Revised Builder’s Guide to Frost Protected Shallow Foundations
Construction issues
The building codes and ASCE Standard deal mostly with FPSF design issues, leaving contractors with numerous construction options. The original FPSF guideline document is available here. It describes the original case study houses and shows a variety of details you can use for construction guidance.
On some projects, contractors use wooden forms for the slab edges; in other cases, the foam sheathing itself is used as the formwork. If you’re installing the insulation after you’ve poured the concrete, it’s important to have a fairly smooth surface on the slab edge.
Also be aware of the placement of the horizontal wing foam, where it’s required. It must be at least 10 inches below finished grade to minimize the possibility of animals or landscapers digging and disturbing it. The distance from the top of grade to the bottom of framing must be least 6 inches, so the slab edge must be least 16 inches thick, or you won’t get sufficient cover.
One patented approach
Though FPSF methods are not widespread in production housing, they have been embraced by resource-conscious builders of passive and super-insulated homes. G O Logic LLC, based in Belfast, Maine, has developed its own shallow foundation system that draws on the general principles of FPSF design, but exceeds the code requirements for frost protection.
G O Logic co-owner Alan Gibson says his company’s projects aim for a passive home model, with insulation that’s optimized for energy savings. “The system we employ now is one that we’ve designed and patented,” he says. “It uses our own foam forms, and includes the soil preparation steps. We put a layer of flowable fill below the foam, then the foam itself, followed by steel reinforcing and the concrete. If people want to use our system, they can. We can specify it for them, or they can go to our supplier for the foam and pay a nominal licensing fee to use it. The advantage is that we can send them drawings and supply the foam forms that will make their job a lot easier and save them a lot of money compared with other ways it can be done,” Visit www.gologic.us for more about the system.
Though FPSF technology has yet to gain a significant share of the mainstream housing market, it’s an option concrete contractors may encounter and should be aware of, as an economical alternative to full basements in cold climates.
Kenneth A. Hooker is a freelance writer based in Oak Park, Ill.
