Frost heave caused by ice lenses can wreak havoc on foundations in cold climates, often driving builders to dig below the frost line. Yet, Section R403.3 of the International Residential Code (IRC) outlines a clever alternative: the frost-protected shallow foundation (FPSF). By placing insulation around a heated slab, the building’s own warmth keeps soil from freezing at critical edges. This article explores the mechanics behind FPSFs, outlines essential code requirements, and highlights what inspectors should watch for in the field.
This article is your walkthrough of the code’s intent, the nuts and bolts of FPSF design, and how to spot problems during inspections.
Very interesting, Ben. The video was well done. The explanation as well well done.
Tutorials like this help embed the Frost Protection Shallow Foundation building process in the inspectors mind and assists them in defect or deficiency determination at the inspection site itself, or in the office building a report.
Not unusual here in far northern MN, just south of the border. In floor heat, shallow footing, foam insulation skirting. My own house of 15 years is built this way. Excellent system when a basement cannot be built (due to shallow soil depth over bedrock).
This is a great article. I have a couple questions. First, if a fancy passive house utilizes a super-insulated slab, such as R40, then how does it avoid frost since almost none of its interior heat is escaping to warm the surrounding soil? Second, your US map for termite infestation is correctly pulling from the ICC page, but the labels seem wrong. Termites are worse the further South you go, not North. This is corroborated by any pest contractor. That image seems to be better associated with concrete weathering probability, seen here https://codes.iccsafe.org/content/FLIBC2010/chapter-19-concrete. Thanks!
See my comment in this thread. Frost is not an issue. Enough heat escapes from the slab to supplement natural warmth from the soil below. R-10 was the requirement under a slab when I built. It’s now R-15. I know of no passive solar house or shallow footing slab with under slab insulation at R-40. That’s 8 inches of foam, making the footing unstable and vulnerable to settling. I would never built that way.
I have a shallow footing home with a 4’ perimeter of foam to the outside. In winter I have no snow melt around the edge of my home. Same is true for houses I’ve built this way when I was a residential contractor.
A 4 foot perimeter of foam placed below grade captures ground warmth and prevents frost from getting under the shallow footing.
Note that even in extremely cold climates like my own, snow often limits frost depth to only a foot or so.
