Ive got a 1902 house in montreal where it gets down to -30C during winter. Usually its about -15C. Its also very humid here. I just bought this place and noticed there is no insulation in the walls which is no surprise. The walls are shiplap cedar. It all looks to be in pretty good shape. The exterior is very old stucco. Might as well be concrete because its so hard. Its never coming off. On top of that is vinyl siding. I wanted to insulate from the outside since the wall cavity is tiny and some places non existant. I was thinking removing the vinyl, then 2inches or maybe more of rock wool then putting up some real cedar siding. I dont think theres an air gap between the wood walls and the stucco siding so that cant be left exposed. I think ill just leave the walls alone from the inside. Anybody know of a problem with this idea?

1 Answer 1


The only issue I can see is that the rock wool is not sturdy which wouldn't give you anything to anchor your new siding too without building some sort of frame to fasten your siding to.

Now, what we (my parents and I) did on a 1903 farmhouse in Saskatchewan was to use that "rigid insulation" and fastened it to the stucco (concrete if you will). We then covered it vinyl siding, but I don't see any issue with going with cedar siding. This has held up just fine for about 8 years now.

  • Thanks for your input. I guess my main concern is creating moisture buildup inside the wall because Ill be modifying the way it used to ventilate itself. Still trying to be more fluent in due point and condensation in relation to wall construction. Rigid roam may work better for mounting the furring as you say, i guess the flammability aspect of it turned me off a bit. I may reconsider and go with foam if mounting furring on rock wool proves to be difficult.
    – Omax
    Commented Jun 6, 2019 at 22:45
  • You can get stuff that has an FR rating - I know we did. But it has been so long that I don't remember what it was (nor did I particularly care I was probably "voluntold" to give up my weekend to fix the house instead of hang with friends I was 16 or 17). And based on this previous post (diy.stackexchange.com/questions/15326/…) as well as some reading online; a lot of rigid foam also works as a vapour barrier. So I think you would be ok with that, in terms of condensation/moisture.
    – J Crosby
    Commented Jun 6, 2019 at 22:51
  • Interesting. I was unaware of the vapor barrier aspect of foam. Although I believe in my situation it would be a case of having the vapor barrier (if any) right inside the drywall. Keeping that warm air from hitting the cold surface which hopefully with exterior insulation would not be inside the wall cavity. I dont even think ill use a vapor barrier. If the wall cavity is warm, i dont think i need one. But im open to differing opinions.
    – Omax
    Commented Jun 7, 2019 at 0:57
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    I would also use foam over rock wool, in the 70’s& 80’s we did a fair amount of siding jobs and rigid foam under the siding was what we used even on homes with some insulation in the walls, I don’t remember anyone complaining about it later that it did not help with the power bill, but there were large energy rebates so it did not cost much, you might check into rebates I know in some cases they pay up to 80% including install costs in my area.
    – Ed Beal
    Commented Jun 7, 2019 at 20:59

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