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Installing drywall ceiling under existing drywall ceiling in bathroom

Grant Robinson | Posted in General Questions on

Planning bathroom reno in main bathroom on second story. There are no existing ceiling fixtures. I went into attic to investigate opportunity to add boxes and run circuits but it’s essentially impossible. Low pitch hip roof and bathroom is at exterior corner so impossible to even crawl to location where fixtures would be or to run circuits from above. Was able to confirm two layers of fibreglass batts(likely original build) on what appears to be 4mil vapour barrier (house built in 1972) above existing drywall celiing. So regardless of what I do with lighting in bath I intend to blow in more insulation in attic.

As running new circuit in attic is not practical am now contemplating strapping the existing dry wall ceiling and hanging a new drywall ceiling below original. Would result in a 3/4″ gap between new and old drywall that would enable electrical fixture boxes flush with new ceiling and a gap to run wiring within the gap. Are there special precautions to ensure I don’t incur a vapour transfer issue? Given there is a vapour barrier above original ceiling should I or should I not include a vapour barrier with the new ceiling? My concern is with the 3/4″ air gap..could moisture migrate there?

As opposed to leaving an unheated air gap between new and old celling a friend suggested I adhere a 3/4 or 1″ foam board to existing ceiling and screw new drywall up through that to avoid creating an air space. Armoured wire would by run to ceiling fixtures through a chase cut onto foamboard. This would result in the original drywall ceiling being sandwiched between foamboard below and vapour barrier above. But is this situation so dire to rule this out as a choice?

Any input or suggestions greatly appreciated.

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Replies

  1. Jonathan Blaney | | #1

    Is the current ceiling on strapping? Can you cut the holes for your fixtures an snake the wire between the strapping. Work from below through the holes. Push the bats up out of the way if you need. Even if you had to cut some work holes, the repairs would be a lot easier than what you are thinking about. LED light fixtures on pancake boxes will get the job done. I have done this.

  2. Walter Ahlgrim | | #2

    I think if you cut in an old work ceiling box and push a fish tape toward the center of the house you should be able to find the tape and pull the wire back to the corner.

    Walta

  3. Grant Robinson | | #3

    Thanks for responses.
    Jon:I am going to use LED fixtures as you suggest. Unfortunately the existing ceiling drywall is screwed direct to ceiling joists, no strapping or gap available there. If there was, I agree your idea of working it all from below would have been quite possible
    Walter: Your idea could have worked but I need to run perpendicular to 3 ceiling joists. To pass by each joist will mean drilling through it or passing over it, all while working from below. I'd be working within insulation, patching vapour barrier and drywall from below and for me that makes the idea of staying completely below the existing ceiling more appealing.
    Given that I will bring in a drywall installer/taper anyway as part of the project (I've learned there are some skills in life I just don't have) the effort of installing a second ceiling is no big deal in the grand scheme and avoids having to work within the insulation from below. So as long as I'm reasonably certain or comfortable that I'm not creating a future head ache, my preference from an effort and impact perspective is to add the second drywall layer and run the wiring in the gap created.
    So if anyone has thoughts on the gap, I'd love to hear!
    Thanks

  4. GBA Editor
    Martin Holladay | | #4

    Grant,
    Adding strapping and a second ceiling below your existing ceiling will not cause any moisture problems. As you probably know, whatever you do, you still want to pay attention to airtightness.

  5. Grant Robinson | | #5

    Thanks Martin. A couple of more questions if I may:
    Would a latex based paint be best on the new ceiling?(e.g. vapour permeable versus comparatively impermeable paint choice for the ceiling)
    Though not required, would it hurt to sandwich foam board between new and old ceiling?
    (why you ask?):
    I have to pull down a bit of drywall above the tub/shower after all to enable replacement of 45 year old exhaust fan and reconnect duct. Once complete will need to seal up existing vapour barrier and restore original drywall ceiling. Thereafter am wondering if a relatively impermeable foam board would be an added vapour defence and modest r value contribution as opposed to leaving 3/4" air gap between the new and old ceiling above the tub. Your thoughts?

  6. GBA Editor
    Martin Holladay | | #6

    Grant,
    Q. "Would a latex-based paint be best on the new ceiling (e.g. vapor permeable versus comparatively impermeable paint choice for the ceiling)?

    A. The type of paint you use is irrelevant. Use whatever you want.

    Q. "Though not required, would it hurt to sandwich foam board between new and old ceiling?"

    A. Add rigid foam if you want, or omit it if you want. Either approach is fine from a moisture perspective.

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