anti passive solar. Every house we buld involves passive solar. Passive solar can be both a benefit and a disadvantage. The trick is to understand the implications of solar gain so we can control it to our advantage.