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Community and Q&A

Do I need a vapor barrier in an unvented roof assembly?

walterbutton | Posted in Energy Efficiency and Durability on

I am building a roof assembly with 2×10 rough hemlock rafters exposed on the inside with cathedral type ceilings and no interior insulation. Rafters are covered with 3/4″ pine shiplap decking, then 5 1/2″ of polyiso insulation (2 layers of 2 3/4″, joints sealed and overlapped), then 1×3 strapping, then metal roofing panels. I’m wondering if I need to include a vapor barrier anywhere in this assembly, and if I should put tar paper on top of the polyiso and underneath the strapping? The polyiso is paper faced. I am in upstate ny.. thanks for any advice!

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  1. GBA Editor
    Martin Holladay | | #1

    Your 5 1/2 inches of polyiso is already a vapor barrier, so you don't need another one.

    What you need is an air barrier. You can either use the polyiso as an air barrier (assuming that the facing on the polyiso is easy to tape), or you can install a separate air barrier under the polyiso (for example, roofing underlayment, housewrap, or one of the European membranes sold by Small Planet Workshop and 475). Pay attention to air sealing any penetrations (like plumbing vent pipes).

    On top of the polyiso, you will need to install a layer of roofing underlayment (asphalt felt or a synthetic underlayment).

  2. LucyF | | #2

    I think this is one of those easy now/hellacious later problems. Easy to put a layer down under the polyiso now (and inexpensive in the scheme of things), seriously difficult to fix later should there be a problem down the road.

  3. user-1109130 | | #3

    Following martin's advice, the trickiest part of the air barrier in this type of structure is typically at the exterior walls. Assuming you want overhangs (who wouldn't), the common approach is to extend the exposed rafters out to form an overhang with the decking continuing to create a finished soffit. Creating a continuous air barrier from wall to roof deck is difficult if the air-barrier is above the decking, plus you have some good thermal bridging. In our part of the country (PNW), plywood sheathing is usually required on top of the decking as the decking doesn't provide adequate shear values.

  4. walterbutton | | #4

    thanks for all the comments. i got an air barrier to put down over the decking. we aren't required to use plywood sheathing, so hopefully the 1x12 shiplap will provide enough shear protection. another question with this assembly is, what is the best way to attach the tar paper to the polyiso? additionally, someone suggested that in this application under metal roofing, it might get too hot for tar paper and it would get brittle and worthless. they suggested we use snow and ice shield instead, but i'd much prefer to use the tar paper as it is about 1/6th (or less!) the cost. do people think tar paper would be adequate here?

  5. GBA Editor
    Martin Holladay | | #5

    Asphalt felt will work under metal roofing; #30 felt is heavier than #15 felt, which is why I prefer to use #30.

    The asphalt felt can be secured with cap nails, which are available is a variety of lengths up to 8 inches.

  6. user-1061844 | | #6

    As Martin mentions you can useProClima membrane under your Poly-iso that we at 475 supply and support in the USA. We can get it to you in a day, and recommend DA or INTESANA under your insulation and SOLITEX MENTO above it as an upgrade to tarpaper (won't embrittle over time). You can then really this a very durable roof underlayment, when only stapling in the overlaps and taping all the seams w TESCON Vana, plus using nail/screw sealing tape (TESCON Naidec under your battens for additional protection.
    When you want to connect the airtight layer or roof underlayment to your walls, you can do this as shown in the photo by clioma house and cut+seal the SOLITEX membrane to the rafters,for continuous weather seal on the exterior (you can do the same w INTESANA/DA for a complete airbarrier from roof to wall).

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