Q: I am in a quandary as to whether to or what to specify in both slab on ground or timber on stump floors. Logic suggests that an insulated floor will perform better when heating is in use for both concrete or timber floors and also when passive solar gain is available and/or inslab heating is used for concrete floors in the cooler months.
However, in the warmer months there is some benefit to be gained from no insulation in the floor with the transference of 'coolth' from the ground to the building. With a lot of insulation choices it seems that one has to design for either cold or hot weather — a compromise.
If only there was an insulation product that could be switched — for instance a membrane that could be filled with water to provide a thermal bridge and then drained out to be replaced with air when an insulative barrier is required.
A: A compromise indeed! It is mostly a problem in southern climes when active heating is introduced, as you noted. This can be countered by increasing the mass above the insulation to provide more of a heat sink in hot weather. However, this also increases the embodied energy — and if just massed conventional concrete is used, markedly so.
Using recycled bricks or concrete slabs on top of the insulation and directly under the slab would be a solution to both those issues. I have used 'water-pods' to do this, with old 20 litre chemical bottles forming the high thermal mass 'void formers' between deep beams, just like a waffle pod slab with high mass voids. Many projects have used pods of old tyres in the same way filled with soil from the site, which could quite happily sit on 40 mm of high density extruded foam insulation.
But the notion of a switchable membrane is another brilliant alternative. A void space which can be filled and drained is good, but even better would be the ability to fill it with warm water in winter and cold water in summer. Sounds a lot like hydronics actually.
The warm water is easy — that technology is appearing more frequently in buildings of all types and sizes. The cold water could be chilled mechanically, but the dew point of the indoor air would have to be carefully monitored to prevent condensation and the plethora of troubles that brings.
Cool water from 3m deep underground would do the job nicely, with no dew point issues. In Sydney, for instance, the ground 3m deep in summer has a maximum temperature (in late February to mid March) of 19°C — perfect for a slab on a hot day and still a few degrees above minimum dew point for that time of year. An underground water tank could also provide the heat sink, with simple heat exchange pipes running through it so the bulk of the tankwater could still be consumed in the usual way.
Dick Clarke is principal of Envirotecture, which provides design and consulting services. He is an accredited building designer with 30 years experience, focusing exclusively on ecologically sustainable and culturally appropriate buildings.
