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Insulating dormer

mfleck | Posted in General Questions on

Hi all,

im nearing the end of my attic cathedral ceiling insulation job. Nearly all of the attic is straight shots that I’ve been able to vent by creating an air gap with cut and cobbled rigid foam. What’s left is a small dormer with no ridge vent and soffits that are blocked off, but I’m pretty sure air is still finding its way in there. 

My thinking based on what I’ve read here is to seal off the bottom of the rafter bays by spray foaming in around a 1 inch rectangle of rigid foam. This would hopefully prevent any air from entering from the soffit (which is blocked off, but I’m pretty sure there is still air getting through there). I have some 2-inch felt faced polyiso left over so I’ll cut and cobble these into the rafter bays (2x4s). Then I’ll screw on a final larger sheet of the same 2-inch polyiso to the underside of the dormer rafters. 

is this an ok approach?

should I foam in the cut and cobbled foam with an air gap between it and the underside of the roof sheathing?  There would theoretically be no way for air to enter or leave this space, but might be helpful anyway.  Alternatively I could also try to pack another 1-inch sheet of foam in there as I have some leftovers of this too. 


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  1. Expert Member
    Dana Dorsett | | #1

    Leave an air gap, and make vent openings at the soffit ends. Even though there won't be convective flow it will have at least SOME drying to the outdoors (by diffusion, if not by convection.)

    For rafter bays with no soffits, drilling some holes in the rafters in the vent space area will allow those blind rafter bays to still dry into the partially vented rafter bays.

  2. mfleck | | #2

    Thanks, Dana

    I’ll open up the soffits and drill some 1-inch holes in the rafters within the vented space. Drilling holes in each of the rafters would connect them to the two bays closest to the house that are vented at the ridge and the soffit. It would be a long way for air to travel, but it can’t hurt. Thanks for the suggestion.

    Since these rafters are 2x4, I was thinking of using the fiber-faced polyiso that I have left over to make the venting chutes. I also have 1 inch XP’s that I could use to make the chutes, but I thought the 2inch polyiso would be better as it’s thicker and would fill more of the rafter bay. Would you use the 2-inch fiber-faced polyiso, or the 1 inch XPS?

    1. Expert Member
      Dana Dorsett | | #3

      The 2" polyiso will thermally outperform stacked 1" XPS over time, not that the difference would be dramatic.

      Strips of 1" XPS tacked to the roof deck adjacent to the rafters as spacers would deliver the code min spacing. If those are full-dimension 2x4s that would leave just enough space to cut'n'cobble layer of 1" too. But clearly it's the continuous layer of 2" on the underside that will be delivering the lion's share of the whole-assembly R.

  3. Jon_R | | #4

    How much convection/diffusion you need to remove moisture depends mostly on how much moisture you allow to enter via convection. So air seal the interior side really well and then test it. Also consider monitoring deck moisture in the minimally vented area so you can address any issue before it causes problems.

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