Replies

1. 
   Jonny_H|Mar 21, 2021 11:02am|#1

   I have asimilar situationand am planning on unvented. If you read therecent 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 toanother articleon 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)包覆在这种stac下面发泄kup 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

2. 
   Daniel Herskowitz|Mar 21, 2021 09:36pm|#2

   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.

3. 
   Daniel Herskowitz|Mar 21, 2021 10:01pm|#3

   Martin Holladay's article//m.etiketa4.com/article/how-to-install-rigid-foam-on-top-of-roof-sheathingrecommends and air barrier below the rigid insulation. But if we are already using an interior air barrier (intello), is this necessary?