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A-Frame re-roof – what type of (if any) air barrier?

BIll Berzins | Posted in General Questions on

I have an A-Frame (steep pitch cathedral roof) built in 1975. It is an unvented roof. Region 5, southern NH. It needs new shingles. The existing roof from inside out is:
5″ x 14″ laminated rafters 14’ OC
2-1/2″ thick T&G decking
2″ rigid insulation
Asphalt shingles nailed through insulation to deck

Removing the old shingles will harm the existing insulation, and I can find no existing air barrier. The roof is leaking.

The proposal under consideration is:
Remove existing shingles and insulation
Install air barrier to deck
Two layers of rigid insulation (polyiso), stagger and tape all seams
1/2” OSB (nail base), screwed through insulation to deck
Roofing felt of choice
Asphalt (or metal) shingles nailed to OSB

The new insulation will yield about R-26. Low for my region, but 6” of rigid insulation (R39) on top of the deck is problematic. Deck is a strong visible detail from inside the house (no insulation to be added under the deck).

My question is, what should the air barrier be? It has been suggested to use either polyethylene or peel and stick ice shield over the entire deck. There is also an argument that I would not need one at all.

Is the proposal in general sound? Another proposal is to use nail base SIPs and convert to vented. I do not think that is desirable.

Thanks for any guidance.

Bill

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Bill,
    All insulated roof assemblies need an air barrier.

    If I were you, I would establish an air barrier at the top layer (the OSB). The best type of OSB in this application would be Zip sheathing, and the seams should be sealed with Zip System tape.

    Your two layers of polyiso with taped seams will also help to greatly limit air leakage. However, the OSB is a more dependable air barrier, because it is more dimensionally stable than polyiso.

    Remember to pay attention to air sealing details at the perimeter of the roof, where the roof assembly's air barrier meets the air barrier for your wall assembly.

  2. Keith Gustafson | | #2

    On my similar but low slope roof I found you could see light between every t+g board, not very air tight. I drilled a 1/4 inch hole at the junction of every board over the exterior wall, down to the wall, and squirted canned spray foam into it.

    If the reason for less insulation is not money, I do not see a technical problem. A pressure treated 'shoe' at the bottom of the roof[almost wall] will keep it from drifting south and they make the screws as long as you please

  3. BIll Berzins | | #3

    Interesting. I was not expecting that for an answer. I may be confusing terms for each other (vapor / air barrier). I was thinking that the air barrier should be on the warm side of the insulation. Looks like I have a lot more studying to do. Can you point me to some suitable reading material? I like to understand these things, if I can.

    For the most part, the roof does not directly meet up with the walls. I have 3 feet of overhang on all sides. I have no reason to believe that the walls have an air barrier.

    Thank you for the guidance.
    Bill

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Bill,
    If you are insulating with rigid polyiso, it doesn't really matter whether your air barrier is on the interior side of the insulation or the exterior side of the insulation. There is a slight advantage to installing an air barrier on the interior side: to prevent convective air currents in the (tiny) gaps between the polyiso sheets. However, since you are planning to install two layers of polyiso with staggered seams and taped joints, that issue shouldn't arise.

    Here are some articles to read:

    Questions and Answers About Air Barriers

    Airtight Wall and Roof Sheathing

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