How extensive should rigid foam be placed under slab on grade frost wall foundations?
Schematic diagrams demonstrating the use of rigid foam insulation under slab on grade frost wall foundations always show the rigid foam under the slab and less frequently under the footer or on the external surface of the stem wall. If the goal is to have an optimal thermal break barrier under this type of foundation construction why not place the foam both under the slab and completely around both sides of the stem wall and under the footer? It seems to me that this is the only way to create a complete thermal break for the foundation. Furthermore if a complete moisture barrier is desired it seems to me that the vapor barrier should also completely surround the entire concrete pour. Is the use of such extensive rigid foam and vapor barrier thought to be not necessary or not commonly done because of cost?
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Tim,
The brief answer is, you're right. If you want a complete thermal break, you should install the insulation around both sides of the frost wall and under the footing.
Most people don't do that, for three reasons: (1) Many concrete contractors and building officials are (wrongly) suspicious of foam under footings, (2) It's expensive, and (3) Energy-efficiency fanatics usually don't choose that type of foundation -- they use an insulated raft foundation instead.
An insulated raft foundation avoids the problem by omitting the frost wall. For more information on insulated raft foundations, see Foam Under Footings.
Foam under footing is what the Passive House folks advocate. In the planning stages of our project, we wanted to do it too. You run into a lot of resistance from code officials, inspectors and probably your own engineer. I think if you choose the right type of foam with the right compression strength, you can get through these obstacles. It will take a lot of convincing though!
Tim: The advertised psi of your foam should be 3x what you need, to avoid creep. The advertised psi is for 5 or 10% compression, which gets into the deformation range. You can read that on the DOW web site, where they suggest the 3x rule. "40 psi" EPS (at 10% compression) has a 15 psi rating at 1% compression; etc. Deformation for EPS starts at about 7% compression. That is from the Insulfoam web site. BTW: Thorsten Chlupp is building on 8" slabs, w/ no footer/stem wall, and 12" of foam outside and under. That is another option.