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Vapor barrier at electrical boxes and pot lights

I have new electrical outlet boxes with the integral vapor barrier and foam seal strip around them. Do I need to tuck tape the poly to these foam seals, or is the pressure of the Sheetrock enough to seal?

Secondly, at the recessed lights (I have IC lights), do I need to tuck tape the vapor barrier to the foam seal strip or again is the Sheetrock enough to seal the barrier?


Asked by Aaron Guest
Posted Tue, 12/31/2013 - 04:59
Edited Tue, 12/31/2013 - 07:03


2 Answers

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Let me guess: you are Canadian, right? (Translation for American readers: "Tuck tape" is a brand of housewrap tape that is manufactured in Quebec.)

First of all, you really don't have to worry too much about having a vapor barrier (except to satisfy your building inspector). It sounds like you are using polyethylene sheeting as your air barrier -- a practice that is fairly common in Canada.

In the U.S., many people use the drywall as the air barrier, especially if (like you) they are using airtight electrical boxes. If you are following the Airtight Drywall Approach, you don't need the polyethylene.

Details for sealing polyethylene to the flanges of an airtight electrical box vary from manufacturer to manufacturer. The manufacturer of Airfoil airtight electrical boxes, Doug McEvers, suggests that polyethylene should be sealed to the box flanges with Tremco acoustical sealant. (See photo, below.)

I strongly suggest that you don't install any recessed can lights in an insulated ceiling, since these lights (even "airtight" versions) are responsible for air leakage and ice dam problems.

If your electrical boxes and recessed lights include foam gaskets, the gaskets are designed to seal air leaks, without the use of Tremco acoustical sealant. However, these gaskets don't create a perfect seal, and air leaks are still possible, even with the gaskets. That's why you still might want to use Tremco at your electrical boxes, and why you still might want to avoid using any recessed can lights.


Airtight electrical box with polyethylene and Tremco.jpg
Answered by Martin Holladay, GBA Advisor
Posted Tue, 12/31/2013 - 07:00
Edited Tue, 12/31/2013 - 07:05.

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To seal the MemBrain plastic vapor retarder to the flanges of the Airfoil boxes in my house, I started using an acoustic sealant, but I found that the bulge of the dense-packed cellulose was lifting the MemBrain off the flanges. I cleaned the sealant off the flanges, trimmed the MemBrain back to expose half to 2/3 of the flange face, and applied four strips of good quality tape, suitable for the two plastic surfaces. That worked well, so I used the tape throughout the rest of the house. The tape remained adhered, was of course not messy to apply, and the time per box was fairly short.

Answered by Dick Russell
Posted Tue, 12/31/2013 - 13:30

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