Foundation red flags?
I’m planning a high performance retirement home in Detroit (Zone 5). The house will be a 26′ X 40′ single story rectangle with a basement. I have extensive remodeling experience, but I have never built a foundation.
After water control and air sealing, thermal bridging seems the most important consideration. Typically, the concrete guys come in three stages: footing, walls, then slab. I rarely find insulated footings described. Besides tradition, an impediment seems to be the necessity of installing a level and compacted gravel base under the footing insulation and another under the slab. Considering the extra steps, does it make sense to install an insulated monolithic slab/footing?
One concern may be the weight of concrete walls loading the perimeter? Which brings me to another crazy idea, permanent wood foundation walls. Online I see a lot of dismissive comments, but few accounts of actual failure. My feeling is that the care and detailing of a high performance house can mitigate potential problems.
Peel and stick waterproofing membrane would be applied to the PWF sheathing extending to the base of the slab insulation, on the way connecting to the poly under slab membrane. Two inches of Roxul from the top plate down would provide continuous insulation, drainage and relief from hydrostatic pressure. This would be bolstered by two inches of XPS from the top plate to 6″ below grade to support synthetic stucco foundation cladding and line up with four inches of above grade rigid foam. With cavity insulation, the wall assembly would be economical and have an R value north of 25 below grade and 35 above.
Does this make sense?