Waffle pods insulate the slab from the ground, while an on ground slab is thermally connected to the ground, or “earth coupled”. This means that heat or cold can move through your floor into your house. There is very contradictory advice on whether to have an insulated slab or not. In part, this is because the slab has a different impact in summer than winter, and so it depends on where you are building.
A concrete slab has high thermal mass, meaning it can absorb a lot of heat or cold, and so can be effective at moderating the temperature of the home. Earth temperatures are relatively stable all year round, with minimum mean surface temperatures in Victoria (BoM) ranging from 5.1 – 13.7 oC. The www.yourhome.gov.au says that uninsulated slab in a good passively designed house has a surface temperature approximately the same as the stable ground temperature at about 3m depth, that is between 16 & 19 oC.
In winter, the surface temperature of the slab can be warmed with direct sunlight, and this added heat is radiated back out into the house as the house temperature drops below that of the slab. If your slab is uninsulated and earth coupled, some of the heat it gets from passive solar design will be transferred into the earth, resulting in a reduction in its capacity to radiate heat and warm your house. Conversely, if the slab is insulated, it will hold the heat, and leave you with a warmer house. So generally, an insulated slab will be better in colder climates, or places where the ground temperatures are low.
Conversely, you do NOT want your slab to heat up in summer – this would just keep your house warmer when the ambient temperature drops. An uninsulated slab will transfer heat into the earth, keeping the slab cooler, and hence your house cooler. An insulated slab will retain heat, and so your house is likely to be hotter in summer.
So typically for hot climates, earth coupled slabs, or light weight elevated construction is best – helps keep your house cool.
For Tasmania, or alpine regions, where earth temperatures are low, insulated slabs are best.
For moderate climates like Victoria, advice is contradictory. There is a good government website, www.yourhome.gov.au that offers advice on building energy efficient houses, but its advice is not consistent. In some instances, for Victoria, it advises earth coupled slabs, in other places it recommends insulated slabs. Concrete slab floors | YourHome.
Hopefully this helps explain why there is differing advice – it depends on your circumstances and where you are building.
We have gone with waffle pod construction – it’s our builder’s standard approach, and what is typically done in the area. As mentioned before, we are more concerned about winter than summer, so it fits. We are now renting a waffle pod house – last summer was cool, so the house has not really been tested in summer.
This winter we noticed that our tiled bathroom floor is MUCH warmer, somewhat ironically, than our floor in Perth which we believe was on a standard slab foundation. The electric ensuite floor was included because of how cold my feet got in Perth – having lived here, we don’t need it.
Although our house orientation is not good for passive design, we have added rotating louvres to our outside living area, to make sure we get morning sun onto the living room floor in winter but block it out in summer, and we have clerestory windows and skylights, double glazed with thermal blockout blinds. We have a large picture window in the bathroom which should keep it warm in winter.
We have made a poor flooring choice for taking advantage of thermal mass – solid wood floors.