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Community and Q&A

To vent or not to vent insulated attic roof

Katherine Rasmussen | Posted in Green Building Techniques on

Currently engaged in an attic remodel where the roof will be the envelope.   Built a dormer on 1/4 of roof on north side with 2×10 joists where we plan to add closed cell foam.  The rest of roof where 2×6 construction original to the house remains, also adding closed cell.  To inc R value at the existing roof, planned to add rigid foam, 2 layers, staggered for air sealing , followed by semi permeable barrier, air gap approx. 1”,  furring strips, metal standing seam.   Running into issues with too high a profile relative to dormer.   Advised to make it an unvented roof, which I am very remiss to do.  Also told that in zone 6, need 1.5” airgap minimum, further getting me away from my end goal of lower profile insulated roof.   Appreciate any insight from GBA minded folk…

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Replies

  1. GBA Editor
    Brian Pontolilo | | #1

    Hi user-7576371. It would be great to know your name.

    Including exterior insulation in an insulated roof assembly is a good way to go, however, you would still need cavity insulation to hit appropriate R-values for your area, which means that you'd be building an unvented assembly either way. If you insulate only the cavities, the trick is to create enough rafter depth to hit appropriate R-values. Venting channels eat into that depth. You have lots of options still.

    I recommend you read this (How to Build an Insulated Cathedral Ceiling) and search the Q&A here to find some solutions that other GBA members have used. Then come back and ask specific questions.

  2. Katherine Rasmussen | | #2

    Thank you Brian. The venting I am referring to would be specifically for the standing seam metal....a cool roof vs a hot roof. Whether I need an air gap vs adding another inch of rigid foam to gain R value.

    1. GBA Editor
      Brian Pontolilo | | #5

      Hi Katherine.

      Thanks for clarifying. I thought you had determined to vent the metal roofing and were wondering about the height of the exterior rigid foam. My bad. Have you checked with the metal roofing manufacturer? Some are specific about these details.

      1. Katherine Rasmussen | | #6

        Roofer says that the metal will be warrantied with either application provided a synthetic barrier goes over insulation or the sheathing

  3. Jon R | | #3

    The window in the dormer is high - so I'm curious about what the dormer/insulation height issue is.

    1. Katherine Rasmussen | | #4

      Good point....behind that dog house dormer is a shed dormer which also has a window lower to the roof line, That is not visible in the pic

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