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Do you recommend closed or open-cell spray foam on roof sheathing in a finished attic space in Chicago that incl. HVAC system?

macvay | Posted in General Questions on

Would closed cell spray foam insulation on roof sheathing impair the ability to detect water leaks via the roof? I’m being told by an insulation contractor that using closed cell spray foam in the roof area would cover up any water damage coming from the roof and the possibility of mold issues would be greater than if I used open cell spray foam. Other insulation contractors are telling me to use closed cell spray foam to R-49 on the roof sheathing in the finished 3rd floor space. Which way is recommended?

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  1. GBA Editor
    Martin Holladay | | #1

    Closed-cell spray foam will perform better and will reduce the chance that moisture will accumulate in your roof sheathing. So I advise you to choose closed-cell spray foam.

    Here is a quote from building scientist Joe Lstiburek: "There has been much discussion about open-cell being better under a roof deck than closed-cell because if there is a leak the leak either dries faster or you can see it sooner. Not true. There is no compelling evidence one way or the other. None. I started all this SPF stuff under roof decks in the early 1980’s. I have been looking at this roof leak issue since the very beginning. I have played with mock-ups and wetted them and watched them. I have taken apart existing leaking roofs all over the place, open-cell, closed-cell, SIP’s, compact, etc. With the mock-ups and the field experience I have not noticed any difference between open-cell and closed-cell in terms of detectability and damage with respect to roof leaks. Did I mention the fact that there is no compelling evidence one way or the other regarding roof leaks and open-cell vs. closed-cell? Ready for this… here it comes: roofs leak. They always have. They always will. Sometime you see the leak, sometimes you don’t. This is true regardless of what system you use."

  2. Expert Member
    Dana Dorsett | | #2

    On the other hand, R49 of closed cell foam is a huge environmental hit, due to the HFC245fa blowing agent's high global warming potential (~1000x CO2), plus the large quantity of polymer involved. To hit R49 without creating a fire hazard as it cures it takes no fewer than four lifts of a couple inches each, with a cooling period between passes. Going to R49 using all closed cell foam is the opposite of "green building".

    To be somewhat nicer to the planet & it's occupants you could install the minimum amount necessary for dew point control in your climate zone, and use open cell foam (or fiber insulation) for the rest. Open cell foam uses about half the polymer per R as closed cell foam, and uses water as the blowing agent, not HFC245fa. (And it's ~30-35% cheaper per R.)

    For the amount of cc foam required for dew point control, consult the IRC:

    In US climate zone 3 (or lower) a flash-inch applied in a single pass would do. In climate zone 7 it would take 5x that much, installed in 3 lifts. It matters where you live.

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