I visited a small, affordable, mono pitch roofed, passive solar house the other day. My dream home! I was inspired by what this young couple had done and it lite a renewed fire in me for working towards my own dream. We chatted awhile about slabs, windows, flooring and other horrendously boring stuff to most people but so very exciting to me! It is still full on winter here in Maine and I was shocked to be standing barefoot inside their very warm and sunny home on a 4” thick un-insulated slab of concrete. I did a bunch of research on this type of cement slab when I got home and learned a lot about frost protected shallow foundations. They are not very common in the united states but more than 1 million Scandinavian homes have been built on such foundations since 1960. They save money, resources and time, yes please tell me more!
A frost-protected shallow foundation (FPSF) makes an elegant building solution from either side of the new home transaction. Homeowners benefit from increased comfort and lower energy bills. Builders profit from reduced excavation and materials costs, as well as shorter cycle time. FPSF uses conventional, readily available materials. This foundation system is a practical option for building with slab-on-grade, stem-wall or floating slab foundations, as well as for an unvented (heated) crawl space and the on-grade side of a walk-out basement. Despite their broad applicability, FPSFs are not used widely in the United States. What’s Different About FPSFs?
Traditional, uninsulated foundations are protected from frost heave because they’re installed below the frost line (up to 48 inches below the surface, depending on geographic region). FPSFs require excavation only 12 to 16 inches below grade because insulation is applied around the outside of the foundation to direct the building’s heat loss downward. For heated buildings, this insulation – along with the earth’s geothermal energy – keeps the soil temperature under the building above freezing, preventing frost heave.
More than 1 million FPSFs have been used in Scandinavia since 1960. For almost 30 years, Scandinavian building codes have recognized the use of FPSFs as a standard practice for houses, townhomes, apartments, stores, schools and low-rise office buildings. Research findings from field evaluations in Denver and Freehold Township, N.J., show that construction and excavation requirements, as well as labor and materials costs, are 15% to 17% less for an FPSF than for a conventional foundation. The savings range – $800 to $6,000 – reflects varying local frost depths, as well as builder overhead and markups. Because an FPSF is insulated along the outside edges, it makes floors at the perimeter of the home significantly warmer. It also reduces heat loss through the foundation.
The above chart shows the frost line and how it can significantly change depending on what’s on top of it. The gravel doesn’t hold on to much heat so the ground freezes at a greater depth while the house is on a frost protected shallow foundation and the earth directly under it never freezes, even during harsh Maine winters!
I am now about to research thermal mass gain and how ostensibly a thick layer of insulation under the entire cement slab could reduce the home’s ability to utilize the earth underneath as a heat sink. This is so interesting to me! It seems counterintuitive, but as I read more and more about the science of heat transfer I can understand why shallow foundations work and also why many people question the technology.