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Sheathing OK between peel and stick membrane and closed cell foam?

user-7402203 | Posted in Energy Efficiency and Durability on

Hello, I’m building a house in Washington State (Zone 5) and 100% of my roof is covered with  TRI-BUILT Self Adhering HT Underlayment (peel and stick) with a Moisture Vapor Permeance  <0.1.   I used TRI-BUILT because my roof sections are all low slope and I have standing seam metal over the underlayment.  My roof sections are all vaulted and made of 14″ TJI560 @ 16″ O.C. and 1-3/4″X11-7/8″ LVL @ 12″ O.C.

I’m considering using 2″ of closed cell spray foam (i believe less than 1 perm) on the underside of the roof sheathing but wonder if this will put my plywood sheathing at risk of rotting both surfaces above and below are vapor barriers.

Trying to decide between 2″ closed cell backed by open cell or batts, all open cell foam, or all batts.  Appreciate your advice.

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Replies

  1. Expert Member
    Akos | | #1

    As long as the roof deck is dry, there are no issue with sandwiching it between two impermeable layers.

    In zone 5, for an unvented roof your options are:
    -exterior rigid above the roof + fluffy bellow
    -closed cell SPF bellow + fluffy
    -closed cell SPF bellow + open cell SPF.

    In call cases you want 40% of your overall R value to be impermeable insulation the rest permeable.

    An all open cell unvented roof in cold climate is an unsafe assembly and can fail.

  2. Expert Member
  3. user-7402203 | | #3

    Thanks for the info Akos and Malcom!

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