Spray foam insulation under concrete slab

[cthomas]

Spray foam guy was here this weekend and said he could also spray the garage floor before pouring the concrete slab. I had planned on using 2" rigid foam, but quote for spraying the floor (1 1/4 - 1 1/2 inches) was the same as the rigid foam alone. Has anyone sprayed under their concrete slabs or heard of this being done? He assured me that the concrete people have no problem with this practice stating it is much easier for them to walk on and easier to roll wheelbarrels on. He also assured me that it easily withstands the load of the concrete and provides a much better vapor barrier. It would save me a lot of time/headache if this is true.

 

[boiler7904]

Assuming its a closed cell product which it sounds like it would be if it acts like a vapor barrier, I don't see why it wouldn't work. Get product info and see what the compressive strength of the material is. Needs to be at least as high as you would get with rigid boards. Make sure the manufacturer permits using it in ground contact applications like this. Find out what the warranty is and what it covers. Lastly, talk to the local building department and see if they will fail you on a pre-pour inspection if you have the spray foam under the slab.

 

[Junkman]

I would install it over a 6 mil plastic vapor barrier to be sure that no moisture can infiltrate it. I think it would be better than the foam boards in one way.... it will be seam free. How will you be dealing with the high and low spots in the foam? Will he "shave" it to a flat surface? Whenever the costs are the same, and someone else does the labor, that is a good thing....

 

[cthomas]

Thanks for the replies. I had a stack of Foamular 150 sitting on the garage floor - gravel at this point - that prompted the discussion. I bought the wrong sheets and was going to return them for the Foamular 250 or Dow Blueboard for its compressive strength. I told him the 250 was rated at 25 psi and he said his product (closed cell) easily exceeds that (I don't recall the exact rating however) stating he has had trucks drive over the sprayed foam without problems as long as they don't turn. As for code, we live in a rural town in Western NY where building codes are very loosely written. In regards to the vapor barrier, he stated that it is not needed and is very difficult to spray over (blows all over the place and has a tendency to melt at times while spraying). As for obtaining a uniform thickness and shaving, I’m not sure how he is going to do that. He just said that the end product is very smooth, but gave no indication that he was going to shave it. The idea of a seamless insulation at no additional cost and no additional labor on my part is very tempting.

 

[Herb]

Thanks for the replies. I had a stack of Foamular 150 sitting on the garage floor - gravel at this point - that prompted the discussion. I bought the wrong sheets and was going to return them for the Foamular 250 or Dow Blueboard for its compressive strength. I told him the 250 was rated at 25 psi and he said his product (closed cell) easily exceeds that (I don't recall the exact rating however) stating he has had trucks drive over the sprayed foam without problems as long as they don't turn. As for code, we live in a rural town in Western NY where building codes are very loosely written. In regards to the vapor barrier, he stated that it is not needed and is very difficult to spray over (blows all over the place and has a tendency to melt at times while spraying). As for obtaining a uniform thickness and shaving, I’m not sure how he is going to do that. He just said that the end product is very smooth, but gave no indication that he was going to shave it. The idea of a seamless insulation at no additional cost and no additional labor on my part is very tempting.

Sounds like a good place for carpenter ants and termites to live.

 

[PAToyota]

Most of the spray foams have a borate additive to keep such critters out.