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Community and Q&A

Flat roof insulation

scottyw | Posted in Energy Efficiency and Durability on

I am about to tear off an old tar & gravel roof as well as the existing sheathing. I know that some of the old fiberglass insulation was water damaged and expect that to be true of more rather than less. The plan is to add a modest 1/4″ per foot slope (that’s all the room we have). I will insulate between the 2×8 16″ on center rafters, deck with 3/4″ ply, add polyiso rigid, another layer of 1/2″ ply, and finally we will lay down a mod bit roof. Over the top will be a modest roof deck. All of this is in a zone 4 region.
Here are my questions:
Shall I lay faced fiberglass insulation into the rafter bays, or unfaced? And should I dense pack it with a 9″ thick R30 product or go with R19 which is made for 2×8 framing?
Shall I add 3″ of polyiso above the 1st layer of decking, or 4″?
And finally I understand that I should install a vapor retarding layer directly above the 3/4″ decking but it’s not clear to me what product would be suitable here. A suggestion or two would be much appreciated.

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  1. Expert Member
    ARMANDO COBO | | #1

    I would use 3" R20 min. taped polyiso (4" R26 better) above the 3/4" decking, and roof with a 60 mil. TPO. No mod bit, no second plwd layer, and no need for vapor retardant other than drywall and paint. Under the roof decking you need R30 min. to meet CZ4 requirement of R49.

  2. GBA Editor
    Martin Holladay | | #2

    In your climate zone, you need to install a minimum of R-15 of rigid foam above the roof sheathing for this type of roof assembly to work. Although you have to de-rate the performance of polyiso somewhat for cold temperatures, I still think that you would be fine with 3 inches of polyiso. If you want to install 4 inches of polyiso, so much the better.

    Here are links to two relevant articles:

    Insulating Low-Slope Residential Roofs

    How to Install Rigid Foam On Top of Roof Sheathing

    Q. "Shall I lay faced fiberglass insulation into the rafter bays, or unfaced?"

    A. If you choose to install fiberglass insulation, either product will work. The kraft facing is unnecessary but harmless.

    Q. "And should I dense-pack it with a 9-inch thick R-30 product or go with R-19 which is made for 2x8 framing?"

    A. If you compress a fiberglass batt, you decrease the R-value of the batt but you increase the R-value per inch. It's fine to compress a 9-inch-thick batt into a 7.25-inch-deep cavity if you want to do it. (By the way, this isn't "dense packing" -- it's "compressing a batt.")

    Q. "I understand that I should install a vapor retarding layer directly above the 3/4-inch decking but it's not clear to me what product would be suitable here."

    A. You are operating under a misapprehension. What you need directly about the lower layer of roof sheathing is an air barrier, not a vapor retarder. (If you are worried about vapor diffusion, rest assured that your 3 inches of rigid foam are already a vapor retarder. For that matter, so is plywood, and so is OSB.) For more information on the need for an air barrier at the sheathing level, see How to Install Rigid Foam On Top of Roof Sheathing.

    To create an air barrier, you can (a) tape the sheathing seams with a high-quality tape, or (b) install a rubberized asphalt membrane like Ice & Water Shield, or (c) install a European air barrier membrane like Solitex Mento, or (d) tape the seams of the polyiso and use the polyiso as your air barrier.

  3. KeithH | | #3

    Hi I'm a diyer and a fan of roxul ...

    Disclaimer above aside:
    Why would choose fiberglass instead of roxul mineral wool batts?? You can order them from either box store pro desk for maybe 20% more than fiberglass

    They make a thickness (2x8, actual 7.25"~) that will provide R-30 without all this squish optimization.

    Roxul is much more moisture and mold resistant than fiberglass. IMO fiberglass loses loft when it gets wet and doesn't regain it. In my experience, Roxul holds up very well.

    And, assuming you aren't contact irritated by the batts (I'm not, a friend of mine is) they are a pleasure to work with compared to fiberglass.

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