Underslab spray foam

[mcbane]

I am considering insulating under my slab with spray foam - the installed price equals the cost of foam sheets and in theory it would save lots of labor vs installing sheet foam and a vapor barrier.

I'm just wondering if anyone has used spray foam under a hydronically heated slab and if there are any gotchas associated with spray foam. For example, any issues with PEX staples holding in spray foam as well as they do in foam sheets? Or any likely issue with foam as sprayed not being flat enough for easy rebar placement?

The specific foam used by the local installer is Demilec HF Pro. The specs look great but I dont know anyone who has used it.
https://www.demilec.com/products/closed-cell-insulation/heatlok-hfo-pro

 

[SGKent]

I am considering insulating under my slab with spray foam - the installed price equals the cost of foam sheets and in theory it would save lots of labor vs installing sheet foam and a vapor barrier.

I'm just wondering if anyone has used spray foam under a hydronically heated slab and if there are any gotchas associated with spray foam. For example, any issues with PEX staples holding in spray foam as well as they do in foam sheets? Or any likely issue with foam as sprayed not being flat enough for easy rebar placement?

The specific foam used by the local installer is Demilec HF Pro. The specs look great but I dont know anyone who has used it.
https://www.demilec.com/products/closed-cell-insulation/heatlok-hfo-pro

does the manufacturer spec it for that use?

 

[mcbane]

Yes. Spec'ed for below grade use and they advertise it as an underslab insulation material.

My reason for posting the question here is that advertisements sometimes differ from user experience.

 

[topp64]

I've seen spray foam insulation used under 2 basement floors. Both had 9' tall poured concrete walls and the foam was sprayed on the gravel subgrade and up the walls. This was for a guy that had been a home builder for 30+ years. The first one was about 4 years ago when he built a house for himself and the other was for his son 2 years ago. No rebar or in floor heat so I can't say how that would work out. I can say that because there were high and low areas of the foam, it took more concrete than what they figured. We had to use a laser to find the high spots and raise the floor up 4" above that. The foam insulation varied around 2" so the depth of the floors were between 4" to 6".

 

[ohioscott]

2 inch foam board is easy to put down

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[Kaizen]

Sounds like someone looking to expand their products usefulness. I like the uniformity of sheets for a flat floor. I. An see it’s usefulness in strange situations like ledge or rock.


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[mcbane]

Slab contractors here recommended the spray foam because in their opinion there is less chance of air pockets under the foam due to base material that wasnt flat enough. Of course you never know if the spray foam guy has some sort of deal with them to encourage such recommendations.

County inspector says he sees a lot of spray foam under slabs these days but the county has no recommendation either way. They just want you to insulate the slab.

 

[yeldogt]

I have never done it .. only because my projects don't schedule properly to do it. The cost of the truck to come just for a small slab ..

I think it's a great idea .. would do it.

 

[Kaizen]

Slab contractors here recommended the spray foam because in their opinion there is less chance of air pockets under the foam due to base material that wasnt flat enough. Of course you never know if the spray foam guy has some sort of deal with them to encourage such recommendations.

County inspector says he sees a lot of spray foam under slabs these days but the county has no recommendation either way. They just want you to insulate the slab.

30 tons of wet concrete will squish any uneven foam pretty easy. Are they giving a gauretee for the foam and slab? I honestly can’t see how the price would be close unless. Around here it’s a buck per inch per sq ft. Hd sells two inch foam board for 34 bucks. Same area for spray would be about 64


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[ransil]

Stuff I got was 2" and came in rolls, had a vapor barrier attached and overlapping tape to seal the seams

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[yeldogt]

30 tons of wet concrete will squish any uneven foam pretty easy. Are they giving a gauretee for the foam and slab? I honestly can’t see how the price would be close unless. Around here it’s a buck per inch per sq ft. Hd sells two inch foam board for 34 bucks. Same area for spray would be about 64


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I don't see how the foam will crush -- the closed is like a rock .... same as with sheet. They do it all the time on large projects with heavy slabs and future use.

 

[mike93lx]

Check out Matt Risinger on youtube (the build show). He recently featured a builder that talked about spray foam under the slab on a build in RI. Mentioned the stone needs to be a certain type and talked about how they feather it up the wall then tie into more spray foam to finish the insulation job later.

 

[Kaizen]

I don't see how the foam will crush -- the closed is like a rock .... same as with sheet. They do it all the time on large projects with heavy slabs and future use.

Not crush. I should have said deform. Op said selling point was no voids. Sheet will flex with weight on it to fill a dip or such in the base. Not saying spray won’t work. Just questioning the need vs sheets


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[kj_mustang]

Screed and compact your base gravel good enough and you shouldn't have to worry about voids under the foam sheets.