Compact unvented roofing assembly: Vapor barrier options
Hello GBA Community!
Long time reader, first time poster. We are still in the early planning stages of a home that will be built in the greater San Antonio, Texas area (Climate Zone 2). My question is about the roofing assembly vapor barrier.
We are planning on constructing what Joe Lstiburek calls a “Compact Unvented Roofing Assembly” in this article <https://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation>.
See Figure 7 for details. This is because we would like to expose the wood decking for aesthetic reasons. The only twist is that we plan on having both exterior rigid insulation (as the article shows) as well as interior spray foam insulation between the true roof sheathing (most likely OSB) and another layer of false sheathing which will be 1x’s strapped across the underside of the rafters to make a nice exposed roof effect while adding some insulation value.
After reading Mr. Holladay’s article about the dangers of using open-cell spray foam on the underside of the roof decking, I am thinking that we will need to use closed-cell spray foam to reduce the likelihood of interior moisture reaching the underside of the roof sheathing/OSB https://www.greenbuildingadvisor.com/article/open-cell-spray-foam-and-damp-roof-sheathing
With that in mind, it seems that the best choice for a WRB on the outside of the roof sheathing would be a relatively vapor-permeable product since it will not be allowed to dry to the inside because of the closed-cell spray foam. Most of the recommendations I have read so far suggest very tight, vapor-impermeable products such as Carlisle WIP 300HT (0.02 perms), PolyWall AlumaFlash (0.014 perms), etc. However these individuals are also usually dealing with a different type of roofing assembly. So my questions for the GBA community are as follows:
1. Am I thinking correctly about the need for a vapor-permeable WRB on the outside of the roof sheathing since the interior side will have closed-cell spray foam?
2. Do you have any recommendations for a vapor-permeable WRB material? My preference would be a peel-and-stick product.
3. Is there a better solution to prevent interior moisture from reaching the underside of the roof decking that would still allow drying to the inside? Thus, avoiding the need to use a vapor-permeable WRB on the outside of the roof sheathing?
I thank you in advance for your time and look forward to your responses! Please let me know if I can clarify anything for you.
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