The advantages

Engineering consultant Jay Crandell was involved in some of the early U.S. research into FPSF and helped develop the original HUD design guidelines. Crandell says, “FPSF not only saves money in construction costs, but it also saves energy, and there are very few technologies that do both — reduce first costs and pay you back with energy savings over time. That’s one reason it’s gained popularity in Scandinavia, and why it may eventually see greater use in the U.S. market.”

Crandell estimates that an FPSF for an average-sized house would cost $3,000 to $4,000 less than a foundation excavated below the frost line. Compared to digging a basement, the savings would be more like $8,000 to $10,000, but he concedes that that’s more an “apples to oranges” comparison.

“There are ways that technologies can be combined to derive greater benefits,” Crandell says. “Take insulating concrete forms (ICF), for example. One of the main benefits is increased wind resistance compared to wood-framed construction. By using a shallow foundation, where you only have to go down 16 inches, you might save enough to pay for ICF walls and the increased resiliency that they provide. It’s a way to make a building more energy-efficient, more resilient to disasters, and not more costly.”

Choosing the insulation

The design guidelines and ASCE Standard 32-01 allow using either expanded (EPS) or extruded (XPS) polystyrene rigid foam insulation in FPSF applications, and provide recommendations for the thickness of each. While the nominal R-value of XPS is higher than that of EPS, practitioners often recommend the EPS version for FPSF application.

Larry Mayer, an energy-efficiency consultant in Fargo, N.D., who has worked with FPSF systems for several years, generally opts for EPS insulation over extruded polystyrene for three reasons. First, it’s less expensive, at only 60% to 70% of the cost. Second, it’s breathable, so if moisture does enter the insulation, it’s more likely to evaporate and allow the panel to dry out. Third, it’s soft enough that you can apply it to an uneven slab edge and expect that it will conform to irregularities in the concrete without cracking.

Alan Gibson, whose firm, G O Logic Inc., designs and builds super-insulated homes, also prefers EPS insulation panels. Gibson cites recent studies suggesting that EPS absorbs less water and retains more R-value over time than XPS does. He also considers EPS manufacturing methods to be more environmentally friendly. Finally, Gibson, and the structural engineer who reviews his company’s plans, likes how EPS can be obtained in 6-inch-thick panels, while XPS panels are typically 2 inches or less in thickness. Using a single layer helps avoid the chance of sideways slippage between panels or voids between them that could eventually lead to settling.

Whichever type of foam insulation you’re using, follow the pertinent design recommendations and code requirements. They have been adjusted to account for the altered conditions represented by below-grade installation.