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Community and Q&A

Unvented Roof Insulation Problem

stephenr | Posted in GBA Pro Help on


I have recently settled on an unvented roof for my 1000 Square foot, 2/12 shed roof, cathedral ceiling, zone 6, coastal Maine.

The roof will have taped zip sheathing, EPS outsulation, ice and water shield (or equivalent) and a standing seam metal roof on strapping.  The rafter span is 28ft with a mid span beam dividing the span in two.. 

I am hoping that the structural engineer specs 2×10’s for this span but with the snow loads, it could be 2×12’s.  Either way, my problem is the same.  I am using dense pack cellulose everywhere else in the building (suspended floor -along with a thin layer of rigid for air sealing- and double walls) and I want to fill my rafter bays with it as well.  According to the laws of this unvented roof assembly, however, I need to maintain at least 51% of my R value on the outside of my sheathing. So, if I used 9 inches of dense pack to fill the rafter bays, I would have an R of 34.  I would then have to create an R of 36 or more with my  EPS. This means 10 inches of EPS for a total R value of 70.

My goal is closer to R-60 and I am not sure that I can afford that much of an upgrade.  Is anyone aware of how I can dense pack my roof without filling the whole cavity?  Its looking like I might have to go with batts, and I am leaning towards fiberglass (although I dislike the stuff) since its cheaper than rockwool and the embodied carbon is about the same.

Thanks, Stephen

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  1. Expert Member


    That's a complicated roof. Thick foam, very long fasteners, dense-pack. Have you thought of how you will handle the deep fascias? From your last posting I thought you were moving towards a simpler vented solution using trusses.

    If you are going with exterior insulation I don't think there is much advantage in using cellulose over batts.

  2. stephenr | | #2

    Hi Malcolm,

    I priced out the trusses and they are prohibitive. The unvented roof seems safer with my roof pitch and climate and since I won't have the benefit of deep trusses, i figure its better.. I plan on using zip sheathing all the way around as my air barrier. I will build it without overhangs at first, tape and seal it, and then prebuild my overhangs and install them. If I go with batts, I could install r-29 between the bays, or possibly less. I would then be looking at just over 8 inches of EPS, so 10 inch screws. Since I will have 8 inches above my sheathing, my prebuilds will seek to minimize my fascia depth, possibly with 2x4's. I would then put an 8inch 2x on edge on the edge of my roof, framing out the rigid, and gusset the edge board to my prebuilt overhangs, which would be my subfascia. I don't like working overhead and my pitch will be easy to stand on, so the assembly seems reasonable to me. Appreciate the feedback.

    1. Expert Member
      MALCOLM TAYLOR | | #3


      It sounds like you have worked out the tricky bits.
      Good luck with your build!

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