Where to put membrane on unvented metal/polyiso roof
Hey folks,
I’m renovating an old house in Tucson, Arizona and I’m just about to put a new roof on. The previous owners of the house knocked out the old drop ceiling, closed off the vents, and stuffed a bit of fiberglass between the roof and a tongue-and-groove ceiling. It’s a very low-sloped roof, so they created a glorious 6″ cathedral ceiling and a mold problem. Hopefully I can do a bit better.
I’ve been reading lots of articles on GBA and I’ve got a plan, but I want to put it up for scrutiny to make sure I don’t overlook anything obvious. Can you give me a quick stupid-check?
From the ceiling up, here’s what I’m doing: keeping the tongue-in-groove, pulling the moldy fiberglass batts and leaving a cavity that is unsealed and breathes to the interior. Then plywood decking, 5″ polyiso, more plywood decking, an impermeable membrane, and metal r-panels.
My biggest question is: should I put a membrane above the first layer of decking and the polyiso sandwich as well? It seems like I would want to avoid having a barrier there and allow the roof assembly to breathe into the interior of the house, but I’ve seen a few diagrams where people are using two membranes.
Thanks.
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Replies
Aengus,
You want an air barrier above the lower level of roof sheathing, and roofing underlayment (for example, asphalt felt or synthetic roofing underlayment) above the upper level of roof sheathing.
Here is a link to an article with more information: How to Install Rigid Foam On Top of Roof Sheathing.
Thanks Martin! I think I'll go with an impermeable, high temperature synthetic for the underlayment and asphalt felt for the lower air barrier. This brings two questions to mind, mostly because I'm curious about the physics of this:
1) Does it make sense to match the permeability of the roofing underlayment with the air barrier I'm putting above the lower level of sheathing? For instance, would an impermeable membrane up top and a (minutely) permeable layer of asphalt felt below lead to moisture passing through the asphalt felt and condensing on the sheathing beneath the synthetic membrane? Does it make sense to go asphalt-asphalt or synthetic-synthetic?
2) It sounds like sealing all of the foil-faced polyiso layers with appropriate tape would not create an adequate air barrier over the lower level of sheathing. At risk of sounding totally naive, why is this?
Thanks again,
A
Aengus,
Again, I urge you to read the following article: How to Install Rigid Foam On Top of Roof Sheathing.
The article explains options for creating an air barrier above the lowest level of roof sheathing. You wrote that you want to use asphalt felt as an air barrier -- but asphalt felt is not an air barrier (because it's impossible to tape the seams of asphalt felt, and because asphalt felt is somewhat porous).
As I noted in the article, there are at least two ways to establish an air barrier above the lowest level of roof sheathing:
If your roof has board sheathing, install an airtight membrane (for example, Solitex Mento, a product available from 475 High Performance Building Supply), a layer of synthetic roofing underlayment, or a peel-and-stick membrane.
If your roof has OSB or plywood sheathing, the panel seams can be taped (for example, with Zip System tape or Siga Wigluv tape).
Got it. I was thrown off by the second bullet point because the full quote reads:
"If your roof has OSB or plywood sheathing, the panel seams can be taped (for example, with Zip System tape or Siga Wigluv tape); after taping the panel seams, install the roofing underlayment of your choice (for example, asphalt felt)."
I interpreted that as a suggestion to put asphalt felt between the lowest level of plywood and the polyiso, which didn't make sense. And, indeed, it doesn't. Thanks for clarifying! Your blog posts have been incredibly helpful.
Aengus,
Many builders like to install asphalt felt above the lowest level of roof sheathing. But the asphalt felt is not an air barrier. The air barrier is the taped sheathing.
Once the sheathing seams have been taped, you can proceed with either asphalt felt (which protects the sheathing from the weather during construction), or you can just install the first layer of rigid foam above the roof sheathing.