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Community and Q&A

Sealing gap between drywall and electrical boxes

doughpat | Posted in Green Building Techniques on

So the drywallers missed the memo that I am a stickler about air sealing.  In the 6 hours between me stepping out to go run errands and when I returned, they managed to bungle up a dozen cutouts, mainly for circular ceiling light electrical outlets.  

They used a rotozip, but apparently used it very poorly.  Some of the gaps between the edge of the box and the drywall are 1/2″!  

I’ll be using surface mounted light “pucks”, so, yes, the gap won’t actually be visible, but from an air sealing perspective I believe it is the weak point in the ceiling.

Ah, but what to use to seal the gap?

Caulk seems like the best performer in terms of flexibility and not making a bunch more work (i.e. trimming back foam).  I’m thinking just a regular latex paintable caulk (i.e. Alex, DAP, Big Stretch, etc.)

Foam seems like it would do a great job with the least amount of material (i.e. cans), but I’m sure every outlet would need to have the trim foamed back as I’m sure it will expand outwards, beyond the surface of the drywall.

Drywall mud would be nice in that it wouldn’t matter if it was accidentally spread onto the surrounding drywall — but it seems like it might be too brittle and I could easily imagine it cracking and falling out, especially during installation of the light pucks.

Any suggestions on a material/technique for sealing these gaps?

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  1. Expert Member
    BILL WICHERS | | #1

    Sometimes the gap will look bigger if they followed a mud ring with their router. Mud rings have a contour to the lip, so they’re a bit bigger at the box then up near the surface of the drywall. It’s not usually a big problem.

    I’ve mudded, caulked, and canned foamed these gaps before. I don’t like to use caulk, because it never seems to adhere well due to the drywall dust on the edge. No amount of cleaning seems to fix that, either.

    I usually use mud. You’re right, it’s brittle, but it fills the gaps. It’s probably not the best air seal though. You can put some mesh tape over the edge of the gap to minimize cracking, but it will never be as strong as uncut drywall, so tightening up electrical fixtures can still crack it.

    Canned foam is probably best for air sealing. The “door and window” version might be better long term for this, but I’ve usually just use the “regular” version that sets hard. I trim it with an handheld fine-tooth loose hacksaw blade, which is flexible enough to hold flush to the ceiling and cuts through foam like butter. Just be careful not to snag a mounting screw or something like that that could cause your hand to slide on the blade. Some electrical tape wrapped around the last few inches of the blade makes a decent handle.


  2. user-2069108 | | #2

    One our Habitat houses, we used Alex with good results. Now, that does nothing for all the holes in the box itself. It is too late for you, but there are at least two manufacturers that make sealed electrical boxes. All that was needed was sealant on a flange that contacts the back of the drywall.

    1. Expert Member
      BILL WICHERS | | #3

      Red silicone fire caulk (not intumescent fire stop) works well for sealing holes in boxes, and it can be applied from inside if needed.


  3. Expert Member


    I wouldn't seal the gap until the drywall had been mudded, and I'd make sure all the boxes were muddied tight to the edge. If the gap is 1/2" they may have to use tape. Maybe next time they will be more careful.

    These are the exterior wall or ceiling boxes we use:

  4. Eric_Powell | | #5

    Mud is good for this. Setting type or durabond is the best in my opinion.

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