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
or
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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Replies
Martin covered this topic thoroughly here: https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs. 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 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.
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