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Warm shingles the only effect of un-vented and no airgap?

mikeolder | Posted in General Questions on

I have a un-vented lap seamed metal roof.  It was re-roofed with osb and shingles, except the box vents installed were never functional..  While re-roofing, I plan to access the attic and install more fiberglass batts since there is no access from below.  I could also add gable vents.

I realize if left un-vented the attic and shingles would be warmer, but I live in zone 5 and concentrate more on reducing heating bills. The shingles left un-vented would be warmer and melt ice and snow, but the water would run straight into the gutter since I dont have cold eave overhangs to refreeze it..  Last reason for no air gap is, I only have 3″ space between top plate and roof deck, and I think reducing the existing R13 down to R7 just to create a air gap is counter productive in zone 5.

My question is, if I leave the space un-vented except for two gable vents at the ridge, what effect does insulation touching the bottom of the roof sheathing have?


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  1. Expert Member


    Can you clarify: You have an attic, but the insulation is not on the floor, it follows the roof line?

    1. mikeolder | | #2

      My attic is insulated with R13 batts laying between the trusses on the ceiling, but also touching the roof sheathing over the top plate. I want to fill the attic space more, and allow new insulation layers to touch the roof sheathing since it already does over the top plate, but want to know if there are any adverse effects doing this with no soffit vents.

      Ice dams and shortened shingle life?


      1. Expert Member
        MALCOLM TAYLOR | | #3


        It isn't ideal, but if the first couple of feet of sheathing are in contact with permeable insulation it usually drys fine to the surrounding space.

        If you haven't had any moisture problems up to now with just the two gable vents, more insulation probably won't make things any more risky.

  2. mikeolder | | #4

    Thanks Mr. Taylor.

  3. Expert Member
    Michael Maines | | #5

    Building Science Corp research shows that even in hot climates like Florida, venting the roof does little to reduce the temperature or lifespan of roof shingles.

    1. mikeolder | | #6

      Interesting.. Did BSC test ice dams with and without soffit vents? This would vary depending on how much R value was below the sheathing naturally.

      IMO 1/150 or even 1/300 is overkill and the main thing is to allow humidity levels to equalize.

      1. Expert Member
        Michael Maines | | #7

        I'm sure I've read their studies that show that venting reduces ice damming. Common sense and anecdotal evidence supports that view.

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