Air sealing the roof from outside on an older house
Oregon Willamette Valley
Climate zone 4c
House has 1″ x 6″ shiplap on roof
Cathedral ceiling with fiberglass insulation in it.
No venting presently.
Want to air seal shiplap, add 1.5 foam board on top of it and then 5/8 CDX with asphalt shingles. I’m also consider a metal roof.
I’m thinking peel and stick for the air seal on the shiplap rather than house wrap.
Should the air seal membrane be breathable or block all moisture?
What brand air seal membrane would you recommended?
Do I need to a vent space between the 1.5 foam board and the 5/8″ CDX?
I have seen both method which is right? Vent or no vent?
If I add a vent what is the screw spacing on the 5/8″ CDX?
If no vent what is the screw spacing on the 5/8″ CDX?
Andy
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Replies
Since most foams are vapor barriers, the permeability of the peel and stick won't matter much.
The item you have to check is that the ratio of the rigid insulation to the existing fiberglass. If your rafters are larger than 2x6, my guess is you would need more than 1.5" of insulation. Check out:
https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing
Going to 5/8 cdx doesn't buy you much, 7/16 is more than enough in this case. Top venting is mostly for areas with a lot of snow. For screw spacing check your local building code (same as roof).
Hi Andy -
For just about any climate, you vent roofs until you can't. Lots of resources to consider on this; I did write a 2-part article on roofs for JLC online relatively recently, trying to be comprehensive for all the different ways one can configure a roof, and for different climates:
- https://www.jlconline.com/how-to/insulation/avoiding-wet-roofs_o
- https://www.jlconline.com/how-to/insulation/avoiding-wet-roofs-part-ii_o
And given our classification of vapor retarders (Class I: < 0.1 perms; Class II: between 0.1 and 1. perms; Class III: between 1.0 and 10 perms; > 10 perms is vapor open), there actually is a significant difference between say foil-faced polyiso (any thickness) = Class I VR; 1.5-inch XPS a bit less than 1 perm so Class II VR; 1.5 EPS around 3 - 4 perms so a Class III VR.
Peter