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Community and Q&A

Airtight Drywall Detailing: Caulk or Compound

843construction | Posted in Green Building Techniques on

Hi all,

I’m currently working on a full gut reno of a 1910’s semi in Toronto, Canada.  The house is something between a balloon frame and modern framing with 1″ board sheathing, what looks like tar paper, 1″ of stucco then vinyl siding.  Before I started any work on the house I had an energy audit done and the blower door test showed around 14 ACH50.  While I don’t want to tear off the siding/stucco and seal from the outside, I do want to improve the air tightness.  I’ve come to the belief that the best way to do this is by using an air tight drywall approach.  My question is this:

From my research, the recommendations seem to suggest that I should caulk any large gaps in the drywall with latex caulk.  As I always fill any material gaps in drywall with setting compound, is the caulk redundant or am I missing something?

Appreciate any help,

GBA Prime

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  1. Expert Member


    The overwhelming majority of houses in Canada use a combined interior air/vapour barrier in the form of poly under the drywall. The main disadvantage is in climates where there is a lot of cooling. The advantages are it is easy to install and test before being covered, and that the details are familiar to every builder.

    You can use an airtight drywall approach for the air-barrier, although to meet code you need to apply a vapour-barrier paint to get the warm side vapour-barrier.

    To answer your question. The caulking used with air-tight drywall isn't a substitute for compound. It is used to seal penetrations, where the drywall meets framing at the base or around windows, and at the top of interior partitions on the upper storey to stop air moving up though those walls into the roof system.

    1. 843construction | | #4

      Thanks Malcolm,

      We did use poly under the drywall but I figured the Air tight drywall would be a bit of a belt and suspenders approach as the design of the walls makes sealing the poly extra difficult.


  2. Expert Member
    Akos | | #2

    The problem with balloon framed houses isn't necessarly air tight drywall but fixing all the leaks everywhere else. Even if you seal up the drywall, there are so many leak paths in the house that your ACH50 will not budge by much.

    To properly seal these, you need to go to either SPF of dense pack. Luckily you have a WRB and vinyl siding, so you can dense pack. Before dense packing, make sure to deal with your window/door flashing details as these are non-existent in these old hoses.

    On a century old balloon framed 2.5, dense packing the walls took the house from ACH20 to ACH4, but more importantly, from a place where you could not sit near outside walls or windows to a place that felt warm even in the coldest of winters.

    1. 843construction | | #5

      Hi Akos,

      I ended up using rockwool as I'm still searching for someone in Toronto who specializes in dense pack. A lot of companies say they offer it but it really seems like an afterthought for them and from my research here on GBA, it looks like it's easy to screw up a dense pack installation.


  3. Expert Member
    BILL WICHERS | | #3

    Electrical boxes are usually big air leaks in drywall. You need to go into those and seal them, ideally from the back if you can get access. If you have to seal from the inside, I like to use the red silicone "high temperature" caulk (NOT intumescent fire stop caulk). Use only a little bit to seal holes, don't go crazy and try to fill the box. You need to seal the edge of the box to the drywall too, since the holes are usually cut oversize and are rarely sealed.

    I like urethane caulk better than latext for sealing the drywall to the framing if you're putting up new drywall, but either will work.

    Take note of what Malcolm said about needing poly in Canada. Typical practice there is to do a very good job of detailing the poly before the drywall goes up. I've been very impressed with how well that's been done in the homes I've seen while I'm visiting (my brother in law is a builder outside Toronto).

    With a balloon framed house, you'll want to install fire stops in the stud cavities if you open anything up. This is really important for fire safety, and it also gives you something you can seal to prevent massive stack effect airflows through all of the otherwise completely open stud bays from the lowest to highest levels of the home.


    1. 843construction | | #6

      Hi Bill,

      We did use those "air tight" electrical boxes which are significantly better than standard boxes but even these don't seem particularly air tight where the wire enters the box. I'll try the high temp caulk to really seal them well.


      1. Expert Member
        BILL WICHERS | | #7

        Make sure to use the high-temp SILICONE caulk that is usually used on things like flues. It will often be found in the section with wood stoves. The usual "fire caulk" is actually an intumescent product (expands when it gets hot), and that's NOT what you want.

        I've never found anything specified as "air tight" for light construction is really all that air tight. "AT" on recessed cans just means "leaks LESS" in my view.

        Commerically, you can get explosion proof boxes and sealoffs, but those wouldn't work well in a residential setting unless you want an industrial look in loft or something like that. You'll also find they suck up a big chunk of your budget too -- they're not cheap.


    2. tjanson | | #10

      Why would you need to use high temp RTV silicone caulk? These are typically rated for 500 deg F. If the point is fireblocking, which I don't believe is required on electrical boxes, a blue PVC box will melt at 200 deg F and you will have worse problems. The Romex insulation is made of PVC too and will melt before that. 3m Fireblocking foam is only rated to 240 deg F!

      I just use polyurethane caulk (OSI Quad is rated to 200 deg F service temperature) to seal electrical boxes.

  4. user-5946022 | | #8

    Is this high temp silicone caulk the type that comes in a caulk tube, or is it the stuff that comes in a regular tube like a toothpaste tube that is advertised as for making gaskets?

    And can you share any tricks on apply it in there? I found the tip of the caulk tube and geometry of the tabs at the back of the box where the wire comes in do not play well together, and I'm not really getting it where I want it, which is at both the used and unused knockouts in the back. The unused ones are comparatively easier to seal - its the used ones that I can't really maneuver to and get sealed

    1. Expert Member
      BILL WICHERS | | #9

      You can get it in a tube for a caulk gun. That's what I've always used. I didn't think of the gasket maker stuff. I'm a little familiar with that stuff, but don't really use it so I can't say how well it would work here. My guess is it would probably work fine.

      It's easiest to apply sealant from the back of the box if you have access. If you can only work from the front of the box, you have to be careful to only seal around the cables and try to not smear sealant all over everthing else inside the box while you're working. The goal is apply the minimum amount of sealant needed to only the area where the cables enter. You want to ensure a good seal without adding lots of bulk inside the box, and without contaminating all the electrical connections and other stuff in and around the box.



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