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Underlayment needed below above-deck rigid foam insulation?

peter2022 | Posted in General Questions on

I’m retrofitting rigid foam above my roof deck.  For the air barrier on the existing sheathing – my contractor is suggesting self-adhered SBS ice & water across the entire roof surface, which is fairly expensive just in material cost, and I’m looking at ways to reduce my total expense.

The existing sheathing is plywood.  If I tape the seams, something like Tescon Vana, is any additional WRB necessary at this layer?

I’ll have additional plywood sheathing with underlayment and asphalt shingles above the foam (2 layers staggered 2″ GPS foam).  Below the existing sheathing are 2×6 rafters filled with cellulose or rockwool.  So about R-20 above and R-20 below the sheathing. This is in climate zone 4C.

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Replies

  1. Expert Member
    Michael Maines | | #1

    Plywood in good condition is a good air barrier, so taping the seams should be fine. I do recommend at least a class 3 vapor retarder on the interior; otherwise enough moist air would reach your sheathing to risk condensation.

    1. peter2022 | | #2

      Thanks. All ceilings are latex painted drywall and I'll be ensuring any penetrations are sealed properly.

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