Frost protected shallow foundation FPSF no heat question

[19D2P]

I am building my shop on a frost protected shallow foundation with the following specifications. 24"x24" thickened edge and 5" slab. 2" XPS under the slab, 2" along perimeter vertically 24" deep, and 2" horizontally 24" out. This all exceeded the design minimums for my zone of Montana. The foundation is on a pad constructed of fill that drains well according to the contractor.

I have been furiously building through spring and summer and finally have the building dried in. I am now waiting for spray foam contractor to get to me. His first available time is first week of January 2023. So the building will be unheated at least through then, and likely longer as I need my electrician to come back after insulation to wire the boiler and such.

My questions: Since a FPSF uses the heat from the building to keep the ground from freezing and heaving the foundation, is the building in danger if left unheated?

Should I hook up the radiant heat and keep the slab just warm enough to prevent freezing?

 

[billconner]

First, I don't know. But you can build a FPSF for unheated buildings. I think I'd run the formulas for that and see how different it is and where different. From my playing with it, in my very cold climate, it seemed to be a wider or longer apron - like 4' instead of 2'. Maybe some temporary insulation - straw and tarp - around perimeter?

My guess is it won't matter. It's covered so won't get real wet.

I'll be following this one.

 

[jack stand]

Sounds like it's been months since the interior has seen rain.
Dry (and well draining) soil won't give you any problems and you've built beyond the specs for your area.
Relax and carry on.

 

[wssix99]

Should I hook up the radiant heat and keep the slab just warm enough to prevent freezing?

Yes, or you could use a construction heater but I would expect a temporary hookup for your boiler would be more efficient, more reliable, and less expensive while you wait for your insulation.

 

[like2wheel]

Could it be relying on geothermal heat from the underlying ground to help raise the frost line at the building?
The cold comes from above.

If it was depending on building heat, why would it have underslab insulation?

 

[billconner]

Different designs. For unheated buildings, it does depend on geothermal. For heated without radiant floor heat, you can omit central underfloor insulation, but he had it. He is close to what is required for unheated.

 

[19D2P]

Yes, or you could use a construction heater but I would expect a temporary hookup for your boiler would be more efficient, more reliable, and less expensive while you wait for your insulation.

I considered that, but my main worry is the ground and slab and it would have to be extremely expensive to get that done with a torpedo heater.

Thanks for all the advice.

 

[rerod]

I considered that, but my main worry is the ground and slab and it would have to be extremely expensive to get that done with a torpedo heater.

Thanks for all the advice.

Any updates on the foundation condition, and what are the differences between a foundation designed for a heated and unheated building? I'm planning a uninsulated slab to stay cooler in the summer, and leaving Iowa november through march.

Thanks

 

[ConCretin]

I didn't get in on the original thread but it's irrelevant whether or not the building is heated. A FPSF works by using insulation under the slab and around the perimeter to retain the natural heat from the ground to prevent frost intrusion. Before the days of ground heaters, we'd spread hay and cover it with poly on construction projects to keep the frost out until we were ready to dig foundations. Same concept.

 

[19D2P]

Any updates on the foundation condition, and what are the differences between a foundation designed for a heated and unheated building? I'm planning a uninsulated slab to stay cooler in the summer, and leaving Iowa november through march.

Thanks

I wound up hooking up my radiant and had the building insulated in December. Zero signs of any heaving or shifting, however Montana was weird this year and we were hit with lots of snow early (insulating the soil?) and despite lots of cold my ground is still thawed everywhere. Not a typical condition for sure...

This will show you the differences much better in your situation than I ever could. Too many variables. https://liteform.com/wp-content/uploads/2017/04/FPSF-Guide-Revised.pdf

I didn't get in on the original thread but it's irrelevant whether or not the building is heated. A FPSF works by using insulation under the slab and around the perimeter to retain the natural heat from the ground to prevent frost intrusion. Before the days of ground heaters, we'd spread hay and cover it with poly on construction projects to keep the frost out until we were ready to dig foundations. Same concept.

403.3 of the IRC is what I was going off. It clearly shows there is a design for a heated building, and a separate design for an unheated building. ASCE 32-01 showed a better (for me) difference in the two design requirements.

 

[ConCretin]

403.3 of the IRC is what I was going off. It clearly shows there is a design for a heated building, and a separate design for an unheated building. ASCE 32-01 showed a better (for me) difference in the two design requirements.

I doubt the basic concept varies much but please post the specific details your referring to.

 

[19D2P]

I doubt the basic concept varies much but please post the specific details your referring to.

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).

Here is a summary of the changes in the design:
Additional measures are required for an unheated building. Unheated garages and other outbuildings rely on heat from the earth, which is a fairly constant 50 degrees, instead of heat from the buildings interior.

While a drainage layer is only recommended under wing insulation for heated buildings, a 6-inch drainage layer is required under unheated FPSF designs. Additionally, the horizontal ground insulation extends not only as a wing beyond the perimeter of the building, but continues under the entire unheated portion of the building. This insulation layer can be installed either directly under the slab, or entirely at one level beneath the lowest portion of the footing. In either case, and it must bear on 6 to 12 inches of gravel, and the compressive load of the building on the insulation must be determined to compare to the compressive resistance of the foam. The horizontal insulation must have a minimum of 10 inches of soil cover.

 

[rerod]

R403.3 Frost-protected shallow foundations.​

Here is a summary of the changes in the design:
Additional measures are required for an unheated building. Unheated garages and other outbuildings rely on heat from the earth, which is a fairly constant 50 degrees, instead of heat from the buildings interior.

Thanks for the summary. Unfortunately these facts prevent me from leaving my home unheated since I dont want buried foam. Rockwool may be a better choice for me.

 

[ConCretin]

19D2P, I'll amend my previous comments. The OP of the thread indicated they were placing foam under their slab and I apparently had that in my head. At that point there isn't much difference between a heated and unheated FPSF. You are correct however that underslab insulation wouldn't be required for a heated building but would for an unheated one.

 

[billconner]

I believe the foam "apron" has to extend a bit further for unheated vs. heated, and equal all around, versus more at corners.

diagram from IRC for a heated building

 

[19D2P]

19D2P, I'll amend my previous comments. The OP of the thread indicated they were placing foam under their slab and I apparently had that in my head. At that point there isn't much difference between a heated and unheated FPSF. You are correct however that underslab insulation wouldn't be required for a heated building but would for an unheated one.

I got ya, I am the OP and it may have been confusing because I was trying to see if a FPSF designed for a heated building would be okay for a period of time in an unheated condition. My concrete guy insisted it'd be okay due to the well-drained rock that was used for the pad, therefore no water to freeze.

I believe the foam "apron" has to extend a bit further for unheated vs. heated, and equal all around, versus more at corners.

diagram from IRC for a heated building




I "think" the most important difference is the requirement on unheated to use drained clean fill under the whole slab and not just the thickened edges. This prevents water which means no frost. On a heated building the main slab doesn't need the cost of the drained rock because it will always be stealing some warmth from the building to stay above freezing.

 

[rerod]

How did you slab fare over the winter? As mild as it was, my luck would be a long and cold winter next year so I'm also wondering because I don't work fast..

 

[19D2P]

How did you slab fare over the winter? As mild as it was, my luck would be a long and cold winter next year so I'm also wondering because I don't work fast..

After the initial pour in November it lived under blankets all winter which I think gave it enough time to properly cure. This last winter was its third, with the last two being heated. I haven’t had any problems and everything seems to have worked just fine.

 

[rerod]

I'm a little disappointed trying to work around my details.. I want a uninsulated slab to keep my shop cool in the summer. But I figured if I installed a good 4' apron which extended up the thickened edge to R30 walls and had a tight building with 2' of blown in that the uninsulated 50 degree slab would prevent my structure from freezing here in Iowa. But after reading you should insulate under slabs in unheated buildings, that blows up that idea.
I just figured a highly insulated and air sealed building above a uninsulated slab but with apron would capture earths heat as well or better than R10-R20 under slab insulation would. The insulated layer would be shaped like a hat.

Your preventing earths heat from reaching the interior with insulation under the slab.

 

[billconner]

The concept is not preventing slab from freezing, but preventing the soil under it from freezing. If the building is unheated, and no insulation under slab, that soil under the slab can freeze.

You present an interesting concept that somehow the air in the garage won't get below freezing. Not sure how you can guarantee that if no heating.

 

[Bert_]

I'm a little disappointed trying to work around my details.. I want a uninsulated slab to keep my shop cool in the summer. But I figured if I installed a good 4' apron which extended up the thickened edge to R30 walls and had a tight building with 2' of blown in that the uninsulated 50 degree slab would prevent my structure from freezing here in Iowa. But after reading you should insulate under slabs in unheated buildings, that blows up that idea.
I just figured a highly insulated and air sealed building above a uninsulated slab but with apron would capture earths heat as well or better than R10-R20 under slab insulation would. The insulated layer would be shaped like a hat.

Your preventing earths heat from reaching the interior with insulation under the slab.

I think perimeter insulation to a depth of maybe 4' would do a lot of good in a situation like this. With a well insulated building it would probably stay above freezing.

 

[billconner]

I feel like Bert is correct, but it may not be as easy to convince a building department if applying for a permit.

Another way to frost protect a foundation is gravel under entire building to frost depth and drainage to daylight.