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Frost protected shallow foundation - insulation under slab

M

MikeC

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May 17, 2012
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NW Pennsylvania
I've been digging, and while I've found this question asked a few times, I haven't been able to find an answer.

Scenario - monolithic pour slab with turned down footings utilizing the FPSF design per section 403.3 of the 2018 IRC. It is my understanding that the heat from the house will conduct through the slab keeping the moisture under the footing from freezing and heaving. The builder wants to put foam insulation under the slab in addition to the foam required by 403.3. I don't see anywhere in the IRC that restricts this, but it seems like a bad idea. Any thoughts?
 
A heated slab requires insulation under it a non-heated slab does not.

A cold slab does not make for a comfortable house nor an energy efficient house.
 
FPSF works by using the earthd heat to keep the ground under the insulation from freezing. Works on insulated heated buildings and uninsulated non-heated buildings. A couple of free guides online that explain it better than IRC. I like ASCE 32-01.
 
bill1952 said:
FPSF works by using the earthd heat to keep the ground under the insulation from freezing. Works on insulated heated buildings and uninsulated non-heated buildings. A couple of free guides online that explain it better than IRC. I like ASCE 32-01.
Click to expand...
I would love to agree with you, but the fact that the IRC restricts the us of FPSF to heated structure leads me to believe that the heat comes into play. I'll see if I can access the standard you referenced and educate myself.
 
The HUD FPSF manual also has unheated buildings. I thought IRC allowed using the ASCE standard, but I can't seem to login tonight to double check.

I any case I'm sure it's based on the earth's heat. When It was looking into it I found an engineer at UW Madison and spoke with them.

And if really interested, look into glavel - foamed recycled glass - as insulation and base.

www.glavel.com

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Pennsylvania 2015 IRC


R403.3 Frost-Protected Shallow Foundations

For buildings where the monthly mean temperature of the building is maintained at a minimum of 64°F (18°C), footings are not required to extend below the frost line when protected from frost by insulation in accordance with Figure R403.3(1) and Table R403.3(1). Foundations protected from frost in accordance with Figure R403.3(1) and Table R403.3(1) shall not be used for unheated spaces such as porches, utility rooms, garages and carports, and shall not be attached to basements or crawl spaces that are not maintained at a minimum monthly mean temperature of 64°F (18°C).
 
Whether or not within a specific jurisdiction a FPSF is permitted for an unheated building with or without a registered design professionals seal or based on ASCE 32, the question was about if the frost protection was from the heated building. I believe ASCE and other research clearly shows in this system the frost protection is from the earth's heat.

iirc ASCE 32 also permits very well drained soil as a valid means of frost protection.
 
I think that this would permit unheated buildings on FPSF under IRC.

From 2024 IRC:

R301.1.3 Engineered design.
Where a building of otherwise conventional construction contains structural elements exceeding the limits of Section R301 or otherwise not conforming to this code, these elements shall be designed in accordance with accepted engineering practice. The extent of such design need only demonstrate compliance of nonconventional elements with other applicable provisions and shall be compatible with the performance of the conventional framed system. Engineered design in accordance with the International Building Code is permitted for buildings and structures, and parts thereof, included in the scope of this code.

and from 2024 IBC:

1809.5 Frost protection.
Except where otherwise protected from frost, foundations and other permanent supports of buildings and structures shall be protected from frost by one or more of the following methods:
  1. 1. Extending below the frost line of the locality.
  2. 2. Constructing in accordance with ASCE 32.
  3. 3. Erecting on solid rock.
In some jurisdictions "designed in accordance with accepted engineering practice" does not automatically require a registered design professional. YMMV
 
The problem would be determining how much insulation would be necessary under the slab and inside the turn-down to keep the soil from freezing when the slab is 0 degrees or less, but it's several feet down to the subsurface constant 50 - 55 degree temperature.

A plan reviewer can't determine whether a frost protected foundation really is "designed in accordance with accepted engineering practice" unless he is familiar with ASCE 32. A registered design professional should be familiar with ASCE 32 before designing a frost protected foundation, although there are a few who go outside their area(s) of expertise.
 
It's no more complicated than the prescriptive requirements for heated buildings in the IRC, just some more tables for unheated.
 
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