Vented or Unvented Roof with Exterior Rigid Foam
okapi222
| Posted in Green Building Techniques on
The question of venting or not has been discussed many times on this site. Unfortunately we still do not have a definitive solution that satisfies both our architect and our roofer. I am wondering if the experts would chime in. We are renovating a wood frame 640 sf cabin in zone 5 with a cathedral ceiling. One of our roof slopes is 2.5/12 the other slope is 7/12.
Roofer’s choice:
Standing seem metal roof
Bitumen hi temp ice barrier
1/2” plywood
4” Polyiso or GPS
3/4” solid wood cladding
8” celulose
Intello vapor barrier
Our architect’s choice:
Standing seem metal roof
“2x 1” furring parallel to the ridge
“2x 1” furring perpendicular to the ridge
A rainproof vapor open or similar membrane
4” Polyiso or GPS
3/4” solid wood cladding
9” celulose
Intello vapor barrier.
- Given that we have the minimum board insulation on the exterior, is venting below the cladding adding any extra value?
- Does the vapor permeability difference between polyiso and GPS relevant here?
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Replies
I have a similar situation and am planning on unvented. If you read the recent article, particularly search for "low slope", the article asserts that "Traditional vented roof assemblies require a minimum roof slope of 3-in-12. If your roof has a lower pitch, you won’t get enough air flow through traditional 1-inch or 2-inch ventilation channels to avoid moisture problems." -- and links to another article on the low-slope situation -- TL;DR don't try a vented low-slope roof unless you have a deep vent area and a cupola. Your 2.5:12 roof is on the edge of "low slope", so you might be able to get away with more, but I'd advise caution & input from actual experts.
Another couple comments on your assembly -- It wouldn't hurt to have an additional air barrier layer between the "3/4” solid wood cladding" and "4” Polyiso or GPS" layers. Also, read the comments (& links from those comments) under my first link for a discussion of the cold-weather behavior of polyiso. We're both zone 5 and the conclusion seems to be that "it'll be fine", but food for thought when choosing insulation types. Also -- in your architect's assembly I don't know why they'd specify "A rainproof vapor open or similar membrane" -- because right under it you have 4" of foam that's not vapor open.
To directly address your two questions (with the caveat that I'm just a guy on the internet who's just in the process of learning this stuff):
1) Venting below the cladding in this kind of stackup could be helpful in areas with lots of snow to help prevent ice dams, but is not necessary for moisture control.
2) I don't feel qualified to take this one :P
To answer your question about why someone would put a vapor open membrane above polyiso or GPS, it is because both of these insulation types are vapor permeability to some degree. But as I understand it, if taped at the seems, they also form an air barrier. So I would like to understand if the vapor permeability is a factor or if that is not an issue because they are air barriers. If they are air barriers, then I wouldn't need an additional air barrier between the rigid insulation and the wood sheathing.
Martin Holladay's article https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathing recommends and air barrier below the rigid insulation. But if we are already using an interior air barrier (intello), is this necessary?