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Community and Q&A

How to Insulate Above T&G Ceiling?

djdoggone | Posted in General Questions on

We are trying to insulate a vented attic in a single-story cottage which has southern yellow pine tongue-and-groove as a ceiling in the living space. We live in central Massachusetts, zone 5.

We have purchased gaskets from Conservation Technology (which we learned about here, thank you very much) and have stapled them to the ceiling (attic side) against the ceiling joists. Then we screwed down 3/8th inch plasterboard between the ceiling joists. On top of the plasterboard will go three layers of unfaced fiberglass batts, each layer perpendicular to the last layer.

We are wondering:

1) Will the plasterboard screws allow air leakage and do we have to seal & caulk each screw? (Please say it isn’t so; we are 64 years old and wondering if we will survive to insulate the basement & crawl space);

2) Will the gasket beneath the plasterboard be air tight or do we have to seal the plasterboard to accept caulking & caulk along the gap where the joists meet the plasterboard? (Ditto)

We are sticking with the least aromatic/non-toxic elements because we learned the hard way — back in January when this project began — that I stop breathing when exposed to the smallest amount of any type of foam. Will be using AFM caulk because I don’t react to it, but it requires the plasterboard be sealed first.

Hoping someone has been here/done this and knows the answer. Thanks & Regards, Art & Deb

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  1. GBA Editor
    Martin Holladay | | #1

    Art and Deb,
    I'm not sure I understand. Are you installing narrow (14.5 inches or 22.5 inches wide) drywall pieces between your ceiling joists? Are you trying to attach these drywall pieces to the attic side of your T&G ceiling boards in hopes of creating an air barrier?

    If that's what you are trying to do -- trust me, there are easier ways.

  2. user-869687 | | #2

    Art & Deb,

    Sounds like you've already completed installing gaskets and wall board in between your joists. This is a little unconventional but you have the right idea about needing to air seal the ceiling. Screws should not be much concern for air leakage but you do need to seal all joints in the wall board. Of course the usual way of sealing these joints would be drywall mud and tape, but caulk might also work.

    It would increase resistance to air flow if you installed a deep layer of cellulose rather than fiberglass batts. Look for detail drawings of vent baffles at eaves, to install in between the rafters. This will contain cellulose while allowing ventilation air to flow below the roof.

  3. djdoggone | | #3

    Martin: The drywall is going on the attic side, cut to size to fit between the ceiling joists. So, when standing in the attic, there is the t&g pine, then gaskets, then drywall (screwed into the pine to compress the gasket). Gaskets have been stapled along the ceiling joists for an air-tight (we hope) seal along the sides of the drywall, and gaskets have been placed at the tops & bottoms of drywall pieces to seal between pieces of drywall. All of this to create an air barrier between the living space and the unconditioned attic.

    If there are easier ways, we'd love to hear them. We struggled for a long time, talking to people at the hardware store, trying to figure out how to do this. This all started with an effort to make our cottage more energy efficient. We read Joe L.'s book about building in a cold climate and learned we were heating the outdoors all this time because the t&g ceiling is not an air barrier.

    We will have a blower test to make sure there are no leaks, but rather than have to move all those layers of insulation to seal/caulk screws & along the gaps where the gaskets are, we figured we would do it now if (and only if) people thought air would leak at these places.

    Please share "easier" ideas. We figure that at the rate we are going it will take us a very long time to insulate the attic, and there is a crawl space & cellar to address.

    Regards, Art & Deb

    P.S. For the part already done, do you think there is danger of air leakage at screws & above gaskets?

    Thomas Jefferson: We are only 1/4 done (and our home is less than 900 s.f.!). Glad to hear you think screw holes are not an issue. See above re: gaskets vs. mud/tape. Do you think we are safe there too? Unfortunately fiberglass was bought last year to take advantage of tax credits so cellulose is not an option now -- and would probably be too expensive for our shoe-string budget.

  4. GBA Editor
    Martin Holladay | | #4

    I wouldn't worry about the screw holes.

    By far the easiest approach -- the one that most people would use -- is it install between 1 and 2 inches of closed-cell spray polyurethane foam against your ceiling from the attic side.

    This would create an air barrier. Then you could pile on as much cellulose as possible on top.

  5. djdoggone | | #5

    Martin: We started with the idea of foam but I am one of those people that can not tolerate any -- and I mean ANY -- exposure to the stuff. And, we are trying to avoid a vapor barrier for fear of mold; that's why we didn't just put down plastic. I believe one of the Building Science gurus has expressed regret that the focus these years has been on vapor barriers rather than air barriers. It has lead people into all sorts of mold problems when curing air leakage cures the majority of heating and cooling loss. That's why unfaced batts.

    Do you think the gaskets will seal the air path between drywall and joists? and between pieces of drywall. No need to caulk too?

    Regards, Art & Deb

  6. GBA Editor
    Martin Holladay | | #6

    Art & Deb,
    You've taken on a very fussy, time-consuming task that will be difficult to achieve well. There will be all kinds of opportunities for air leaks using your system -- especially because the drywall can't be installed tight to the joists. The cracks at the edges of the drywall will be hard to seal with gaskets.

    If you don't like spray foam, why not just install a new drywall ceiling (covering up the T&G boards)? Of course, then you won't have a board ceiling anymore -- but you'll have an air barrier, which is something far more valuable than a board ceiling.

  7. Richard_in_Maryland | | #7


    Do you think Art & Deb might be able to use the approach you suggested to me? You wrote:

    "If you are worried about your air barrier, why not establish an air barrier with plywood? Assuming that you have access to the attic that allows you to bring the plywood up there, just fasten the plywood to the top of the joists and seal the seams with high-quality tape. Then you have an air barrier, and you can blow as much cellulose on top as you want."

    Instead of drywall, use plywood?

  8. GBA Editor
    Martin Holladay | | #8

    That approach would work, as long as the plywood details were truly airtight -- especially at the perimeter of the attic. However, it might be tricky to maintain air-barrier continuity between the plywood and the wall air barrier -- but it's possible with attention to detail.

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