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Community and Q&A

Perm Rating of Roof Underlayment

Chris_in_NC | Posted in Energy Efficiency and Durability on

I’m plodding along with my shed/workshop, and still using it as a test case for materials/techniques/education for the future shop/garage and for future house improvements.  I’m now in a orthopedic boot for a while, and it’s been 100degF weather in the Charlotte area, which is slightly slowing progress.

I need to get the roof rough dried-in this weekend.  I’m going to add underlayment for roof deck protection until the foam and metal roof can get installed.

Can I just grab whatever readily available synthetic underlayment meets the requirements for weather resistance, foot traction, etc.?

My reasoning:
Knowing that rigid foam is very low perm, and that the sheathing is designed to dry to the interior, I infer that underlayment perm shouldn’t matter?  Most details only callout “self adhered air control membrane”, or “peel-and-stick membrane”, or roofing felt, so vapor perm doesn’t seem critical.

Currently I have the roof deck on, which is 1/2″ plywood with ZIP tape on the seams.  Almost no water leaks during the heavy rain we just had.

It’s a 3/12 shed roof, nice and simple.

Roof stackup will be fairly standard:
-Metal roof
-Furring or upper layer of ply/OSB, with underlayment
-R-5 or R-6 rigid foam, likely polyiso or XPS
-Synthetic underlayment for dry-in
-1/2″ plywood deck, taped seams
-Fluffy stuff in 2×8 rafter cavity

Sunday update: Titanium UDL 30 is down on the deck with cap nails for dry-in.  That stuff is amazing, I’ve never had such good traction on underlayment before.  Worth every penny to not have any slipping issues with my ortho boot.

Now to figure out the rest of the foam stackup.  More questions to come.

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  1. user-2310254 | | #1


    I know it's a little late, but I will give your post a bump. I suspect that you are fine with the UDL 30.

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