Here’s a common question that is not often answered adequately. Should we insulate underneath a concrete slab on ground? The short, inadequate answer is, ‘it depends.’
Before we get into the details we need to establish what our starting point is. The most common construction method in Australia is a raft slab with edge beams and stiffening beams extending to founding depth. The second method is a waffle pod slab which is concrete poured over polystyrene pods in a waffle pattern.
Both of these methods are capable of being insulated, (the waffle pods are inherently insulated) but both are coupled to the earth due to the concrete being a direct pathway from the floor to the earth. Due to structural reasons the area of possible insulation for a raft slab is restricted to the space between the edge & stiffening beams. The area of insulation for a waffle pod slab is restricted to the area of the pods. The volume of concrete from the floor to the earth is a thermal bridge. (Click here to my previous post about thermal bridges).
I recently did a heat transfer calculation on a raft slab, taking thermal bridging into consideration. The edge and stiffening beams represented 27% of the slab area and I nominated R1.0 insulation under the slab. The result was pretty bad. The insulation lost 85% of its effectiveness. Battling the earth’s thermal inertia is a tough fight, and when our temperature difference (DeltaT) is low it might not be a fight worth fighting. We also need to consider to what degree we are conditioning the space. Are we constantly heating, or just turning the heater on when we feel cold?
This is where the, ‘it depends’ answer comes in. My own conclusion is that if a heating system is NOT being installed in the slab, sub-slab insulation is largely ineffective. This is unless the concrete is coupled to the internal conditions by being completely thermally isolated from the ground, (which would involve re-designing the slab & footing construction method).
As always, I’m happy to be proved wrong. So far the advice from my North American building scientist friends is that insulating the external perimeter of the slab is beneficial due to the higher fluctuation in ground temperature at the surface, but apart from that we have bigger fish to fry. We need to start dealing with our leaky buildings first in the order of water, air, vapour and thermal.
UPDATE: Since I wrote this post in 2019, BRANZ has done some reseach and measurement regarding slab insulation which draws the same conclusions that I assumed. Link to article below.
Insulated raft slab video below.