One of the most popular foundation systems used in my Minnesota market (climate zone 6), and one I’ve been using for more than a decade, is the frost-protected shallow foundation (FPSF). In part, that’s because code requires footing depths of 5 ft.; and digging, constructing, and insulating footings and a foundation system that deep is time-consuming and expensive. A shallow-foundation system can be a substantial savings for a new build.
The International Residential Code (IRC) covers FPSFs in section R403.3. There is a lot of information specific to this type of foundation that differs from other systems. For instance, when using a FPSF, the average mean temperature of the building needs to be maintained at a minimum of 64°F (there are ways to design a frost-protected foundation for unheated and semi-heated building, but this article will concentrate on continually conditioned dwellings). There are also requirements for rigid insulation R-value derating and protection for buried insulation.
What is an FPSF?
A frost-protected shallow foundation system is typically a monolithic concrete pour, sometimes called a turned-down slab, where the footing and slab are constructed as one. Insulation is placed against the slab edge as well as a “wing” of insulation that extends out, away from the thickened edge, to prevent frost from driving under the slab during the winter months.
How much insulation is required at the slab edge and on the horizontal or wing, and the minimum footing depth is determined by the air-freezing index for your location. The map is backed up in the code with tables showing the air-freezing index for the individual counties in each state. The air-freezing index is defined as cumulative degree days below 32°F. It is used as a measure of the combined magnitude and duration of air temperatures below freezing. The index was computed…
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Thanks for the comprehensive overview.
Part of what makes me cautious about using foam outside the building envelope (or under load-bearing components) is seeing how destructive carpenter ants can be to foam in the PNW. Do you have any thoughts on that? Maybe it's a technique best limited to certain climates?
We do see carpenter ants here in Northern Minnesota as well. I have been involved in removing XPS from exterior foundations in the past, have not seen any ant damage. It may be around, but I haven't seen or heard of it yet. I get your concerns.
Something that has surprised me is how much moisture I've seen these insulations take on. One of the XPS removal projects, the insulation was vertical, against a foundation wall. One person could not lift a single 4 x 8 x 2" sheet by themself. There were several issues with the landscaping around the building along with the soil type used as backfill, water was not moving away, I'm sure that was the biggest reason for the oversaturation. I've had other contractors quiz me on saturated insulation as well. I have a feeling it's a pretty common occurrence.
Thanks for the comment!
That's interesting about the XPS. I used to do work for a nearby resort that had 24 cabins all with hot tubs. Within a year the lids would become too heavy to use. They were large slabs of EPS foam encased in poly. It worried me as I use EPS under slabs.
I assumed the EPS took on water because it was somewhat permeable, and that XPS would fare better. Your experience seems to suggest otherwise.
I wonder what the saturation does to performance?
I think a lot of the issues with saturated sheet insulation has to do with water management around the site. Keep the FPSF's out of areas with high water tables and use stone or washed gravel under the horizontal wing insulation should help keep things dryer.
I also wondered about using Rockwool in this application. Again, it needs to be kept out of water that does not drain out of the site, but if it does become saturated during a wetting event, I think it has a better chance to dry. I haven't seen any testing with regards to R-values when wet and it's not listed for use with the prescriptive path, would need some engineering.
Is it possible to use foamed glass as the insulation? Or does that defeat the cost savings?
I don't think you could use foamed glass in the prescriptive path, but it could be a solution to saturated insulation.
Another reason for the saturated XPS is not providing proper water management and drainage for the site. There is a section in the FPSF code that I did not mention in the article covering drainage (R403.3.3). It basically says to install 4 inches of washed gravel or stone under the horizontal or wing insulation. Haven't seen this in my market.
I have used the system for years in Colorado, Wyoming, and Nebraska, even on four-story condominiums. I never had a problem except convincing the building department it was in their code. Recently I discovered FoundationPro a foam product with a vinyl skin designed specifically for this application. It worked well for me.
Two questions: HAve you used a FPSF for a garage? Any special considerations there? I'm planning to use one for my garage/workshop (and for my house, next year).
Also, you said you protect the exposed insulation with metal flashing. How high is that? How much of the slab/insulation is above grade?
You can use it for an unheated building, but under a different standard than the IRC, you would go by the provisions of ASCE 32 FPSF standard. If the garage is heated through the winter, then you can use the IRC. I would not use metal for the exposed area, although this is common and I have done it many times -- I build with FPSF routinely. I use a product called FoundationPro from Progressive Foam. You would not need anything on the walls if your building is not heated. Just under the slab. In the case of ASCE 32, you're capturing geothermal heat, and not the heat of the building. I did an article on FoundationPro if you want to see this material. You would use it on your house under the IRC. See https://www.greenbuildingadvisor.com/article/a-new-vinyl-laminated-exterior-foam-foundation-insulation
"And finally, with FPSFs there’s no place to shelter during severe weather. "
I do not understand this statement.
I assume he's referring to there being no basement.
Yeah. It took me a while, but that was what I also concluded. Thanks.
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