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Community and Q&A

Insulating a brick wall from the inside

user-982243 | Posted in Green Building Techniques on


We have a 100 year old row house in Montreal, (zone 5000). Most walls abut the homes on either side, but we have a bump out in the back with two exposed walls. The walls are two bricks thick, (I think), with plaster on the inside, and I think a layer of drywall on top, (judging by how the walls are actually proud of the original baseboards).

The walls in question are in our bathroom, which gets quite chilly. I’d like to add foam board insulation on top of the walls, and then drywall on top of that again (we can live with the resulting deep sills in the windows).

My concern is moisture build-up between the foam and the drywall due to condensation in this damp environment. Should I install furring and have some sort of small gap at the top and bottom to allow moisture to evaporate? Or should I just make sure it’s totally hermetically sealed by using a lot of adhesive? Any other suggestions?


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  1. user-2310254 | | #1

    You should read this article on GBA's Martin Holladay ( Do you have the option of applying exterior insulation? That sounds like a much safer option in your climate, which I believe is 6A.

  2. GBA Editor
    Martin Holladay | | #2

    Condensation occurs on cold surfaces. You won't get condensation on the rigid foam -- but you might get condensation on the cold bricks, especially if there is a small air gap between the rigid foam and the bricks.

    As I explained in my article, Insulating Old Brick Buildings, with the type of building you are describing:

    1. It's always better to install insulation on the exterior side of the bricks than on the interior side of the bricks. (Think EIFS.)

    2. If you must install insulation on the interior side of the bricks, the safest insulation is closed-cell spray foam.

    For more information, read the article.

  3. user-982243 | | #3

    Thanks Steve and Martin, the article was excellent and an eye-opener. Freeze/thaw never occurred to me, I thought that condensation and thus mould between the bricks and the foam would be the big problem.

    Unfortunately, I can't insulate on the outside as I live in a conservation district.

    Sounds like the best thing to do is to leave well enough alone, although at the very least I may move the radiator onto an interior wall as I am probably losing a lot of heat through the bricks to the outside.

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