Old apartment renovation – tyvek on interior side – cold climate
First time posting ! I’m an architect in Canada currently working on the renovation of a second floor apartment in an old building in Montréal (zone 6). I want to improve the insulation and indoor comfort by doing some work on the exterior walls affected by the renos (walls are not bathrooms walls).
The current composition of the existing walls is (from in to out) : plaster on lathe, old 2″x3″ stud wall (cavities very partially insulated with pink wool), tar paper, 3″ thick solid squared timber wall , air gap (I assume) and brick (unfortunately haven’t seen what’s between the timber wall and brick). The walls seem very dry and there are no signs of mold. The walls are very old, the tar paper is dry, ripped and broken in many places and there is considerable air leakage everywhere.
We have a very very limited budget, the client doesn’t want “serious” interventions and we can’t build new walls in front of the old ones (apartment much too narrow) I’ve tried to find an inexpensive solution to improve the insulation and comfort and reduce the heating costs, but I’d love some feedback on it. I think it is unconventional and I’m a little stressed about how it will perform …
To start, I assumed the existing walls have had no issue staying dry up to now because they are very porous and “breathe” a lot. Only adding new insulation in the cavities (and nothing else) could potentially “slow down” the moisture-laden air leaks and potentially lead to condensation in the new insulation. Adding a polyethylene vapor barrier on the interior (as is commonly done here) would probably block vapor diffusion very well, but I worry a lot more about air leaks than diffusion, and I seriously doubt the vapor barrier would be installed sufficiently well to prevent air leaks. This improperly installer vapor barrier would inhibit any drying towards the interior, which I suspect might have played a significant role in keeping the walls dry throughout the years. The budget is a real issue, and we (the client) unfortunately “cannot” afford smart membranes. With this in mind, I’ve proposed a solution adding insulation and reducing air leaks while still allowing potential drying on both sides. In practical terms, I’ve proposed adding rockwool comforbatt insulation to tightly fill the cavities of the existing stud wall, then applying tyvek over the insulation on the interior side (this is the controversial idea…), then furring strips and drywall, painted with latex paint. I would spray PU foam around W&D openings as well
I realize the tyvek will not prevent vapor diffusion through the wall (it will act like the “summer mode” of smart membranes), but, again, I am much more worried about wintertime air leakage of moisture-laden air in the walls than vapor diffusion. I think the latex paint will prevent vapor diffusion through the walls sufficiently. The tyvek would allow us to wrap and air-seal around all penetrations, at odd junctions and at the bottom and top of the walls much better and easier than with drywall, making it an efficient air barrier to stop air leaks. My thinking is also that if any condensation were to occur inside the wall, the rockwool comfortbat would not readily mold and could dry towards the exterior (very porous exterior side …). During summer, the walls could also more easily dry towards the interior, since the vapor drive will be reversed (although the latex paint could potentially inhibit this ?).
This is where my reflexion has lead me. I’m not a specialist on these questions and my thinking or my proposition could have flaws. I’d really welcome any feedback on this (validation, alternatives,potential problems, etc.). Thank you all very much in advance !
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