High R-Roofs Case Study Analysis Detail
Before asking my question I want to thank many of you for your willingness to take time to share your knowledge, it is truly an enormous help for those just starting out. Though sometimes Martin and Robert’s discussions leave me more confused than when I began (which is a good thing).
I am planning to build a house in New England, Zone 5, and have been basing most of my plans from research done by The Building Science Corp. I am leaning toward using exterior foam (4 or so inches) with cellulose in cavities for roof and walls (unvented walk-up attic). I just read the draft for “High R-Roofs Case Study Analysis” from the Building Science Corp website and was curious about their detail using exterior foam. It shows 2 layers of lapped peel and stick membrane; one on each layer of OSB sheathing (I plan to use plywood over OSB due to the enormous fear this site has instilled me with) . One serving as an air barrier and the other as the drainage plane.
Aside from the added cost of using two full layers of peel and stick membrane, I am concerned about the potential for trapped moisture. I assume I am misapplying something but am curious if this detail is “correct” or if I might be able to avoid one or both layers of peel and stick by using different details or products to serve as my air barrier and drainage plane.
I am curious on your thoughts. Thanks.
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Replies
Kevin,
Peel-and-stick membrane is an excellent air barrier, but I'm not sure why BSC thinks you need two layers.
I might use it to cover the lowest level of roof sheathing, assuming you are planning to install 4 inches of rigid foam above it, but I don't see any reason for a second layer of peel-and-stick on top of the top layer of roof sheathing.
Kevin,
If you can share the link to that BSC roof section, we can better respond. A search of their site for "High R-Roofs Case Study Analysis" came up empty.
If you are going through all that extra work and materials why not use ready made SIP's it must be so much cheaper. http://www.sips.org/ It seems like you are doing an overkill unless you are planning on a passive house, http://www.passivhaus.org.uk/
The first layer of peel and stick acts as both the air control layer and vapor control layer. It also makes the roof constructible as the first layer provides rain water protection during the construction process. We recommend that the roof assembly be dried in as quickly as possible especially in a multi layer roof assembly and peel and stick membranes provide the best performance in this regard. Non adhered membranes have not performed as well in this application.
The second layer of peel and stick - the upper most layer - provides rain water control to supplement the shingles which tend to leak. It has been argued that this layer can be replaced with typical roofing paper. I disagree. Typical roofing paper tends to absorb too much water and is dimensionally unstalbe.
The concern about a double vapor barrier is not born out out of experience. There are millions of square feet of compact commercial roof assemblies with double vapor barriers dating back to the mid 1950's. In any event the question is moot since the shingles are impermeable in there own right.
In terms of SIP roof assemblies - they have problems of their own and you can see a discussion relating to this at the following link: http://www.buildingscience.com/documents/insights/bsi-036-complex-three-dimensional-air-flow-networks?topic=doctypes/insights
Thank you for the feedback. I feel as though I hit the Holy trinity of building science.
I am unable to find the link on the website, it was under "New Documents" as part of a longer article. That and and a few other links I saw yesterday appear to be missing. So let me apologize for not being able to add a link and let me also apologize if my question resulted in some information from the buildingscience.com website being taken down. I assure you that was not my intent.
Nice to see a Lstiburek posting.
And nice to see that Bruce Brownell's methods for 30 years are somewhat now known to Joe.
http://www.aaepassivesolar.com/