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Community and Q&A

Roof Membrane/Underlayment Recommendations

Stolzberg | Posted in General Questions on

Hi, we’re going to turn our uninsulated attic into conditioned space.  It’s a 1900 era farmhouse in northwest Massachusetts with board sheathing over real 2×6 rafters.  12:12 pitch.  Current plan is to remove existing slate (it’s in pretty rough shape) and put down peel and stick membrane as air barrier, then 1.5″ polyiso insulation between horizontal 2x4s, another layer of 2.5″ polyiso, then another layer of 3″ foil-faced insulation.  (Total 7″ of rigid insulation outside sheathing.) This will be covered by a layer of plywood sheathing screwed through to the 2x4s, then underlayment and finally standing seam metal roof.  We’re also planning to put batt insulation between rafters and cover with shiplap or bead board.

I’m looking for recommendations for 1) the peel and stick membrane to go over the board sheathing and 2) for the underlayment to go under the metal roof.  Do we need a high heat underlayment under the metal roof (planning to use plain galvalume, which should be pretty reflective)?  Should the underlayment be permeable? Is stick down better for the underlayment?

I was planning to run 3′ of ice and water around bottom of roof over plywood sheathing and in dormer valleys, then run underlayment over that.  Is that a good idea?

Any advice much appreciated.

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  1. Expert Member
    Akos | | #1

    Not sure if 7" of rigid+~R22 of batts is worth it in your climate. The overall R value is impressive but the ROI is probably measured in centuries.

    I would stick to only enough rigid insulation for condensation control, take a look at Table 1 here:

    For the board roofing you want a peel and stick with either acrylic or butyl adhesive. The one you want to avoid is SBS ashpalt as it can react with sap and ooze.

    The underlayment under the metal roof can be any of the many synthetic roofing underlayments. I prefer to use a permeable one (ie Deckarmour or BreathX) as it is minimal extra cost and does allow for a bit of drying of any trapped water. The underlayment does not need to be self adhered. A strip of high temperature peel and stick (ie Grace HT) is a good idea around the roof valley.

    Unpainted galvalume does not hold up all that well. You are better off with one that is pained to look shiny if that is what you are looking for.

    P.S. For your attic ceiling, you still need a decent warm side air barrier. The simplest is either OSB or drywall but can also be one of the variable perm membranes.

    1. Stolzberg | | #2

      Thanks very much for your feedback. I've already got a lot of recycled insulation for this project and figured 7" of recycled rigid plus batts would get me well over R49. Why do you suggest a warm side air barrier? I was assuming any moisture that gets into batt insulation between rafters could dry to inside and 7" of rigid insulation above sheathing will prevent condensation on interior of sheathing. I'm planning to spray foam around perimeter where ceiling meets floor to air seal top plate and connect walls to air barrier on top of roof sheathing.

      1. Expert Member
        Akos | | #3

        The warm side air barrier is still needed for the hybrid assembly I linked to earlier.

        With 7" of exterior iso, you might be fine. If I remember correctly you need somewhere around 1.5x interior insulation R value on the exterior to skip interior barriers.

        If your board sheathing is in good shape, with the amount of iso you are putting above the deck, you can also skip the interior fluffy. For interior finish you can paint/stain the boards and rafters and skip any wood paneling. I did this for my wife's studio (mostly to save time/cost) and it turned out pretty good. As a bonus the extra rafter space also makes the space feel bigger. You do have to take a bit more care with running wires as they are now visible. MC/AC/EMT pained tends to blend pretty well.

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