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Community and Q&A

Insulation on a 2/12 pitch

Robertsoave | Posted in Green Building Techniques on

Hi all I do love this site it has so much information on it
I am in the planning stages of a new home hope my last stop before I leave earth
I is a mono pitch it will span 32 ft at its widest part I will be using 24in parallel trusses we are in lower michigan north of Detroit.
My thinking is 18in of fiberglass it will leave me with 6 in of air on top and a vent running all the way along the soffit and Ridge
Will this work?
I was thinking if I do need more air in the middle I can put a small step in the roof to grab more like 2 in or so.
Doing that I can also use shorter steel roof easer for me to handle I will also have 3ft overhang s
Also what would be the best way to seal the celing ?
Thanks in advance

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, I advise you to read this article: Insulating Low-Slope Residential Roofs.

    A few reactions to your plan:

    1. Your plan won't work unless you include one or more "doghouse" vents (ventilated cupolas) in the center of your roof.

    2. Cellulose insulation will perform much better than fiberglass in this application.

    3. The best air barrier for your ceiling is drywall. You should minimize the number of electrical boxes in your ceiling; you should specify airtight electrical boxes; and you should seal any penetrations in your ceiling air barrier.

  2. Robertsoave | | #2

    Thank you
    If I were to put this step in the roof the entire length half way up would it take the place of the cupola
    2in by 30ft long

  3. bfrench1 | | #3

    Hi Bob,
    I am hoping to start a house this spring with a similar roof assembly. Wondering what you decided and if you have any other thoughts. Thanks!

  4. Expert Member


    How risky your assembly is depends in part on your climate, and the site conditions (south facing, treed lot, etc.). You can go some way to alleviating the risk by maximizing the vented area at the eaves and peak, and leaving a deep ventilation gap between the insulation and sheathing above. That said, you are still using an assembly at the margins of it's abilities to perform well over time.

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