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How to glue XPS together – turn 1″ into 3″

mikeysp | Posted in General Questions on

Hi, I am in zone 4a and I just picked up a bunch of recycled 1″ XPS (25psi).

I would appreciate a sanity check on my strategy.

I am going to stack 3 layers of it under my slab to create a 3″ insulation total. 

Since it is fairly thin layers, my strategy is to make a very flat small gravel layer of rock under the foam. Then to stagger the joints as I install it and glue the layers together, so it withstands walking on it for installing plastic, rebar, pex, and concrete pour. 

My fear is that 1″ thick will be more prone to break, whereas a 3″ layer will be much stronger. 

I was thinking to use my spray foam gun and run a zig zag pattern of the spray foam to act as a glue while staggering the seams. 

CONCERN: While spray foam glues very well, my concenr is that of expansion and creating air gaps. I am thinking??? that I can just ensure I put a thin layer while zig zagging the foam???

Alternatively, I thought I could make 3″ thick boards the same way and not have staggered seams. 

FYI, these boards came off an old chicken house and are 4ft x 27ft x 1in and are essentially fomular 250 (25psi foam).

I figured the density by cutting a 1″ x 12″ x 12″ piece, weighed it and multiplied the weight by 12 to get a 1.6lb weight which is the density of fomular 250. fomular 150 is 1.3lb per cubic foot of foam and it has a 15psi load limit. 

Thank you for your advice.


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  1. Expert Member
    BILL WICHERS | | #1

    Test your gun adhesive on a sample of the XPS and see if it melts the XPS or not. The first thing you need to do is make sure whatever glue you use is chemically compatible with XPS. Mostly this just means “make sure the glue doesn’t melt the XPS”. My guess is your gun adhesive will be ok.

    If your gun adhesive melts the XPS, then I’d use PL foamboard adhesive. Don’t worry about air pockets and the like, the slab will be plenty heavy to squish the foam layers together. You really just need something to hold the foam in place while you’re assembly the insulation layer and pouring the slab. Once the slab is in place, the foam won’t be able to move around enough to matter.


  2. jberks | | #2

    I don't think you need to glue it at all. Especially since it's going under a slab.

    But if you do still glue it, my first thought is to use foam board glue out of a caulk gun, because I know they make adhesives for foamboard in caulk tubes. You don't need much, just a bit to hold it in place. However, obviously dispensing from a foam gun is easier/faster, but you'll have to dig or test to find which adhesive works with the foam. Again, you don't need much, just enough to hold it from sliding while you walk on it.

    I glued 3" layers of polyiso on my roof with a 2 part adhesive from the foam manufacturer. that mildly foamed, and I found it was lifting the foam boards a bit so I had to weigh it down with leftover I-joists while it cured. It was moderately annoying. Caulk style foam adhesives don't foam so that solves that issue. Or if you have a bunch of long boards kicking about, then sure try the foam gun.

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