Has anyone studied or quantified how XPS foam shrinks?
I am working on the foundation for a new house I am building for my family. Under the pad I am putting a 3.5″ layer of reclaimed XPS. Then I got thinking that I should put something over top of that to overlap any seams. So the lumber yard had 4X8 sheets of .5″ of XPS which isn’t a lot of R-value but I mostly wanted something over the seams in the thicker reclaimed XPS. I know EPS is better for the environment, but they didn’t have any. Anyway I took my time and really made sure the joints were tight and everything laid in nice without any gaps. I didn’t tape it because I figured It is under poly and concrete, it shouldn’t move much and I’m not sealing anything. Luckily there was no wind and the foam stayed in place for about 5 sunny days before a layer of poly was put over top of it. What I noticed though was that the foam shrunk, or footers grew, but I don’t think so. There were gaps all over the place. I know Joe Lstiburek has mentioned this before but I had no idea it would happen that fast. Has anyone studied this? I plan on applying two layers of 1.5″ XPS to achieve 3″ on the exterior of my new home. Would laying out the layers shrink the foam ahead of time? That would be a pain in the butt. I would think the thicker sheets would probably take longer to shrink. Also what is the relationship causing the shrinking. Temperature? Does the sun have anything to do with it? Other than staggering and taping seams is there anything I can do to prevent or minimize these gaps from occurring over the long run? Some other things worth noting is the pile of new .5″ XPS sat in a stack outside for about 2 weeks before it was laid out, I have no idea how long it was at the lumberyard. Also worth noting is the 3.5″ reclaimed XPS was out for a few days in the sun and did not shrink at all. I imagine is has already shrunk. On a last note the reclaimed board is of the blue brand and the shrinking .5′ XPS was of a green brand. Thanks for any input.
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