Q&A: Slab insulation


I’m planning to build a small house in Castlemaine, with a concrete slab area of 60 m2 and a perimeter of 38.3 m—giving a perimeter:area ratio of 0.64. The house will be oriented to get good solar access onto the slab during winter and will be shaded in summer. However, I’ve read that the higher the perimeter:area ratio is, the less the thermal resistance of the slab. Therefore, my question is, given the high perimeter:area ratio for my planned build, is there any point in having the slab at all? And would it actually provide much additional benefit to the alternative—an insulated wooden floor?

Also, I totally get the logic to insulate under the slab in terms of winter and retaining heat. But in summer, would the fact that the slab is not in contact with the ground work against me, in that it could stay too warm in the house?
—Amy Butcher, VIC


Concrete does not have great thermal resistance i.e. it is a poor insulator. It does have great thermal mass though. In this situation you will gain/lose more heat at the edges due to the greater perimeter:area ratio. Insulating the slab edge will dramatically reduce this heat loss. See www.bit.ly/2sxdCqG for more information.

From what I know of your climate I would insulate underneath the slab as well. You will pay a penalty in summer (but if well-shaded and insulated this should be minimal) and get a greater gain in winter.

As your home is going to be relatively small, having a well-insulated envelope on all six sides (roof, walls and under and around the slab) will make it easier to control the indoor environment; if you do need to add or remove heat, it will be a minimal amount. The thermal mass of your slab will stabilise temperatures and add to comfort.

A timber floor can work but the lack of thermal mass will give you greater temperature fluctuations. If your site is quite flat then the underside of your timber floor structure will need to be 400 mm above ground i.e. floor level would be 650+ mm. This is for termite inspections.
—Andy Marlow, Envirotecture

Read more Q&A in ReNew 144.

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