Spray foam under slab

[nemise]

I noticed that a lot of people use XPS rigid foam boards under the slab, and that most recommend using boards with higher PSI than 25 so the slab and cars do not crack it underneath. I see that foamular makes 40, 60, and 100psi boards. Am I better off going with spray foam underneath? Does spray foam have a PSI rating as well? Where would I find that info?

My thinking on using spray foam instead of separate 4x8 boards is that the foam would be all one complete unit, rather than about 30 separate boards, making it more level without high/low spots, reducing the chance of cracking.

Thoughts?

 

[BlackTalon]

Might want to consult a spray foam applicator. I would think that moisture in the subgrade would cause a ton of blisters in the foam. Also not sure if you can reliably get a compressive strength anywhere close to the 100 psi XPS boards available from OC/ Foamular or Dow; at least for roofing applications a 40 psi strength seems to be the norm (some samples can be much higher, but you cannot reliably/ consistently achieve that).

 

[anthony666]

My thinking on using spray foam instead of separate 4x8 boards is that the foam would be all one complete unit, rather than about 30 separate boards, making it more level without high/low spots, reducing the chance of cracking.

Thoughts?

woah .. deja vu

 

[nemise]

@BlackTalon - Makes sense, thanks for the input. What PSI rigid foam is recommended underneath a 6" slab? Is 40 the lowest I should go with, but prefer 60?

@Anthony66 - I had originally got the idea of using spray foam under the slab from you, haha It seems most people are saying not to do this though. I even had a spray foam company say they will pass on the job because they do not recommend it under the slab.

My problem now though, is that I had them leave 3" for insulation and 6" for a slab, so now I need to either purchase a 2" and a 1" board for each spot, buy more crushed stone, or do a 7" slab.

 

[anthony666]

My problem now though, is that I had them leave 3" for insulation and 6" for a slab, so now I need to either purchase a 2" and a 1" board for each spot, buy more crushed stone, or do a 7" slab.

lol .. i'm gonna decide for ya; put an inch of sand down, put vapor barrier down and make sure it goes up the walls past finished height, tape that, put 2" pink board, edge it with bubble wrap insulation, put mesh, tie your tube to that put 3/4" electric conduit elbows where the pex come up out of the floor, put a extra pex tube in for slab sensor chase .. pour concrete, pour a whiskey, relax

 

[BlackTalon]

FYI, for XPS, the compressive strength ratings are at 10% deformation. There has actually been a big stink in some jurisdictions recently about prohibiting the use of XPS in egress areas due to the potential for tripping hazards. The main issue is there is really no elastic deformation under load; once it compresses it stays that way. As a result, they (Dow) recommend using a design value of 12 psi for the 40 psi PlazaMate board.

So looking at it from a loading standpoint, a 4x6 inch tire contact patch on a 6-inch-thick slab can weigh about 3,500 lbs before exceeding the 12 psi compressive strength of the '40 psi' insulation board.

So for all practical purposes, a 40 psi plaza-grade XPS board should be fine. Maybe at most consider going to a 60 psi board, but 100 psi would be way overkill.

Also, these boards should be available in 3 inch thicknesses. You probably just need to check with a roofing/ waterproofing distributor to get it in that thickness.

 

[rburke65]

IMO.....even the 25psi stuff is over kill......? What are you parking on this a Saturn V rocket?

 

[earthworks]

when you do the math, if an eight thousand pound truck's tires has a square foot of tire footprint, then that truck plus the weight of a 6inch rienforced concrete slab would load the base under each tire at 14.93psi. also 6 inch is overkill for slab thickness.

 

[BlackTalon]

Drop the slab thickness below 6 inches and the psi loading on the XPS increases (the loading increases pretty substantially if you drop to 4 inches). So all-in-all not too terribly difficult to excess the 12 psi load rating they suggest staying within for the 40 psi boards if you do not want permanent insulation deformation (and potential floor settlement/ differential settlement).

 

[Pointbock]

Here's some under a shop in northern MN
http://www.24hourcampfire.com/ubbthreads/ubbthreads.php/topics/9275470/11

 

[rburke65]

Just for my own knowledge, who is THEY that suggest the 40 psi? And thanks for the info.