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Community and Q&A

Sealing Walls Open in the Attic

jabraben | Posted in General Questions on

I live in north central Ohio in a house build in 1900 and the interior walls are open to the attic, though blown in insulation lays on top of the wall openings, by no means sealing the tops. Consequently, a lot of cold air makes it into the second floor walls in the winter. An energy audit camera revealed cold air in those walls, which is why I thought to look at the tops of the walls in the first place. What should I use to seal the walls in the attic?

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  1. Expert Member
    BILL WICHERS | | #1

    When you say the walls are "open", do you mean with penetrations like holes for wires and pipes, or is the entire stud cavity open, because of a lack of top plate? Usually the top plate would fill the bulk of the opening, so air sealing mainly involves leaks from holes for stuff passing through. I have seen walls that are not even with the attic floor though, so the top plate is not flush with the attic floor, leaving portions of the stud bay open. This is an issue for fire spread, so you normally seal these off with plywood (or OSB), or pieces of 2x4 depending on how the structure is in the areas you're concerned with. Once you've done that, air sealing can be done the usual way with caulk and canned foam.

    Do you have any pics or a drawing that can clarify things so that we can offer better advice?


  2. jabraben | | #2

    I'll go up tomorrow and examine along a stretch of wall. I presumed what I saw represented how all the interior walls are done but I haven't confirmed that. The section where I pulled the insulation back, I saw a horizontal board maybe 6" above the opening with gaps on either side of it. Are you suggesting that I might find that the gap between the opening and the horizontal board might be closed up on the sides in other places and that what I found might only be here and there, maybe for access?

  3. jabraben | | #3

    I was mistaken. The board is not directly above the wall cavity. This is what it looks like. It's completely open except that they put some kind of paper over it, held down only by the weight of the loose insulation.

  4. Expert Member
    PETER Engle | | #4

    That type of wall is somewhat common with old houses and yes, it allows tons of cold attic air down into the walls. Diligent air sealing of the top of the walls is in order. Building performance contractors have lots of tools in the kit for this sort of work. In short, anything airtight can be used. If you want to boost fire safety (and you should), then sheet metal or 3/4" plywood would be best, sealed around the perimeter with caulk, adhesive, or spray foam depending on the size and shape of the gaps. Once the walls are sealed, bury the tops in insulation.

  5. Expert Member
    BILL WICHERS | | #5

    That looks like the back side of a wet plaster wall. It may be difficult to seal that up with wood fire blocking due to the irregular surface, but you could stuff in mineral wool batts instead. Mineral wool can also act as a fire stop, but you CANNOT use fiberglass batts in this case, only mineral wool. You need to stuff in enough to solidly "seal" the gap, which probably will end up being close to a foot or so of batt stuffed into the top of the wall cavity. Note that when I say "stuffed in", I mean packed tightly. You can't just use a regular size batt and work it into the wall. I would get whichever mineral wool batts were the cheapest, then rip out chunks and stuff those into the top of the wall until I had a well packed layer to act as a fire stop. Remember that the mineral wool is acting as a fire stop here, not thermal insulation. Mineral wool is commonly used for this purpose commercially with larger penetrations, usually with fire putty pads over the top to seal it off.

    I'd spray foam over the top of the batt-stuffed cavity for an air seal after that, applying around 1" or so of spray foam to act as an air seal. Once that's done, you can blow in more loose fill insulation in the normal way to get up to whatever your target R value is (typically R49 as a minimum).


    1. jabraben | | #6

      I've done a quick drawing that I hope will help. The point of view here is looking horizontally at the wall from the end of the wall. You mention having to contend with the irregular surface of the plaster. I was thinking that I would instead place a board on top of the wall, covering the gap and laying on top of the boards that sit on top of the plaster and lath on both sides of the wall cavity. You can see one such board in the photograph I attached previously, just on top of the uppermost layer of plaster and below the insulation. Does this make sense? Would this not work?

      1. Expert Member
        BILL WICHERS | | #7

        If you don't have lots of obstructions in the way, I see no reason your retrofit top plate idea wouldn't work. My thinking was just in terms of what would be easiest to install in the typical miserable working conditions in an attic. The advantage to mineral wool and spray foam is that you can stuff it around obstructions and not have to cut anything -- nothing has to be precision fit anywhere. With a board, you need to have a flat surface between the old and new. I would overlap 1" or more on either side of the gap, lay down a bead of sealant, then nail the new board over the top. If you have a relatively unobstructed area over the top of that wall, putting the board in might actually be easier. I'd use 1x stock for this, or ripped pieces of 3/4" plywood.


        1. jabraben | | #8

          Yeah, it's going to be a slog, either way but your recommendation would save me a lot of time and aggravation. Thanks for the advice.

        2. Expert Member
          PETER Engle | | #9

          I agree with Bill. Stuffing mineral wool and spraying foam for the air seal will give you both fire protection and air sealing. Not quite as rugged as the wood cap, though that might not matter. In most attic floor retrofits, you end up using a number of different approaches in different areas. Just find all of the "holes" in the attic floor and fill them with something before installing the thick blanket of insulation over top.

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