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Sealing knee wall and top plate – checking my technique

MichiganHouse | Posted in General Questions on

I’m sealing attic knee walls in the attic over my garage. I’ve completed sealing most of the wall space, including the floor plate area, but want a second opinion on how I’m doing. Have I sealed up around the framing in the right way? 

I also want some thoughts on how, if at all, I should go about sealing the top plate. This portion of the attic has soffit vents, but does not have exhaust vents (except a single small gable vent). Rather, it seems the air has to travel up through the fiberglass in the top plate and then up to the turtle vents near the peak of the roof. The fiberglass in that area is dark, which seems to confirm this. I believe the right answer is to insert some kind of baffles into these areas and then to use foam board beneath them, but the space in there is so confined and is punctuated with roofing nails, so I don’t see how this would be possible without removing my roof or the interior drywall . Any other solutions available? Or perhaps I’m better off just leaving those as is. 

See photos attached. Thanks for all the help

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    Looks OK to me, but you should ideally be sealing the gap around the perimeter of the foam with canned foam.

    Note that I see batts stuffed between rafters going to the upper attic. You need to allow vent space there so that air can move from the lower attic to the upper attic, where I assume you have a ridge vent. My own home was the exact same way, and I pulled all the stuffed batts out (which were used to keep the blown cellulose in place during installation), then put in site built vent baffles made from 1/2" polyiso up from the edge.

    Bill

  2. Expert Member
    BILL WICHERS | | #2

    Looks OK to me, but you should ideally be sealing the gap around the perimeter of the foam with canned foam.

    Note that I see batts stuffed between rafters going to the upper attic. You need to allow vent space there so that air can move from the lower attic to the upper attic, where I assume you have a ridge vent. My own home was the exact same way, and I pulled all the stuffed batts out (which were used to keep the blown cellulose in place during installation), then put in site built vent baffles made from 1/2" polyiso up from the edge. Try to keep the top plate insulated on top when you pull the batts out, you don't want to leave the framing exposed up there.

    Bill

    1. MichiganHouse | | #4

      Thanks Bill. To be clear, I've not actually used the foam gun to seal around the foam boards yet. First because I wanted confirmation from people here that I wasn't screwing up my install, and second because it has been much too cold (currently 4 degrees) to apply the foam. I'm going to wait for a day around 30 degrees and hope that the interior temp in my attic might be a bit warmer than that thanks to the sun, as I understand Handifoam can cure properly over 40 degrees.

      Using a thermal camera I've seen almost no difference in temps so far despite the application of foam board. Where the wall meets the floor is still coming in around 37 degrees. See attached. Is that to be anticipated given the significant air gaps around the foam boards degrading the R value of the insulation, or is that indicative of the cold infiltrating from a different area?

      The fiberglass batts in the top plate are original to the 1995-era house. I handled some plastic baffles at the store today and found they were extremely flimsy - much more flexible than even a plastic takeout container. For that reason I can't imagine how I'd go about securing them into the bays now. How did you go about retrofitting those into the cathedral ceiling are (if that's the right term for it) in your home?

      1. Expert Member
        BILL WICHERS | | #6

        The air leaks hurt you here, but you'll also have some thermal bridging from the framing in those peripheral areas. That hot (cold) spot in your pic looks like an electrical box to me, so the batt insulation will be thinner in that area. You could try putting some rigid foam behind the box to help mitigate that if you want.

        Bill

  3. Expert Member
    Akos | | #3

    Can you do a pencil sketch of what you are trying to seal up? Looks like truss room, but not seeing much sealing. Lot of batts in places which don't do any sealing.

    1. MichiganHouse | | #5

      Glad to try, see below. I am not an artist. The black is wood framing, red is insulation, and the blue is where I've put foam board.

      I've also included a couple of photos that, when looked at together, help to make some sense of the situation. It's very tight quarters so tough to get a good photo.

      Also to be clear I've not sprayed foam around the foam board yet, both because it's too cold (4 degrees) but also because I don't want to seal things up before getting confirmation I've done the work the right way.

      Thanks

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