Typical cold climate construction: Drywall, 6mil vapour barrier, fiberglass batt insulation. How do I retrofit a junction box and maintain the vapour seal? To be clear, by 'retrofit', I mean the drywall is up and I'd like to install this without having to repair any drywall.

Typically for new construction, you'd use an insulated and sealed box:

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The vapour barrier is sealed against the gasket when drywall goes up.

Obviously, there's no way to get this box into a wall without removing a large chunk of drywall.

Does a product exist for this? How do you get a box into the wall and maintain the seal with the vapour barrier, without any access to the inside of the wall beyond the hole the box will fill up?

(actually fishing the wire to this box is beyond the scope of this question)


This worked brilliantly.

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I cut a square hole out of the existing vapour barrier, then pushed the vapour box in behind it and spread it out. I then used tuck tape to attach from the back of the existing vapour barrier to the vapour box (using several small overlapping pieces), and had enough room to fit in a regular old work box.

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  • Are you asking about keeping the seal to stop air infiltration into the living space?
    – Spark
    May 7, 2013 at 18:54

1 Answer 1


I think you'll have to get one of these vapor barrier boxes, and fit it before installing your electrical box.

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Seal up where the cable penetrates, then install your electrical box.

The other options would be to cut a larger hole and patch drywall, or seal it up from the back side (but that would require access to the other side of the wall/ceiling).

  • Ah, that is brilliant, I totally forgot those existed. After that I can just use a regular old-work box (or even low-voltage box, if applicable) and it should be fine.
    – gregmac
    May 7, 2013 at 21:55
  • where do you buy these?
    – DMoore
    May 8, 2013 at 5:55
  • 2
    I just got a couple at a big orange box store last night, on the shelf next to the junction boxes. True story: the girl at checkout picked it up and said "woah, there's a barcode on this? I was about to throw it out, thought it was a piece of packaging or something"
    – gregmac
    May 8, 2013 at 15:42
  • 1
    @Tester101 What are the ramifications of installing the electrical box in an outside wall and not patching the vapor barrier or using a vapor barrier box? Does it just reduce the efficiency of the insulation or is it a code/safety issue?
    – Matthew
    Jan 16, 2017 at 4:56

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