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Community and Q&A

Roof Underlayment Perm Rating?

q8mb7QkQSz | Posted in General Questions on

I am in my final push of re roofing. Martin, Thank you again for answering my earlier questions.

My cathedral ceiling timber frame from inside out has:

tongue and groove
vapor barrier


tongue and groove
vapor barrier
Metal Roof

We tore the shingles off to find a fair amount of air leakage and are strapping and adding venting to the assembly.

My question is below the strapping will be an underlayment. My roofer suggested Grace Tri Flex which has a perm rating of 0.54 My concern is moist air hitting this and not being able to dry to the exterior. Drying to the interior does not seam possible do to the plastic VB against the tongue an groove. Is there a better material? Deck guard? Felt? Or is it fine?

Thank You. I sincerely appreciate your ideas.

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  1. bdrfab | | #1

    I'd probably side with you. A perm rating under 1 can be considered a vapor barrier iirc, so I'd probably stick with good old felt paper.

  2. albertrooks | | #2

    T White,

    Yes, a higher perm would be much better for the durability of the plywood.

    The upshot is that it would be best if you could seal the T&G as airtight as possible. The rest of the assembly won't be moving much vapor through it.

    For the underlayment you can use felt or something more durable such as a roofing quality membrane like SIGA Majcoat. Materials such as these are noted by a high perm rating of 34 perms but are still driving rain proof as well as airtight.

    It looks like the steel roof is on the strapping rather than additional plywood sheeting. In this case it would be good to use a membrane instead of felt for durability. I can't see your climate zone, but I'm guessing the back side of the steel roofing will get some regular condensation and wet the membrane seasonally.

  3. GBA Editor
    Martin Holladay | | #3

    T White,
    1. You don't need a "vapor barrier" on top of your T&G ceiling boards, since polyiso is a vapor barrier. What you need is an air barrier. If you call it a vapor barrier, you misunderstand its function and you might end up being sloppy with air sealing. Airtightness, airtightness, airtightness!

    2. I hope that your polyiso is thick enough to meet the minimum code requirements for ceiling R-value in your climate.

    3. I hope you have at least two layers of polyiso, with the seams of the second layer staggered with respect to the seams in the lower layer. Tape the seams of each layer.

    4. In your case, the vapor-permeance of your roofing underlayment doesn't matter too much -- but asphalt felt is better, because it won't trap moisture in the plywood.

  4. q8mb7QkQSz | | #4

    This is not new construction. The home was built in 1985. I do not have the money to rip off the whole roof and redo it. What I am calling a vapor barrier is a plastic sheeting.. I am trying to air seal as much as I can get to without ripping everything. My main concern is the long nails that hold the assembly together are leaking air/moisture and I want the plywood to be able to dry. If the Tri-flex will let it do that then great.

  5. GBA Editor
    Martin Holladay | | #5

    T White,
    The Tri-Flex has very low permeance, so it isn't the product you want. That's why I advised you to use asphalt felt.

    For more information on roofing underlayments, see Synthetic Roofing Underlayments.

  6. q8mb7QkQSz | | #6

    Would GAF Deck Armor be a better choice? Given its perm rating of 16?

  7. GBA Editor
    Martin Holladay | | #7

    T White,
    Your choice -- but it will cost you more than asphalt felt.

  8. q8mb7QkQSz | | #8

    Thanks Martin.

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