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Community and Q&A

Shed roof insulation strategy

kwig | Posted in General Questions on

Hi all, I’m building in Maine(zone 6) directly on the coast. The roof is an unvented 3 pitch shed roof. It was my intention to have my builder insulate the top of the roof deck with rigid board before the metal roof went on and additional insulation in the roof assembly to the inside. After a conversation months ago I thought we were on the same page. What has been done is, ice and water directly on the Zip sheathing and metal on top of that. After speaking to him about it, he wants to spray a heavy layer of closed cell on the underside of the sheathing and fill the rest of the rafter with probably fiberglass before a layer of strapping and then blueboard and plaster. I feel like i’ve lost an opportunity to have a great roof insulation job. I’ve done a little reading on this but i’m getting lost in some of the details. One concern i found was, too thick a layer of foam could be problematic, among other things… My hope was to have a continuous insulation layer on the outside so i’m wondering if what he’s suggesting makes sense and would it be wise to add an additional layer of continous rockwool to the bottom of the rafters before strapping? thanks in advance, K-Wig

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  1. Expert Member
    Michael Maines | | #1

    It is unfortunate to lose the opportunity for continuous insulation but I'd say your best bet at this point, if you really want an unvented assembly, is a flash-and-fill system as your contractor proposes. There is not a performance or durability concern with additional foam, only an environmental impact concern. At least 50% of the total R-value needs to be in the foam layer, in Maine's climate zone 6, regardless of the total R-value of the assembly. You could add a layer of continuous insulation at the interior as you suggest, as long as it's vapor-open, so mineral wool would work.

  2. plumb_bob | | #2

    Thermal bridging with this assembly is more of an issue if solid lumber rafters are used, and less of an issue if Ijoist rafters or engineered trusses are used.

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