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Community and Q&A

Polyiso insulation in rafter bays

Samuel Koerber | Posted in Energy Efficiency and Durability on

would it be exceptable to use Polyiso foam sheathing in rafter bays without provisions for venting? I imagine that as long as the fit was really snug that you could get a tight enough seal to eliminate condensation problems. Another option I’m considering is placing 8″ of polyiso in the 2x10s, tight to the sheathing, followed by 1″ of closed cell foam to seal off any airflow. I think that this may be the only viable way for us to achieve a high R value in the roof assembly at a much lower cost than deep closed cell foam. We’ve got cathedral ceilings and a fully livable roof space and no means to put polyiso on top of the sheathing. Boone NC zone 5. Thanks for any insight.

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  1. GBA Editor
    Martin Holladay | | #1

    Either of the two options you listed will work, as long as you are meticulous about air sealing. Ideally, you should air seal each layer of foam, to minimize the chance that you are leaving hidden channels where air can move around.

    Most people who do the work you suggest find it easiest to cut the foam a little small and loose, to make it easier to seal the edges with canned spray foam.

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