I am building a SIPS house with SIPS foundation. I plan on putting radiant heat in the walkout basement. We get quite cold here in MN and I am wondering is I could use closed cell foam under the slab/on top of the gravel layer? I plan on having polished concrete floors in the basment and will be living down there and would like to minimize cracjing of the concrete floor. I just thought that closed cell foam might conform better and be a firmer surface for the slab. Any thoughts? Thanks

Sometimes, I wonder about the spray foam and it's ... I digress...

Anyway, XPS is generally what is used under slabs, and it's the high PSI type I would wager, there are multiple PSI strengths of XPS foam, type IV or V is the strongest if I recall correctly with a 100 or 125 psi strength per square inch. Want it warmer, just use more. While i might be possible, I highly doubt that anyone has ever used spray foam under a slab.

If there is any chance that the spray foam could get wet, then do not use it.  If you do use spray foam, then of course use closed cell.  It has a higher PSI than open cell and a higher R-value.

As a general rule, expanded (EPS) or extruded (XPS) polystyrene should be used under a slab.  Extruded has a higher R-value but it does cost more than EPS.

I believe that you will find that 1.5 pound EPS will be sufficient under a slab.  At least I have on projects in the Southeast U.S.

The EPS on top of the compacted gravel will have a good stable bed.  Since this is a basement application, I would have a tendency to place the waterproofing membrane under the EPS instead of on top next to the concrete.  This will help keep ground moisture from getting into the EPS and lowering its R-value.

Thanks for your advice. I will nix the idea of spraying closed cell foam under the concrete slab. 

 

On the same note, I am having trouble finding out the ideal amount of foam to put under the concrete slab in a MN home.  This will be a walkout, with infloor heat in the basement.  Would it be best to use 2" or 4"? 

 

 

Thanks for your replies.

 

Ellen

Hi there,

foam under slabs has been done fairly often. Here is a link to sprayfoam.com for a short discussion on the matter.

http://sprayfoam.com/mnps/fullthread.cfm?threadid=6407&mnboardid=5&mnforumid=2&startat=1