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Community and Q&A

Plaster air sealing

Andrew207 | Posted in General Questions on

We just acquired an 1900s duplex with plaster and lathe. It goes through a tank of oil a month in the Maine winter. I’d like to cut down on this, but it presents some unique challenges. It has newer windows and had blown in cellulose in the walls many years ago (I do not think they did a good job). The basement has a very thin 1/4 inch of spray foam, which seems to do a decent job of air sealing it because the basement is always very warm.

There are two priorities that come to mind. First I believe it may have balloon framing because you can look up into the exterior walls in the basement. I’m thinking I should plug those up to limit the stack effect.

The other issue is potential air leakage in the attic. The original ceilings are plaster and lathe, but they added a secondary drywall ceiling after. Because of the older ceiling I cannot access the newer drywall ceiling to air seal it. The other problem is there is 6 inches of cellulose in the attic, which makes it hard to do air sealing work. I was thinking about removing the cellulose or blowing it out of the way and put down a layer of spray foam, but I’m unsure if it is worth the time/expense.

I’d appreciate any ideas.

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  1. Expert Member
    BILL WICHERS | | #1

    Are you sure the spray foam in the basement is only 1/4” thick? I’m not sure it’s even possible to apply such a thin layer, at least not without all kinds of gaps and irregularities. If it really is that thin of a layer, all it’s doing is air sealing. R value would be around maybe R1.5 at best. The 1/4” fanfold XPS is R1 for comparison. Not much.

    If you have ballon framing, the studs will go the entire height of the building walls, basement to attic. You’ll get massive amounts of air movement through these cavities due to stack effect unless you close them off. Best is to put in 2x4 blocking which will also serve as fire stops, then air seal the blocking with canned foam. Stuffing mineral wool in will also help, but isn’t a true air barrier unless you cover the exposed end with spray foam.

    If your drywall is well installed, taped and mudded, it’s a pretty good air barrier. If you want to air seal top plates and electrical/plumbing penetrations in the attic, you’ll need to move the blown in insulation out of the way. 6” of blown insulation isn’t very much, so after you air seal it would be a good idea to install more. While adding more blown insulation you can even out whatever you’ve disturbed while air sealing too.


  2. Andrew207 | | #2

    Thanks for the reply. The foam in the basement-it's really very thin. Maybe slightly more than 1/4", but no more than 1/2." It's a bad job. I've picked a piece off here and there to test the depth.

    In terms of the balloon framing, I cannot access all of stud bays in the attic because some walls meet sloped ceilings that cannot be accessed from anywhere. Is it worth it to air seal these cavities from the basement?

    I do plan to add more cellulose after all of the air sealing is done. But once again in the attic the older plaster and lathe prevents me from finding and sealing penetrations and top plates that go through the drywall barrier. I've taken canned spray foam and injected foam into the region between the drywall and plaster where penetrations go through, but there is no way for me to confirm that I was successful. I'd have to cut the plaster and lathe out of the way first. This is why I thought I should add a layer of spray foam over the entire surface of the attic floor.

  3. Expert Member
    BILL WICHERS | | #3

    Yes, it is absolutely worth it to seal the stuf cavities from the basement. Best is to do attic and basement, but basement only is a big improvement over nothing. With top and bottom sealed, you also help with any air leaks from electrical devices in the walls (like switches and receptacles). With nothing in those stud bays, they’re acting as chimneys sucking air from your basement up into the attic. It’s exactly the same process at work that makes natural draft chimneys function.

    If you air seal penetrations in the plaster ceiling, you’ll stop any air from getting through from inside just as if you sealed the drywall. All you need to do is seal any holes you find. Every hole you seal makes a little improvement, just do the best you can. You could spray foam the entire attic floor, but I don’t think it would be worth the cost. A thick layer of cellulose will help to slow any remaining air leaks. Cellulose is much better than fiberglass for this, but still not as good as spray foam (or canned foam in holes).


  4. user-1072251 | | #4

    Yes, it will be well worth taking the time and expense. Most people underestimate the value and the comfort of a home with no drafts or cold spots, and with temperatures even and steady, until they live in a tight house. I’d stick with 2” for a minimum depth of spray foam in your attic, and ideally, cover that with 14-16” of loose fill cellulose insulation. Stopping air flow in the stud bays is also critical to the overall performance of the house.

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