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Typar Roof Underlayment for Unvented Conditioned Attic

2screws | Posted in General Questions on

Hi Team,
A contractor recommended Typar roof underlayment for the roof of my planned Nonvented conditioned attic in CZ3 (fiberglass shingles, oc spray foam underside of OSB).

Vapor perm is very low at 0.41 perms. Typar rep said, “Surround VR roofing Underlayment can be used under non ventilated roof as the perm rating of the OSB or Plywood used is less than one. In many cases a ventilation system is recommended for the best results.”

I’m familiar with the history of this topic. I just wanted to know if anything changed since Martin’s 2015 response to a similar question, “… Most manufacturers of synthetic roofing underlayment do not allow their products to be installed on unvented roof assemblies… I suggest that you install good old-fashioned asphalt felt.”

Chris

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Chris,

    I will give your post a bump. What does the shingle manufacturer allow or require?

    By familiar, I'm assuming you know about the potential for damp sheathings (as detailed here: https://www.greenbuildingadvisor.com/article/open-cell-spray-foam-and-damp-roof-sheathing).

  2. 2screws | | #2

    Thanks Steve. Yes, I also received info from roofing contractor who said Certainteed shingles told him to use Epilay--- which is also nonpermeable like Typar!
    From my research, it seems that felt or reinforced felt (Shingle-mate) with perms of 5 or 6, or DeckArmor (16 perms) are the best choices.
    I posted the question to see if anything was new since 2015.

    (I'm also planning hvac supply air to attic and an Ultraire 120h whole house dehumidifier.)

  3. Steve Knapp CZ 3A Georgia | | #3

    It sounds like you have a good plan.

    My recollection was that asphalt shingles have limited permeability. To confirm, I searched GBA and found this post: https://www.greenbuildingadvisor.com/question/asphalt-shingles-gaf-deck-armor-astm-d-226-30-type-2-asphalt-felt-or-underlayment-permeance-really-doesnt-matter.

  4. 2screws | | #4

    Thanks Steve. The shingles are nearly vapor impermeable, but I'm assuming the reason for a vapor permeable underlayment is to give the sheathing at least some chance to dry to the outside?
    I also sent a message to Certainteed asking for their recommendations and to maintain warranty.
    Chris

    1. Expert Member
      Malcolm Taylor | | #5

      Chris,

      From what I can gather the answer to your question is also a bit climate dependant. Looking at test roofs in Vancouver, RDH concluded that the plywood sheathing did stay bit dryer with permeable underlayment, but in that wet climate that benefit was outweighed by the increased risks of bulk water intrusion through the shingles.

    2. Steve Knapp CZ 3A Georgia | | #7

      Hi Chris,

      I'm debating how to do the roof assembly for my next build (also CZ3A) and leaning toward using one to two inches of HFO closed cell in combination with cellulose or rock wool. If that gets too complicated or expensive, I'll probably go with open cell and focus on managing interior moisture to minimize the risk of damp sheathing.

  5. 2screws | | #6

    Followup from Shingle manufacturer recommended "...a standard 15 lb felt underlayment. It has a variable perm rating between 5 and 60. "

    Malcolm, I'm in CZ3a, hot humid summers and occasional snow in winter.

    Chris

  6. 2screws | | #8

    Steve, I thought about those options too but decided that closed cell foam would make any roof leaks harder to find. I really like Rockwool but with a truss built roof, contractors told me I wouldn't be able to hold the thickness of the batts between the smaller lumber trusses to achieve the needed R values under the roof deck. If the attic had wider lumber "stick built" rafters, then it could probably work.
    I haven't started building yet, so my experience so far is theoretical!
    I am trying though to do the right things to avoid a crawl space and have HVAC in a nice,energy efficient conditioned space.
    Chris

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