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Community and Q&A

Service Cavity OK for Backsplash and Mirrors?

lance_p | Posted in Energy Efficiency and Durability on

Climate Zone 6A, Ottawa Ontario

My proposed wall is double stud cellulose with a 2×3 service cavity. The service cavity would be filled with a compressed R12 fiberglass batt for around R9-R10. Vapor barrier will have variable permeability, probably Membrain.

With a fiberglass filled service cavity on the inside of my vapor barrier, should I be concerned with moisture buildup behind a kitchen backsplash/cabinets or bathroom mirror? Should I be using green drywall in the kitchen just in case?

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  1. Expert Member
    Dana Dorsett | | #1

    Unless the backsplash is a large fraction of the full height of the wall you need not worry about moisture accumulation. Fiberglass will wick and redistribute adsorbed moisture that gets into the wall.

    A tall mirror on glass is a true vapor barrier, but as long as there is capacity to dry toward the exterior it's not likely to be a problem. (Most new homes in Canada are built with polyethylene vapor barriers on the interior side of the assembly. MemBrain won't stop moisture from drying in either direction, but limits the rate of diffusion unless the proximate air is really humid.

  2. lance_p | | #2

    Thanks Dana. Yes, with Poly on one side and a mirror on the other, there would be no way for moisture to get in OR out of the drywall in that case! :-)

    Honestly, I'm more concerned about the kitchen than anything. With cabinets (coated fiberboard) on the top and bottom and a backsplash in between, I'm concerned about that very large area having somewhere to dry to. Good point about drying back to the outside through the Membrain, but drying back through 2.5" of dense fiberglass, then Membrain, then 12" of cellulose, then OSB... that's a long way to go.

    Would Green drywall be any benefit in that scenario? I'm under the impression that green drywall is intended for use in damp locations like bathrooms, but I don't know if just the paper side is different or if the whole product is more moisture resistant?

  3. Expert Member
    Dana Dorsett | | #3

    Don't sweat the drying rate aspects of the kitchen- it's going to be just fine. If you would sleep better using green wallboard, go ahead, it's not a big upcharge, but unless you're doing three meals of pasta per day boiling it with the lids off and no kitchen exhaust fan running the kitchen moisture issue won't be dramatically different from most other rooms.

  4. lance_p | | #4

    Thanks again, Dana. Now if I could only convince my wife to boil pasta with the lid ON... she's set in her ways with some things!

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