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Adding insulation to plaster walls – worried about moisture

GreenStrathcona | Posted in Energy Efficiency and Durability on

Edmonton, Alberta, Canada. Zone 3b-4a. House is a 2×4 framed 1942 bungalow, 800 sqft + 750 in the finished concrete foundation basement. All the walls on the main floor are lath + plaster, basement is drywall (finished about 15 years ago).

We recently had the deteriorating stucco on the outside removed down to the sheathing boards, and replaced with tar paper + 1″ high permenace polyisocyanurate insulation (IKO ener-air, >1 perm WVTR) + vinyl siding to both improve look and hopefully make the place a bit more efficient. We wanted 1.5″ but the company ordered hte wrong material and didn’t realize til the stucco was all off, and we couldn’t wait the six weeks for new insulation in that state. So 1″ it is.

I still notice that the walls get fairly cool. Currently it’s -20 °C so we’re feeling it a bit. I believe that the house may have very minimal or even non-existent insulation in many of the exterior walls. This would be pretty typical of that vintage.

I’m curious about adding some blown-in cellulose, but there are some conflicting pieces of advice on this with plaster walls due to the possibility of moisture issues.

My question is basically this: while we have lath + plaster walls, the exterior is now updated in a way that (I believe) allows better vapour permeance and should mitigate issues with moisture retention. We do not have big roof overhangs, but with the siding we got the flashing updated as well which should help.

Would it be safe to add blown-in cellulose to the walls? Would it cause moisture issues?

Many thanks!

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  1. Expert Member
    Dana Dorsett | | #1

    In IECC climate zoning, avearaging a bit more than 5000 HDD (base 18C) Edmonton is zone 7B:

    For a milled 3.5" deep 2x4 wall with R13-ish cellulose fill, in Table R702.7.1 the IRC calls out a minimum of R10 c.i. on the exterior to be able to use a Class III vapor retarder (eg standard interior latex) on the interior side of the walls:

    For full dimension 2x4s it would need to be about R11.5.

    But a house with plaster walls built in 1942 probably has multiple layers of low-permeance alkyd paint (possibly even leaded paint) on the interior, so as long the walls are reasonably air tight the low-perm paint blocks interior moisture drives from reaching the stud bays and sheathing. (If the walls have been stripped and re-plastered, "vapor barrier latex" primer would drop to about a half-perm.) With the semi-permeable foam the assembly dries toward the exterior.

    The first most important thing to get right is the window & door flashing and other exterior-side bulk water handling details, followed by air-sealing (particularly the interior side.) Vapor diffusion through the paint comes in at a distant third as a moisture risk. Cellulose insulation is very air redardent and while not adequate as an air sealer on it's own, it limits the amount of air-transported moisture from incidental minor air leaks.

    Better still, cellulose can buffer a substantial amount of moisture in it's fiber without damage or losing effectiveness, sharing and redistributing the seasonal moisture burden with the structural wood- it's fairly protective on it's own. There are 100 year old houses still standing in Saskatchewan that have done quite well that were insulated with cellulose back in the day, well before hand wringing about vapor barriers entered the conversation. If the bulk water management details are in good shape your risk of creating a problem is pretty low.

  2. agooch | | #2

    I always hear the “old builder talk” (which I know to be often wrong) about how insulating plaster walls is asking for mold. I imagine you could get your vapor perm perfect and still potentially have issues simply because a house old enough for plaster is likely to have its weak spots for leaks?

    Always curious why blown mineral wool isn’t discussed more. Seems like it could be a simple solution.

  3. GreenStrathcona | | #3

    Dana: thanks very much for the reply and all the detail. This helps a lot.

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