How to handle vapor barrier around electrical boxes?

July 16, 2006 6:30 PM

How do I seal holes in the vapor barrier around electrical wires?

So I'm going to insulate part of my 3rd floor/attic. I'm putting in pot lights and some of the outlets will be on outside walls and all those have to be in side the vapor barrier. (I'm ripping out the existing plater and lathe and drywalling it, so I figured I might as well fix the electrical stuff while I'm in there)

So I assume I have to poke a hole in the vapor barrier to get the electrical wire through, but then what? Do I just leave a hole in the barrier? Or do I have to seal it with something? Google is no help. This is in Ontario if it makes a difference from a code point-of-view.

posted by GuyZero to home & garden (10 comments total)

I would think you'd use putty. Any serious hardware store carries it and can make suggestions.
posted by Steven C. Den Beste at 6:58 PM on July 16, 2006

You should seal the barrier. When you knock outlet boxes through vapor barrier, you put them in a plastic "Lessco box" which has flanges you can tape or spray-adhesive the barrier to - I've seen this done with Tyvek tape or with 3M "92" spray adhesive. Lessco boxes are only a couple of inches deep though - how deep are your pots?
posted by nicwolff at 7:01 PM on July 16, 2006

Or, you could consider insulating with foam instead of fibreglass - foam blocks air and water well, and prevents condensation - not really a DIY though.
posted by nicwolff at 7:09 PM on July 16, 2006

Those vapour ready boxes are silly expensive. I use a regular box and a heavy clear plastic box surround (looks like a clear plastic fedora). To seal the vapour barrier to the box surround and around wire protrusions I use black acoustical sealant because it is really cheap (but can be messy). There are all sorts of other products in caulking tubes that do the same job for lots more money.

If your pot light mounting box requires a plywood enclosure then you can just seal the plastic to the plywood at the edges. I like the aluminium metal tape for this purpose as it isn't degraded by heat. Again seal where the wire comes through the plywood with acoustical sealant.

I like to back the ceiling boxes with a 2X6 between the joists just in case you might want to hang a ceiling fan in the future. I also run 14/3 rather than 14/2 to the switch for flexibility in the future.
posted by Mitheral at 7:28 PM on July 16, 2006

I'm using the stock vapor barrier boxes you get at a big box hardware store (Rona, because it's literally 100 feet from a Home Depot and is, at most, 1/10th as busy. I don't get it.) for both the outlets and the pots. The ones for the pots are huge, like 1'x2'x1' and the ones for the outlets are just big enough to get a standard outlet box into.

I understand how to attach the pre-made vapor barrier box to the main vapor barrier (or rather, I don't have to understand really, because my drywall guy will do that). But I'm still not sure which of the many possible products I could use that I should use. Putty seems... too stiff. At least, if it's like the glazier's putty I have. Though obviously glazier's putty probably isn't the right thing. Caluking? Is it rated for use around electrical wire? Spray foam? Seems a bit much for a little hole and again, I'm not sure if it's rated to 90 deg C like the wire, boxes, etc. Electrical tape? Seems like it wouldn't seal well. Chewing gum?

Anyway, thanks for the input so far. Maybe I should just go ask my local hardware store (the big box ones... ugh)
posted by GuyZero at 9:09 PM on July 16, 2006

Sealing around the wire entering the box is one of the uses of the Acoustical [PDF] Sealant [PDF]. See also: NRCAN, Rona, Lessco {1st step, second section}.
posted by Mitheral at 6:51 AM on July 17, 2006

You want the stuff pictured here. It's made by "Canada Technical Tape" and is product number 205-02 (can't direct link on their site) It's called "Tuck Tape", any building center will have it.

That's what I used on my vapour barrier. The stuff sticks like the dickens. I taped around the wire going into the plastic box surround as well as over the flanges against the main vapour barrier.

If you watch shows like Holmes on Holmes, they use the stuff everywhere - every seam or penetration through the vapour barrier is liberally covered with the stuff.
posted by davey_darling at 7:27 AM on July 17, 2006

Yeah, I have to get cable again just so I can watch Holmes on Holmes. Thanks again!
posted by GuyZero at 7:42 AM on July 17, 2006

If I'm missing something, I'm sorry, but wouldn't caulk do the job? If you are looking for insulating as well as sealing the vapor barier, there's always expanding foam such as Great Stuff.
posted by kc0dxh at 1:21 PM on July 18, 2006

The issue with caulk and expanding foam is if they comply with electrical or building code - they may not be rated to 90 deg C, they may be conductive, whatever.
posted by GuyZero at 2:21 PM on July 18, 2006