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Logistics of exterior roof insulation

Hari Kamboji | Posted in General Questions on

Dear GBA readers,
I’m starting the process of finding a roofer who can add up to 6 inches of insulation on top of the roof. I have an unvented cathedral ceiling in the attic, with rafters filled with mineral wool. I would like to get a clearer picture of what is involved before describing my needs to prospective contractors.

I’m in climate zone 5 and I’ve read Mr. Holladay’s post about performance of different types of foam in cold weather. I’m leaning toward EPS at this point. Or maybe 2″ polyiso and 3″ EPS on top. But I’ve also heard a bit about dense mineral wool board for exterior walls. Is there a similar product for roofs? (If so, would you recommend investigating this further?)

Also, I don’t really have a clear picture of exactly how this is done. From what I’ve read, the foam is laid in overlapping layers, taped, and nailed to the rafters with long nails. Then new sheathing is laid on top -presumably on furring strips so that the top roof is vented like an umbrella over the foam. Is it really structurally sound for this entire assembly to be held in place with nails passing through foam?

I haven’t found anything online that really spells out this process- if anyone has such a link it would be most appreciated. The attached photo from one of Holladay’s articles gives a very good idea- I certainly plan to show it to contractors. It doesn’t seem to have any space between the foam and the top layer of sheathing though. Maybe that’s not necessary?

Many thanks!
Hari

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Hari,
    The best place for you to start is probably to watch this GBA video:
    Exterior Insulation Retrofit — How to Install Foam On a Roof.

    If you have any follow-up questions after watching the video, feel free to post them here.

  2. Hari Kamboji | | #2

    Thanks a lot Martin- that 2 minute video was well worth the price of my free GBA Pro trial membership ;). Very helpful. The method in the video is not entirely consistent with the jpg image above. In the image one sees a wooden frame that holds the foam in place along the bottom edge.

    I'd still be interested to hear whether mineral wool board might be a viable alternative on the roof. Roxul makes a product called Comfortboard IS which seems to be marketed for exterior walls; not sure about on the roof.

    It still seems as if the whole roof assembly being held in place by screws through thick foam may be structurally unsound, though I have no experience with this kind of stuff.

  3. GBA Editor
    Martin Holladay | | #3

    Hari,
    Q. "I'd still be interested to hear whether mineral wool board might be a viable alternative on the roof."

    A. Mineral wool insulation is commonly used on the exterior side of roof sheathing, but these installations are usually on flat (low-slope) roofs. See the attached images for typical installations. If you are interested in using this approach on a steep roof, I would contact mineral wool manufacturers for advice.

    Q. "It still seems as if the whole roof assembly being held in place by screws through thick foam may be structurally unsound."

    A. You are mistaken. As I wrote in my Fine Homebuilding article on the topic, "The type of fasteners used and the way they need to be spaced for the top layer of sheathing depend on the pitch of the roof and roof loads, particularly wind and snow loads. It’s fairly easy to find screws with a pullout-resistance rating exceeding 400 lb., even when they are secured just to plywood or OSB sheathing. The fastener rating increases if the screws are driven into the rafters. (Fastener sources include Wind-lock and FastenMaster, which manufactures HeadLok and OlyLog screws.)"

    If you have further questions on engineering or structural details, consult an engineer.

    .

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