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Bad Flash and Bat job in skylight shaft

patrick1 | Posted in General Questions on

I’m planning an attic air sealing and insulation replacement job for my 1984 house that we bought 3 years ago.  While I was up there planning the job out, I also had a look at how the skylight shaft is insulated.

At some point, someone did an “upgrade” through a flash and bat with the “flash” (closed cell foam) against the interior drywall and bats exposed to the attic.   Since the batts are exposed on one side, I would assume they are not getting anywhere near their rated r-value right?

I would like to upgrade the insulation and I was hoping to follow the detail drawing here:

But because of the SPF against the interior drywall, I would end up with double vapor retarder (SPF against the  drywall and rigid foam on the exterior), which I imagine is a risky assembly for moisture.   Can the spray foam be removed, or am I likely to just tear the paper facing off the interior drywall?  

If I don’t want to remove it, what’s my next best option to get to say R40?  It’s hard to tell because they foamed all over the framing but I think the shaft was originally framed with 2×4’s  on the flat and then they furred out with 2x6s oriented the usual way to retain the batts.

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  1. Expert Member


    I'm not sure I'd throw the baby out with the bathwater. The existing spray foam provides both good air-sealing and some reduction of thermal bridging. The reduction in R-value of the batt insulation may not be as great as you think (see tables 8 and 9 in this. link)

    You can easily make up for it by using thicker batts, or further reduce it by either switching to mineral wool, or wrapping the shaft in house-wrap.

    1. patrick1 | | #2

      Thanks for the pointer, I wasn't aware that Mineral wool was so much less susceptible to wind washing. Still there's an important caveat to the results that might matter for my particular situation:

      "It is assumed that openings are relatively small and that
      eddies within the wind are not carried into the cavity and the
      momentum of the wind is not impacting the material directly.
      Hypothetically, for large openings (such as slots larger than
      50 mm [2 in.]) these phenomena could have significant scouring effects on fibrous insulations, increasing heat loss. These
      cases should be explored through further analysis and field

      So, as you suggest it is probably prudent to cover the back of the insulation (attic side) with some kind of vapor permeable air barrier.

      1. Expert Member
        MALCOLM TAYLOR | | #3


        Something like taped Tyvek would probably be the easiest.

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