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Has anyone studied or quantified how XPS foam shrinks?

dvaut | Posted in Green Products and Materials on

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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  1. GBA Editor
    Martin Holladay | | #1

    I discussed your question in my article, Using Rigid Foam As a Water-Resistive Barrier.

    Scroll down and locate the heading, "Do rigid foam panels shrink?" The rest of the article below that heading discusses your question. Note that there are several photos of XPS panels that have shrunk.

  2. dvaut | | #2


    I have read your article and it was helpful to review again. Luckily the foam is not going to be part of my WRB. Though I still am wondering what is the relationship of the foam shrinking. Wood dimensions changes with MC, Metals and vinyl change dimension with temperature. Could there be multiple things causing dimension changes in foam? Does foam change sizes with changes in temperature, or is it only off gassing and shrinking after it is unpackaged over time? Are there conditions that speed up or increase the shrinkage? Would immediately covering the foam up with a house wrap slow down the off gassing and hence the shrinkage? While your article discussed foam that had shrunk over years, I was very surprised to see the .5" thick foam shrink noticeably in just days. I imagine the generic answer is going to be every manufacturer uses a different formula, so each manufacturers foam will behave differently, but I thought maybe someone had some insight to this.

  3. GBA Editor
    Martin Holladay | | #3

    Like you, I would love to know the answers to your questions. I've tried to track down answers to some of these questions, but technical experts employed by rigid foam manufacturers have not been forthcoming with answers. The issue is complicated by the fact (as you correctly guessed) that blowing agents and manufacturing processes are subject to change and are closely guarded industrial secrets.

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