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Community and Q&A

High performance flat-ish pitched roof enclosure

Dylan Kinsey | Posted in Energy Efficiency and Durability on

Hey There Fellow Tradespeople!,

Wondering which GBA articles I should go after to study best practices for insulating and venting a 3/12 pitched roof deck, for a high performance enclosure. We’d like to avoid the rigid foam board approach and use dense packed cellulose, if possible. ( I’m imagining a netted open webbed floor truss as rafters, or a TJI rafter, perhaps? ) I refuse to use spray foam on this build.

Thanks Friends!

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  1. Expert Member
    Malcolm Taylor | | #1


    I clicked on this thread expecting another 1/12 proposed roof pitch, as they appear like locusts here on GBA. At 3/12 you are laughing. Either TJIs or parallel chord trusses work fine. Run baffles at the top chord and fill with cellulose. You don't need to dense-pack to the same extent you do with walls, as there will be much less chance of settling. Strap the bottom if you prefer a service cavity.

    1. Dylan Kinsey | | #2

      Hey Malcom,
      No 1/12 pitches here. Thanks for the advice. I'm thinking something similar to what you describe. We've learned that it's crucial to compartmentalize double wall and parallel chord type assemblies with Insulweb to ensure a manageable dense pack. Thanks a bunch!

  2. GBA Editor
    Brian Pontolilo | | #3

    Hi Dylan.

    Malcolm's strategy seems like the way to go if you want to avoid foam and use cellulose, which is a noble goal. In terms of articles, you could look at all of these for helpful info: How to Build an Insulated Cathedral Ceiling, Insulating Low-Slope Residential Roofs, and A Vent for Every Roof.

  3. Dylan Kinsey | | #4

    Hey Brian,

    Great! My goal here was to narrow my search to less than 10 articles, to get started, and you've helped with that. Never underestimate your fellow GBA folks. Thanks! : )

  4. Robert Swinburne | | #5

    We just did a very long (50') single pitch 2 1/2 in 12. We did mento over the insulation cavity then (2) 2x4's on the flat so we had a serious 3" vent space running the long way. then board sheathing and metal roofing. Hope we did good. It's the Sugar Bush House if you've been following that.

    1. Dylan Kinsey | | #6

      Hey Bob,
      You were on my list to see how you pulled that one off! Good method. I struggled with Mento as a temporary WRB on my roof, as it got soaked, but if the sequencing was quick, it wouldn't be exposed for long enough to be a problem during assembly.

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