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Follow-up issue with 6 inches of open-cell foam

cobragrover | Posted in Energy Efficiency and Durability on

Following up on the earlier question someone had about 6 inches of open cell foam on the underside of the roof. Icynene is saying 5 1/2 inches of open cell spray foam (R20) is equal in energy efficiency as R38 cellulose or fiberglass because the foam blocks all air infiltration as opposed to the cellulose or fiberglass. I may be over simplifying the answer so you can read it for yourself here. I hope this link works.

I have a question about this. If I do the spray foam and create a conditioned attic space can I add unfaced fiberglass batts or cellulose at a later date to the floor of my attic or will 2 layers of insulation cause an issue? I could raise the total R value to R38 or R50, or whatever is required. I can’t imagine what kinds of problems it might cause but I thought I’d ask.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "If I do the spray foam and create a conditioned attic space, can I add unfaced fiberglass batts or cellulose at a later date to the floor of my attic?"

    A. Yes, you could do that. I assume that this is an attic with ductwork or equipment in it. Otherwise, you should just put all of the insulation on the attic floor.

    If you have ductwork in the attic, your approach will work. Remember that in Climate Zone 5 (or anywhere colder), it's possible to get moisture accumulation in your sheathing if you spray the underside of your roof sheathing with open-cell spray foam. This problem is best addressed by installing interior drywall covering the foam, and then by painting the drywall with vapor-retarder paint-- or by using closed-cell spray foam instead of open-cell.

  2. cobragrover | | #2

    Thanks Martin. It seems there is a big debate on foam on the underside of the roof concerning, amount, type(closed cell/open cell). My insulation guy said he didn't like closed because you would never know if you had a roof leak. You never know if they are giving you the corporate crafted answer or if it is truly the best thing to do.

  3. GBA Editor
    Martin Holladay | | #3

    I recently attended a building science conference (sponsored by ORNL and ASHRAE) in Clearwater Beach, Florida. One of the papers presented at the conference showed the results of modeling studies looking at moisture build-up in roof sheathing with spray foam on the underside of the sheathing. In all climate zones, open-cell foam was riskier than closed-cell foam.

  4. wjrobinson | | #4

    Just a FYI, thousands and thousands of homes in climate zone 6 have been spray foamed with open cell at 5-6" .I built two that are performing very well and far superior to batt days of yore. Martin's advice is conservatively correct, but ..... Just saying.... good.... Better.... Best.

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