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Community and Q&A

Old apartment renovation – tyvek on interior side – cold climate

slarc | Posted in General Questions on


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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  1. brendanalbano | | #1

    Am I understanding correctly that the only purpose of the tyvek is to serve as an air barrier? Would an airtight drywall approach work and simplify things a little?

    While it isn't an air barrier, the cheapest class II smart vapor retarder that I know of is a kraft-faced fiberglass batt. Could be worth considering. I'd imagine you'd save money with kraft-faced batts compared to mineral wool, although it may be more difficult to achieve a really high quality installation. The latex paint is a class III vapor retarder.

    Am I reading correctly that there is 3" of solid wood, like a log cabin or mass timber building? What an unusual wall!

    1. slarc | | #4

      Hi Brendan and thanks for your reply. Yes, the tyvek's only purpose is as an air barrier. I agree that an airtight drywall approach would probably be the best and optimal solution, but I have good reasons to doubt the workers on the project will do this properly... their abilities are very very limited and they haven proven multiple times that they know very little about lots of things ... The other thing that would worry me about an airtight drywall approach is that the building seems to have moved considerably throughout the years and I am worried the drywall might not provide much resilience and potentially lose its airtightness pretty quickly. Maybe I'm wrong, but it's a hunch. In any case, I will definitely try to convince the workers to install the drywall as "airtight" as possible and seal at every possible end, but the tyvek in behind might block most of the air leakage.

      Oh, and yes, the solid wood wall is unusual I guess, but here in Montreal, a lot of old homes are built this way. I guess it's because we have so many trees here ! We call it "carré de bois" in french ("wood square"). See attached picture for an example.

  2. Expert Member


    I agree with Brendan that air-tight drywall probably makes the Tyvek redundant, but because all the trades in Canada are familiar with how to detail and work with interior poly, substituting Tyvek makes sense to me.

    It's hard to predict how these old building will react to changes, but your analysis rings true to me - although I'm no expert in these sort of renovations.

    1. slarc | | #5

      Hi Malcolm, thanks a lot for your reply. I'm wondering if you forgot to write "but" after "redundant" ? Your second paragraph highlights the exact reason why I wrote this post ... I've heard horror stories about people insulating old homes and having catastrophic mold problemes only a few years after, and other stories where the same intervention in what seemed like the exact same setting and situation didn't create any problems. There is a lot of ways to look at the problem and I feel like the outcome is very difficult to predict ... Thanks for your comment !

      1. Expert Member
        MALCOLM TAYLOR | | #7


        Yes I did and have edited my reply to reflect that.

        Good luck with your project - from an ex-Montrealer.

        1. slarc | | #8

          Thanks !

  3. Expert Member
    BILL WICHERS | | #3

    Tyvek is only an air barrier and WRB, it's vapor open, by design. You could try a smart vapor retarder (MemBrain, Intello, etc.), which would help limit moisture ingress into the wall from the livings space while still allowing for some inward drying potential. I would avoid polyethylene in this wall assembly as I think it would increase the risk of moisture problems as you suspect.

    You could drill a small hole in the timber to probe what's behind it. I would drill a 1/2" or so hole through the timber, being careful not to blow out the back and potentially damage anything in the mysterious space behind the timber. Take a piece of scrap wire (solid electrical ground wire of 14 or 12 gauge works well here), bend a small "L" into the end. Poke this wire through the hole until it stops -- that's the back of the brick, or at least the back of whatever the next layer of the wall assembly is. Now pull the wire back while dragging it on the edge of the hole until the "L" catches on the lip of the hole. That's where the back of the timber is. The difference between those two stop points tells you if there is an air gap, and how big of an air gap you have if there is one. When you're done, pull the wire out and fill the hole with caulk to seal it. This is a pretty good minimally invasive way to test for things like this. I've done this myself to find out how deep the rafters in old cathedral ceilings are too, but usually without the bent "L" for that.


  4. slarc | | #6

    Hi Bill and thanks for your reply,

    At first, I proposed a smart vapor retarder like the ones you mentioned, but the client and contractor were not interested and thought it would mean a very expensive solution ("smart" always sounds expensive...). Anyways, they said no ...
    Thanks for the idea to measure the gap between timber and brick, you're right that it's definitely not too invasive and I'll try it out next time I'm on site.

    1. Expert Member
      BILL WICHERS | | #9

      MemBrain is probably the cheapest of the "smart" vapor retarders, but it will still cost more than a sheet of polyethylene. Intello performs better, but costs more. Either offers additional protection against moisture problems in the wall though. It might be worth at least pricing out MemBrain to give your customer some real numbers before they decide against using the product.


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