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Venting 1:12 roof assembly in cold/high humidity climate

user-3923583 | Posted in GBA Pro Help on

Planning to vent a 1:12 shed roof at eave at top of peak. Due to high humidity/cold climate in SE Alaska
plan is to use

slope shield SA by vapro shield underlayment 
12” I joist w/ either 
10” HIgh density fiberglass batt R-38
(2” vent space underside sheathing)
w/ 1-1/2” poly-iso VB at underside rafter
6” closed cell spray foam installed to bottom of rafter (6” vent space underside sheathing)
w/ 1” poly-iso VB at underside rafter

mech. Seamed metal roof 
w/ or w/0 vaproshield “ vapro shim” 1/8” epdm shim at roofing brackets to provide vent space at roofing panel  to substrate?

please advise

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  1. Expert Member
    Michael Maines | | #1

    Martin covered this topic thoroughly here: The short version is that your first option does not allow for enough vent space to be effective. Your second option allows enough space but so little moisture will get through the spray foam that having a vent space probably isn't necessary, and in a damp climate it may be worse than having an unvented roof.

    Is there a reason you couldn't make the rafter cavities deeper so you could use an insulation with low environmental impact and a deep vent space?

    1. Expert Member
      Akos | | #2

      +1 on bumping up your I-joist size and sticking with batt insulation. With taller I-joists you can go for larger spacing so the overall cost is about the same.

  2. user-3923583 | | #3

    Ok thanks, joists are in place. I can fir down joist if needed 3.5”. Looking to avoid redundancy, but hot roofs have never panned out well in this region. Well detailed Hrv system is also planned to mitigate moisture in the home

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