Zip Roof, Unvented, with Shingles
Good Afternoon!
We are currently building in Climate Zone 5. Zip System for Walls and Roof. Due to the complex roof, our builder would like to do an Unvented Assembly. Plan is to use Closed Cell Spray Foam under the roof deck, and add fiberglass to meet the R-Value requirement.
My husband wants to use Ice & Water Shield in the critical areas of the roof, such as in the valleys, to add an extra layer of protection, NOT over the entire roof assembly.
My concern is creating a double vapor barrier. The roof needs to dry in at least 1 direction, and it can’t dry to the interior. Would placing the Ice & Water Shield be a problem in only the critical areas?
Also, we are using Shingles, and have called to make sure they can be installed in an unvented assembly, which they can. Do you recommend a layer of asphalt felt under the shingles? Or should the Zip System already meet that requirement?
Thank you in advance!
BrendaK
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Replies
BrendaK,
Martin addresses the issue in this blog: https://www.greenbuildingadvisor.com/article/sandwiching-roof-sheathing-two-impermeable-layers
"I recently telephoned Joseph Lstiburek, a principal at Building Science Corporation, and asked his opinion on the matter. “We have double vapor barriers in commercial roofs all the time,” Lstiburek told me. “When people say, ‘The roof sheathing has to be able to dry in at least one direction,’ I say, ‘That’s ridiculous.’ It doesn’t have to be able to dry out."
Malcolm,
Lstiburek continued, “That said, I like it when the roof sheathing can dry in at least one direction, because I always like a little bit of redundancy and safety. But you don’t have to have it. Remember, an impermeable layer of peel-and-stick really is impermeable. But with 4 to 5 inches of spray foam, you’re still going to have more permeance than with the peel-and-stick. There will be some limited drying downwards through the spray foam — it’s just not going to be very fast.”
I have taken apart a roof like this in zone 6 and seen rotten framing that no one new about. If the roof has feet of snow on it, the shingles may be above freezing and ice dams will force water in and no one will know the roof is leaking because the spray foam comceals it. The water has no where to dry to and its a bad day. If his roof is "complex", that to me means dormers and valleys, which leads to greater risk. The approach may be fine, but it has no forgiveness.
I would suggest an over roof layer of strapping and another layer of sheathing to create a vent space. I think one leak would be more expensive than the cost to install in the first place. Both situations can work, but I wouldn't want to be the builder responsible for that.
One mans opinion...
conwaynh85,
I wouldn't be comfortable with an unvented roof with no path to dry either, but this assembly is now widely used - mostly with open cell foam (and all the possible problems that brings) - so it's hard to make the case that it's particularly problematic. Once you go the spray foam route, what are your options? If the roof is too complex to vent, over-venting the sheathing might be equally ineffective. The safest solution is adding sufficient rigid foam over the sheathing, but that adds complexity and expense.
Malcolm,
Agreed. Thank you for the discussion.
conwaynh85,
What I don't know is when over-venting complex roofs works and when it doesn't. It would be great to avoid the spray foam altogether and use something like this: https://www.greenbuildingadvisor.com/article/building-a-vaulted-high-performance-and-foam-free-roof-assembly
Thank you. I am trying to read the article, and have a prime membership trial, but it's loading properly. Waiting for Customer Service to get the issue fixed. Hoping that article will help settle my mind.