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Bathroom renovation, double brick walls

Chris_herrmann | Posted in Energy Efficiency and Durability on

I am gutting the bathroom in my house. All the plaster on lath walls were installed over 3/4″ furring strips, nailed to the exterior brick walls.

My tub is on 2 exterior walls.

I would like to insulate the room, if possible, but am not sure on the best way to do so. I can increase the spacing from the brick a bit and still hold my interior dimensions (plaster was 2″ thick in spots, with 3/8″ tile to boot).

Looking for advice on:
– insulation material
– vapor barrier requirements
– insulating one room and leaving adjacent rooms uninsulated (could this have unforeseen adverse consequences?)

Anything else I am surely overlooking.


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  1. brendanalbano | | #1

    You'll get more complete responses if you share with folks where you are located (climate zone and/or zipcode).

    After you've removed the interior plaster, am I understanding correctly that the existing wall in question is just two wythes of brick, nothing else?

    There are a bunch of articles on GBA and Building Science Corporation about old brick buildings. Here are a few to get you started in your research. Do keep in mind that there is a lot of nuance to it, so gathering as much info as you can and running it by some experts is wise!



  2. Chris_herrmann | | #2

    Thank you, Brendan!

    My house is in Baltimore, MD. The bathroom is on the southeast corner of the building. We have a large oak tree shading it to the south, a holly tree to the east (this side is gabled in the attic), and my bedroom projects past the bathroom to the southwest.

    The walls, to my knowledge, are two wythes of brick.

    I have attached photos of its current state.

    Thanks again!

  3. Expert Member
    Akos | | #3

    The way I've done something similar is by getting one of the smaller spray foam kits and spray foaming between the strapping. If you want extra R value, a layer of foam over the strapping than drywall/cement board directly over the foam. The spray foam keeps interior air and moisture away from the brick, and seals everything up nicely. This is very hard to do any other way.

    Which small depth, it is hard to get the SPF even. If you are quick after the foam sets up a bit where it is no longer tacky but still soft, you can push it flat with a piece of plywood with poly on it against the strapping. Otherwise need to trim it flush after it sets.

    1. Chris_herrmann | | #4

      Would this technique be considered pretty standard practice?

      My tub, which is cast iron and on two exterior walls, has a large void space beneath it. I was thinking about applying spray foam in this cavity -- would you recommend this or another method?


      1. Expert Member
        Akos | | #5

        I don't know how standard practice it is but with older brick houses you are limited to how much you can move the tub. This lets you put at least some insulation in there VS none before.

        For the famed area you definitely need to air seal, this has good details.

        I'm in the land of interior poly, so the air barrier typically is a well sealed ploy sheet.

        Extra R value in corner bathrooms never hurts, so if you have the space a layer of rigid insulation over the studs with the seams taped and edges caulked can also work. You can notch the studs and run a horizontal 1x4 for the tub lip, then install 1/2" rigid foam bellow and Kerdi backer above. Going up to 2" Kerdi is even better but harder to find.

        You can spray foam the stud cavity, but it is generally not worth the cost.

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