Australian Passive Solar Home

When you use the concrete slab as the heat / cool storage mechanism in a passive solar house, it’s important it can’t exchange heat readily with the outside environment. Thus explaining the 75mm extruded polystyrene insulation used around the edge of the slab, as shown here before the concrete pour. But what about under the slab?

That’s more complicated – much more complicated.

First, our engineer wouldn’t sign off on insulation supporting the slab.

Second, in our Canberra climate, it’s not completely clear how much benefit would actually result from the insulation under the slab. Software modelling showed a gain – but in our case, where many poured concrete piers support the slab, that would be greatly devalued by the thermal bridges through the insulation.

Furthermore, our measured performance in summer strongly indicates that the soil beneath the slab is being heated and cooled – that is, it’s acting as a thermal mass extension. (In fact, the house’s summer performance is better than modelled.)

There’s no doubt in my mind about the importance of having slab edge insulation in all Australian climates, but underside insulation is much more debateable.