I'm still studying that. The slab seems easy enough; Dow is now selling a two-inch thick high-density polystyrene sheet intended to be placed under slabs.
I've seen a few different techniques for the footings. One just stood high-density polystyrene on edge inside the footing. I'm having trouble picturing how the junction between the slab and footing is handled if you do that. Seems like that would be a large heat loss area unless the styrofoam actually separated the slab and footing at the finished floor.
The other scheme used polyisocyanurate insulation on the outside surface of the footing, covered with a stucco-like covering (Dryvit?) where it was above grade. Seems like a lot of work, and a potential maintenance headache.
We have a HVAC contractor who does a lot of radiant floors, geothermal, etc., and I planned on having him involved when it's time to make those decisions.
There are two low-tech (and cost) methods I know of. Both assume the insulation board is on the outside of the frostwall/slab.
One is to offset your bottom sill plate over the outside edge of the slab or frostwall. The offset amount will be the thickness of the extruded poly board minus the thickness of your exterior sheathing. In other words, when it is all done, the outer surface of the insulation board is flush with the outer surface of the exterior sheathing. Your exterior finish can then lap over this butt joint.
You put aluminum trim coil on before your exterior siding, it can be nailed to the exterior sheathing and glued to the insulation board. There are specially formulated glues that don't eat the foam board and actually stick a little bit.
Once you back fill, you will see a ribbon of aluminum between the ground and the lower edge of the exterior finish. It comes in white plus few other colors and is paintable.
The drawback to this method is you may want to use larger dimensional lumber for the wall studding to compensate for the fact that about 1 1/2" of the stud width is not supported by the foundation.
The second method uses aluminum trim coil also. Build the framed wall and sill plate as normal, no offset. Before attaching the trim coil to the exterior sheathing, use a brake (sheet metal bender) to put two 90 deg. bends in it. The first bend extends the aluminum out over the thickness of the insulation board, the second bend turns it down toward the ground.
When finished, you see a ribbon of aluminum, then a narrow shelf of aluminum, then the exterior finish material.
The aluminum trim is by no means a rugged material. It kinks and gets dinged very easily. You might use the same technique with a sturdier material.
It is water tight and won't rot or rust, and protects the insulation which will be worthless in about 3 yrs if left exposed.
The stucco might be a great choice in dryer climates. I don't think it would hold up here for this use.
Dave.
