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Community and Q&A

Tips for rigid foam installation

ecoRetro | Posted in Green Building Techniques on

I’m retrofitting a 1960’s ranch house with two layers of 2″ foil faced polyiso rigid foam. I have a few questions about the detailing:

– what is the minimum amount of seam staggering recommended between the two layers of foam?
– since neither the insulation sheets nor the house is perfectly square, what amount of gap between sheets is acceptable to simply tape over?
– for larger gaps would you suggest filling them with regular spray foam in a can or use the low expansion stuff?
– I have been using 3″ roofing cap nails to secure the first layer of foam, but these nails tend to bend if I don’t hit them exactly square — are there better first layer fasteners to use?

Thanks in advance for sharing your tips and techniques on how to do this job both quickly and correctly!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Lesley,
    Q. "What is the minimum amount of seam staggering recommended between the two layers of foam?"

    A. There is no rule. I would aim for at least 4 inches if possible.

    Q. "Since neither the insulation sheets nor the house is perfectly square, what amount of gap between sheets is acceptable to simply tape over?"

    A. I would say less than 1/4 inch. If it is more than that, I would squirt some canned spray foam in the gap.

    Q. "For larger gaps would you suggest filling them with regular spray foam in a can or use the low expansion stuff?"

    A. Either one. After the foam has cured, trim off the excess.

    Q. "I have been using 3-inch roofing cap nails to secure the first layer of foam, but these nails tend to bend if I don't hit them exactly square -- are there better first layer fasteners to use?"

    A. You can practice your nailing technique until you get better. Or, if price is no object, you can always use a different type of nail (or screw) with fender washers or roofing buttons.

  2. ecoRetro | | #2

    Thanks for the quick response Martin! Any other tips or best practices from the forum members?

  3. GBA Editor
    Martin Holladay | | #3

    Lesley,
    If you haven't read it yet, you might be interested in this article: How to Install Rigid Foam Sheathing.

  4. user-1037041 | | #4

    I just finished installing two 1 1/2" layers of polyiso. Like you, Lesley, I had cap-nail problems. And supposedly, I do this kind of work for a living! I thought I was being clever when I used 16d box nails with fender washers for the first layer and 20d box nails with fender washers for the second layer. But it seems that our fearless leader (and many others, no doubt) had already thought of it!

    Anyway, it worked great for me. Held my panels in place until I got my rainscreen up. I'd do it again in a quick second.

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