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Community and Q&A

Insulating bathroom

schatzyman | Posted in General Questions on


I know this has been debated, but I am just curious as to some more outlook on the best option here. I just ripped out a bathroom with one exterior wall. The wall was insulated with foil-faced fiberglass batts — after ripping them out I saw they had mold on the side contacting the house sheathing. The home is 100 years old — wood sheathing with stucco outside.

I have already created an unvented cathedral ceiling (using 2″ of closed cell spray foam followed by fiberglass batt and 2″ of rigid foam for thermal bridging). Any advice on the best way go about insulating the walls? I will be covering them with cement board, Kerdi membrane, and tile.

Spray foam for this wall isn’t an option…. rigid foam + fiberglass? fiberglass alone?

I am in climate zone 4a….


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  1. GBA Editor
    Martin Holladay | | #1

    First of all, make sure that the sheathing is dry before you insulate.

    Rigid foam makes more sense than fiberglass batts in this case. Be sure to install the rigid foam -- at least 2 inches, but the entire stud cavity if you have the patience -- in an airtight manner, by sealing the perimeter of each piece of rigid foam with caulk, canned spray foam, or high quality tape.

  2. schatzyman | | #2

    Thanks Martin -

    Generally, in remodeling (when it isn't practical to put foam on the outside of the sheathing) in this climate - is rigid (air tight) or spray foam the best general practice for wall cavities?

    It seems to me that providing this air sealing at the sheathing with foam is easier & more effective then batts with poly vapor on the inside.

    I'm assuming if I only used 2" above I could safely fill the rest with un-faced fiberglass?

  3. davidmeiland | | #3

    In 4a I don't think you want or need the poly.

  4. GBA Editor
    Martin Holladay | | #4

    Spray foam is easier to install (especially around wiring) and is more likely to provide an airtight installation. The reason I didn't mention it in my first answer is that your wrote, "Spray foam for this wall isn't an option."

    You are correct that either spray foam or air-sealed rigid foam will perform better than fiberglass batts.

    And David is right that you shouldn't include interior polyethylene in your climate zone.

    Good luck.

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