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Community and Q&A

Installing Rigid Foam Over Roof Sheathing

alanferg | Posted in Energy Efficiency and Durability on

Does anyone have practical experience, tips, or suggestions for installing thick rigid foam insulation on a roof?

We’re about to close in our timberframe in climate zone 7 and my next step is ordering insulation. Our plan is three layers of 3″ GPS rigid foam, staggered, for a total of 9″. However, we do have the option of going as thick as 12″.

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  1. user-2310254 | | #1


    Tip. This article has some links for suppliers of long screws (

    1. alanferg | | #4

      Steve, the article you shared is quite useful, I've referred to it frequently over the past couple years while designing my plans. I'll be using GRK RSS 16" to fasten 2x4s to the timberframe through the 9" of rigid foam and roof deck.

  2. Expert Member
    Akos | | #2

    At that amount of insulation, it might be simpler to over-roof and insulate with batts.

    You can nail up 14" I-joists and insulate with to R45 batts. This would leave you enough gap above the batts for venting which is generally needed in heavy snow country to limit ice dams. The I-joist can be simply nailed up through their bottom flange into the timbering bellow, way less work than dealing with 12" or so timber screws.

    Timber frame generally means T&G ceiling.

    These are extremely leaky, all those gaps between boards can add up to a very big hole. The best is a peel and stick membrane over the whole deck. Make sure the peel and stick has either acrylic or buty adhesive as the modified bitumen (ie standard I&W) can react with sap in the wood an ooze.

    The T&G also allows for 3D airflow and can also leak sideways along the grooves, this has to be sealed up above any outside wall. The best is a flex caulk or spray foam injected between each board over the outside wall top plate.

    In zone 7 getting the place air tight is just as important for energy usage and comfort as insulation, I would say even more so.

    1. alanferg | | #3

      Akos, yes I'm very much focused on the air sealing aspect of this build. You're correct I'm installing T&G for ceiling and roof deck. The entire build will be air sealed and the roof will be vented to avoid ice dams. I hadn't considered I-joists, thanks for the suggestion, I'm going to look into it. Perhaps a combination of joists with batts, air-sealed sheathing, thinner layer of rigid foam (but enough to avoid condensation in the joists), ventilation channel, deck and roofing.

      1. Expert Member
        Akos | | #6

        Not sure where you would put rigid with the above assembly. There are no issues with a vented roof with fluffy insulation. The thin OSB web on I-joists is a pretty small thermal bridge, you get about the same performance as continuous rigid insulation.

        Since the batts will be installed from above, it is easy to make sure the vent gap is clear before the sheathing goes on, saves having to deal with any baffles. Most batts, especially high density ones, are not effected by wind washing.

        Bonus, you can also run ceiling wiring up there

  3. Expert Member
    Deleted | | #5


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