For many years, my company built conventional poured-concrete foundations, always aiming to be below frost depth (4 ft. in our area). But as we got into high-performance building and changed the way we built walls, our foundation system had to evolve. We started with double-stud walls about six years ago, and were able to pair those with 4-ft. frost walls and maintain a thermal break between the interior slab and the outer frost wall. But as we strived for even higher levels of performance, we began looking at exterior insulation and Larsen truss assemblies that necessitated a different type of foundation. Further driving this evolution, many of our customers started requesting houses without basements.
Our standard high-performance house uses a 2×6 structural wall with Zip System sheathing and 12 in. of exterior insulation supported by vertically mounted I-joists. For optimal performance, the insulation layer needs to be continuous from the walls to beneath the slab and over the building (a separate discussion). A frost-protected shallow foundation (FPSF) is an efficient way to handle the underslab part of this. The approach has been around for years, and there are a handful of off-the-shelf systems that can work well.
The problem we encountered was finding an FPSF system that would work with our various wall assemblies. No two clients want exactly the same thing, so we need the ability to vary the level of performance of the foundation system along with the walls. We looked all over, and talked to a number of suppliers of ready-made systems. Some had some of the features we wanted, but none touched every base. We may be picky, but I have to stand face-to-face with our customers and answer the hard questions when something isn’t just right. Accepting a foundation system that doesn’t check all the boxes isn’t an…
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