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Rigid foam and drywall directly on top of plaster/brick interior?

westeringman | Posted in General Questions on

Howdy folks,

New here, hope this is the right format and question category!
I recently bought an 1887 brick Victorian in Colorado. Unfortuately it is covered with years of ugly previous retrofits such as aluminum siding, very dirty fiberglass bats and such. It felt very drafty and the sheetrock was done very poorly. I tore out the sheetrock, framing, and fiberglass in one room to reveal original plaster walls covered with wallpaper. I have read extensively on rigid foam retrofits in old houses, but have come to no conclusion on better insulation options.

I would like to install rigid foam directly to the plaster and wallpaper, use masonry screws to secure furring to the foam and wall, and then install drywall directly over that. No vapor barrier, because it is brick. Brick needs to breathe I hear. Aluminum siding installed over the brick keeps them dry outside, and I think they are in fine shape (I tore the siding off a small section to inspect the brick). I would like to remove this siding at one point but that is another project. There are no exterior moisture penetration issues I can see. According to some posts foam over plaster/brick would be fine, and a vapor barrier would not be nessassary. Other articles state that a vapor barrier is required when foam is applied directly to brick walls covered in plaster, due to moisture accumulation from changes in air tempuratues.

Vapor barrier, or no vapor barrier? Is this this best solution to insulate these type of walls? I’ve also read that the plaster may have to be removed. Should I take the walls down to brick?

Walls seem to be very dry, but do display very old water damage that I am certain were the result of a past roof issue that has been fixed. I am in the highest city in the country, and relative humidity is always pretty low except in the spring during snowmelt. But the climate zone is probably just under taiga/ arctic because I am above 10000ft.

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  1. GBA Editor
    Martin Holladay | | #1

    Here is a link to an article that explains your options: Insulating Old Brick Buildings.

    A few principles:

    1. Adding insulation to the interior of an old structural brick wall can be risky. The risk is lower if you keep the aluminum siding on the building; the risk is higher if you remove the aluminum siding.

    2. Installing insulation on the exterior side of your brick walls is much safer than installing insulation on the interior side of your brick walls.

    3. If you install rigid foam on the interior of your walls, there is a risk that there will be small air channels between the rigid foam and the bricks, and that these air channels will permit condensation against the cold bricks. That's why it's always better to install closed-cell spray foam on the interior rather than rigid foam on the interior.

  2. Dana1 | | #2

    If you remove the siding and replace it with new back-ventilated siding it'll still be fine, but if it's exposed brick you would be well advised to back-ventilate the brick with at least a 3/4" cavity vented to the exterior at both the top and bottom.

    The location in Colorado makes a bit of a difference too, since your local climate could be anywhere from US zone 4B to zone 7B.

    How much roof overhang do you have at the eaves, and how many stories?

  3. westeringman | | #3

    Thank you for the answers Martin!

    And Mr. Dorsett, it is a two story brick and my eaves are somewhat minimalist, perhaps 14 inches or less. I would worry some about rain and snowmelt infiltration. But things do dry out extremely quickly here.

    I am starting to consider simply adding insulation in the attic (r-38+) and minimizing drafts around the windows and floor gaps; and leave the walls un-insualted for the sake of the brick. I hear other brick places (mostly commercial) in town have done the same, and Dr. Joe Lstiburek in the Old masonry insulation article seems to suggest that this is best for a historic brick building. Simply attic and basement insulation, nothing else, and tighten up the gaps. Even if I go no insulation, I would still want to put up drywall, perhaps with 3/4 inch gap, as you suggest, just without the insulation. Would this be a good compromise for the sake of the historic brick? Just drywall with a 3/4 inch vapor gap? The brick and paster walls alone are probably no more than r-4 if I am lucky, correct? Its 2 wythe, probably 8-9 inches thick.

  4. GBA Editor
    Martin Holladay | | #4

    An insulated wall is always better than an uninsulated wall, as long as the insulation won't harm the bricks. You can either follow the advice in the article I linked to, or simply leave the wall uninsulated; the choice is yours. But if your investigation reveals that insulation won't harm your bricks, then you won't regret installing insulation.

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