Frost Protected Shallow Foundation Questions
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Hey all,
I am planning to build a 20×36 two story house this year in Maine (climate zone 6). I am planning to do a frost protected shallow foundation insulated underneath and around the edges with R-10 xps. It will not have radiant heating.
I am working with a local contractor and he said he pours his slabs 6in thick evenly, no thickening at the perimeter. It was my understanding that the perimeter should be at least 12in thick. Any thoughts on this?
My other big question is, is a frost protected shallow foundation suitable for a two story home?
Appreciate any input!
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Replies
MaineHomestead,
The are a couple of aspects to this.
The first is that the depth at edges of a slab with integral footings needs to be determined by the structural requirements, not rules of thumb. If they can show it meets code, and applicable engineering standards, then fair game to them, but I very much doubt that it is possible.
Codes also typically require a 6" gap between the siding and grade. If the slab was that same depth, the bottom would be at grade - which makes no sense.
Are you sure it isn't a misunderstanding, and they think the slab is going to be enclosed by stem-walls?
The gap from siding to grade isn't solely determined by the slab thickness. You could easily elevate the house with insulation and gravel to make the gap whatever you want.
Trevor,
You are still left with a slab that is unrestrained horizontally floating above grade on foam. It's not a great detail.
That's the first I've heard that concern voiced. There are articles and discussions on here about them, and I don't think that's been brought up. There are lots of houses on this kind of slab in Europe and a fair number in North America too. I don't think there are any reports of them moving about.
My house is a 2-story, slab on grade. It's a flat slab with no thickened edge, but it's 8" thick. I'd encourage you to use EPS instead of XPS. It's orders of magnitude more environmentally friendly, and it won't degrade in R-value over time like XPS. I'd also encourage going to at least 4" thickness, if you're building a pretty good house. 2" is basically code minimum.
Where I am, all FPFS need an engineer's stamp.
We were planning to get the XPS from a local supplier who repurposes it. But now you have me wanting to go with EPS instead. Any ideas where I could buy it from here in the northeast? The repurpose suppliers around here currently don’t have any EPS.
As you may know, per the HUD FPSF design guide, minimum depth below grade is 10"+ inches increasing from there depending on your design criteria, climate, and insulation selection. So you are correct that 6" won't work. My "heated" FPSF slab ended up being 14" below grade per the HUD guidelines.
6" depth sounds more similar to a "floating slab" which relies on good drainage, weight, and slab strength to keep movement to a minimum.
Thanks for the replies. I may be getting my foundations mixed up. I thought a floating slab was a frost protected shallow foundation? What is the difference?
The distinction in my mind is, a FPSF follows IRC / HUD design guidelines aimed at keeping the ground underneath the slab unfrozen. A "floating slab" is more of just a thickened edge slab on well draining gravel. The design relies on local knowledge of what's necessary to have the slab not move "much", and if it does, it stays in one monolithic, stiff, piece. I think floating slabs are pretty common in my area, Northern Vermont, for unheated outbuildings.
That's about right. A floating slab on a gravel base is appropriate for unheated outbuildings in cold country. I would not consider them for a two story house. For the house, a reinforced slab with thickened edges is a must, or a floating slab on the inside with cast stemwalls surrounding it. In either case, the frost protection prevents the ground under the "footing" from freezing. As mentioned earlier, the IRC has some standard requirements for FPSF. There is also a FPSF engineering design guide with more flexibility. Most jurisdictions will require an engineer's sign-off on the design and installation.
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MaineHomestead,
You may find this useful: https://www.homeinnovation.com/-/media/Files/Reports/Revised-Builders-Guide-to-Frost-Protected-Shallow-Foundations.pdf
Big red flag: frost protected foundations in Maine need to extend at least 14" below grade, more in colder areas; in no location in Maine can you legally have a monolithic 6" slab unless it's part of a system where the foam insulation extends deeper below grade. (All of Maine is legally required to follow the IRC code, though small towns are not required to provide enforcement, that doesn't mean it's not still required.)
You can have a floating slab in an unheated building less than 600 sq.ft. Your house is probably larger, and definitely heated.
Footings can be 6" thick, even for 2-story buildings. (See table R403.1(1): https://codes.iccsafe.org/content/IRC2015/chapter-4-foundations#IRC2015_Pt03_Ch04_SecR403)