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Community and Q&A

Vapor Permeability for Metal Roof Assembly

georgehart | Posted in Green Building Techniques on

Hey all! I just bought a house in Buffalo, NY climate zone 5a. I have several projects on my radar but I am zeroing in on the first being a pitched roof (6/12) retrofit. I would like to do a full tear off down to sheathing, roll a peel and stick membrane over it (provided it is in good shape) to simplify my roof air sealing. Then go over it with rigid or semi rigid insulation, attach spacers and bug screen, then metal roof.

My question is, in this assembly, should I try to keep the whole assembly vapor permeable, including the membrane?

I haven’t lived in the house long so I don’t have a good idea of what my vapor situation looks like now. Any advice?
Thanks so much,

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  1. GBA Editor
    MIKE GUERTIN | | #1

    You didn't mention how the roof is framed and the inside is configured. Is there an attic? Is it vented? If so, how are you planning to detail the air seal (roof membrane) to the walls? What interior insulation (if any) are you planning? You mentioned exterior insulation but didn't note the R value . And are you installing insulation inside the roof rafters? If so, what type and R value.
    The answer to your question about vapor permeability of a peel-and-stick over the roof sheathing in part has to do with the other details.

    1. georgehart | | #2

      Hey Mike,
      In order:
      Attic: yes, that will become conditioned living space. house is a single story Cape that I know Martin Holliday just loves.
      Vented?: currently yes with unconditioned knee walls I hate because they are poorly air sealed. will become an unvented assembly with proposed peel and stick membrane.
      Wall air-sealing: membrane will meet the wall on the exterior wall sheathing. Another project is to air seal the exterior wall sheathing when I re-do the exterior. There the 2 air sealing products will meet, hopefully under an over hang, flashing, with the proper lighting, and a fine Italian dinner.
      Exterior insulation: enough to coordinate with the interior insulation value per code, hoping to hit R 50+ for the entire assembly.
      Interior insulation: Batt or blown in insulation into the 2x6 cavity, coordinated with the r value of the exterior insulation per code.

      Does the above help?

  2. GBA Editor
    MIKE GUERTIN | | #3

    Since you'll only get about an R20 on the inside with cavity insulation you'll end up with R30 on the exterior. That level of exterior insulation will eliminate the potential for condensation in the roof framing. So installing a vapor impermeable membrane on top of the roof sheathing should be fine. This assembly has been used successfully in deep-energy retrofits for many years and has been researched extensively in similar climates to yours. One important detail is to seal the roof membrane to the exterior air control - often sheathing or another peel-and-stick membrane. The rafter tails can be problematic and why many of DER practicioners cut off the tails to simplify the sealing. Then the sleepers/furring installed over the exterior insulation are extended beyond the wall to attach a new eave to.

    1. georgehart | | #4

      Thanks Mike appreciate the feedback!

    2. Expert Member
      MALCOLM TAYLOR | | #5

      Really nice to see Mike posting here. I've enjoyed his articles in FHB over the years.

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