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Foam board built up roof questions

jpeterersen | Posted in Green Building Techniques on

I have read the articles and threads I have been able to find. I could not find answers to a couple of questions. I am turning a small old barn 20’x30′ into a guest house near Madison WI zone 6A. I want to preserve the inside appearance as much as possible, hence the exterior insulation. Per code allowance for the vault and insulation extending over the exterior walls I will need to achieve R33. The roof needs to support a dead load of 10lbs/sqf and snow load of  26lbs/sqf, when corrected for pitch. I have not decided on EPS vs XPS vs ISO. I am open to mixing as well, such as ISO on the warm side and one of the others toward the cold side. Do I need to select a foam boards with 36lbs/sqf + compression strength. It does not look like ISO can meet that parameter.
Also is the vapor retarding qualities of any or all the options sufficient or will I need a vapor barrier on the warm side. If these question have already been addressed please refer me to the thread/blog
Thanks Jerry

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  1. Jon_R | | #1

    A GBA search will yield lots of information about roof insulation and code requirements (eg, IRC 806).

    If walls are involved, I'd follow the guidelines here:

  2. jpeterersen | | #2

    Thank you, I found the answer to the vapor retard/barrier, sorry for bringing it up yet again
    I have not found recomendations for density/compressive resistance for above roof deck foam board insulation based on snow loads, despite using the search function. The search generated a number articles, but none addressed this question. Is the typical big box store rigid board isulation with a compressive strength listed at 15 lbs/sqf or less acceptable where the snow load exceeds this number?
    Thanks Jerry

  3. Expert Member
    Akos | | #3

    Rigid insulation ranges from 10 PSI to 40 PSI, which translates into 1500 to 6000 lb/sqft. No amount of snow load will dent it, so you can use any foam you want.

    I like to use roofing polyiso (rated for 20 PSI) which is pretty stiff and can be walked on without damage.

  4. jpeterersen | | #4

    Thank you for setting me straight, I have been misreading the foam compressive ratings as PSF, sort of explains why there is no literature on it. Its a non issue

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