Australian Passive Solar Home

One of the interesting things about the different approaches to energy efficient homes is that those who do not read widely tend to think the approach they’re using is the only one, and therefore the rules that they follow apply in all cases.

For example, many people assume that to give best energy efficiency, a home must be sealed-up as tightly as possible. That is, any loss (or gain) of air represents a loss or gain of heat as well. Ipso facto, all houses should be well-sealed to stop such air movement.

But this is a slightly circular argument, because it depends on how much heat a house can lose or gain without actually changing much in internal temperature. In turn, that depends on how much heat (or the cooling equivalent – ‘coolth’) is stored within the house. In turn, that depends on thermal mass – the placement inside the insulated envelope of materials that can store heat and coolth. A concrete floor slab is one example.

If the house has little thermal mass (e.g. it’s all lightweight construction), any air leaks will rapidly change the interior temperature. But if the house has a lot of thermal mass, air leaks make much less difference. Any heat lost or gained is much better compensated for by the internal heat (or coolth) available from storage.

This is not just theory. In our passive solar home, that has plenty of thermal mass, I can leave a window fully open on a very hot day (e.g. 38 deg C) and the internal temp is only a measured 1 deg C higher than it is in the same conditions with the window shut. For example, 25 deg C versus 24 deg C. (No air con running of course.)

So the idea that a house must be tightly sealed applies only to houses that have little thermal mass.

But why wouldn’t you want to tightly seal all houses anyway? Primarily because such sealing has downsides: higher cost for the sealing membranes and labour, and another higher cost for the required mechanical ventilator. Also, some people (like my wife and I) really hate the idea that having safe air to breathe is completely dependent on the operation of a mechanical ventilator. The only real advantage of a mechanical ventilator is that the air can be filtered, and that can be achieved if desired by portable in-room filters like the ones that Ikea sell.

So our house in the Canberra climate zone, that so far over spring and summer has performed extremely well, is not tightly sealed – and it doesn’t need to be.

That is a completely different reality to one that is peddled by so many.