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What is the proper way to insulate a relatively flat roof system?

Normanm | Posted in Energy Efficiency and Durability on

This is a new two bedroom home to be built in northern NH, Climate 6A, that has been designed with a fairly flat roof system.

The roof structure is designed with an open-web, tapered truss, which spans 26ft. There is a difference height of 6″ from one end to the other. The exterior sheathing will be 3/4″ T&G advance which will then have 4″ of closed cell foam roofing product sprayed onto it and sealed with an applied silicone coating to resist the elements. The underside of the sheathing will then have 4″ of closed cell foam applied, as well as on the ends where the trusses meet the wall sheathing. The remainder of the cavity was to be filled with dense pack cellulose to come in contact with the drywall ceiling below.

I am looking to know if anyone has a thought to this design.


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  1. GBA Editor
    Martin Holladay | | #1

    Your R-values are high -- 8 inches of closed-cell foam plus an undisclosed amount of cellulose. High R-values are good.

    It's expensive, because it calls for a lot of spray foam.

    I don't like the idea of sandwiching your roof sheathing between two layers of spray foam, because it is unforgiving of roof leaks and hard to repair.

    You could save money by switching to 4 inches (or more) of polyisocyanurate on top of your roof sheathing, followed by your choice of roofing membrane. Once the polyiso is on the roof, you are free to fill your open-web roof trusses with dense-packed cellulose.

  2. Normanm | | #2

    I apologize for not mentioning the average depth of the truss which is 20".
    The foam and silicone that is going to be applied to the topside of the roof sheathing, is in fact the roofing membrane. With this being said, spraying our 4" foam membrane over lets say, 2" of polyiso. and eliminating the underside foam, then fill the whole truss with cellulose...... works for me. If I am understand you right, this eliminates the sandwich and puts more effisist on thermal bridging.

  3. user-659915 | | #3

    Norm: I've no experience with exterior sprayed-on roof finishes so forgive my ignorance, but I'd be concerned about two things: first would be UV degradation of the foam, presumably that's what the silicine seal is designed to prevent but I wonder about its longevity. Second would be that a sprayed insulation would have some degree of irregularity in its top surface which could lead to ponding at this very low slope of around 1/4" per foot. And I was always taught that ponding was a Very Bad Thing on a roof. Why not go with the much simpler, probably more durable and almost certainly less expensive solution as Martin suggests: just polyiso + membrane? The exterior spray foam sounds like a solution in search of a problem. You don't happen to own the company by any chance?

  4. GBA Editor
    Martin Holladay | | #4

    I knew what you were talking about. Like James, I think my proposed solution would be less expensive.

    If any GBA readers want to know more about using spray foam as a roofing material, here's an article I wrote on the topic:

  5. user-869687 | | #5


    It would also be better to increase the slope, even just an inch or so at the high side. In the long term there can be ponding when the slope is less than 1/4" per foot, because the structure may sag under snow loads or gradual creep due to age.

  6. user-659915 | | #6

    Thanks for the link, Martin. Now I know much more than I used to. The system makes more sense with a parapet wall I think than with an overhanging eave. I note the silicone needs recoating every ten years. Also that you can use the foam to increase the slope of the roof.

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