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Exterior roof insulation overhang?

shedworkshop | Posted in General Questions on

Does continuous exterior insulation on a roof need to extend into the overhang spaces, or can those be blocked off? (saving on insulation costs / sq footage).

I’ve found diagrams specifying both ways (see attached images below).

This is for a small 8’x12′ shed roof with 1′ overhangs on all sides. With overhanging insulation I need 150 sq ft of insulation. Without the overhangs I only need 104 sq ft.

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Replies

  1. Expert Member
    Michael Maines | | #1

    Research by Building Science Corp shows that in areas that get snow, sun can hit the exterior wall, heat up and rise into the soffit where it melts snow and contributes to ice damming; insulating the soffit, including by the methods you're sharing, limits or prevents this from happening. With continuous exterior roof insulation, it's usually easier to simply extend the insulation to the roof's edge. But with a shed it's probably not necessary unless you have concerns about ice dams.

    1. shedworkshop | | #4

      Thanks Michael! Not a whole lot of snow here, but it does happen. It sounds like extending the exterior insulation is the safer bet.

  2. Expert Member
    MALCOLM TAYLOR | | #2

    shedworkshop,

    If this is for the un-insulated shed you discussed in your recent posts, and the insulation is there to keep the sheathing warmer, then yes it needs to be extended over the whole surface.

    If the foam is there as part of the insulation for your conditioned shed/workshop, then it only needs to extend to the outside walls.

    If

    1. shedworkshop | | #3

      Thanks Malcolm! It's for the conditioned shed/workshop with all insulation on the exterior of the building. The design is slowly evolving over time :) Hoping to submit a full list for final critique (part II) soon.

      It sounds like extending it over the whole surface is the way to go, just in case of ice dams. Should add ~$200 to the cost since I'm using rockwool now.

      1. Expert Member
        MALCOLM TAYLOR | | #5

        shedworkshop,

        I have an un-insulated tool shed, and a heated she-shed. The she-shed is insulated with exterior foam. I didn't extend the insulation past the exterior walls, and recently have seen some grey mold on the underside of the soffits, which may be related to that. Extending the insulation certainly does no harm.

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