What is the best way to insulate the exterior walls of an older home?
I have an older home in Toronto with plaster lathe walls with no insulation behind them. I want to insulate the walls but not remove the plaster as it's still in excellent condition. My house goes right to the property line on two sides and the exterior shell is cedar board/insulbrick covered by vinyl siding. I would like to stip it to the cedar board, putting a wrap over it, but then what would you recommend for insulating after that given my narrow confines? I really can't extend much more than a couple of inches without encroaching, three max...
If you have on older home plaster lathe then it is a pretty safe assumption you don't have an incorporated air barrier or vapour barrier. Though you do in ‘effect’ likely have a makeshift vapour barrier as there is likely several coats of oil paint. Along with insulation I would propose an air barrier of some kind, either inside or out. There are a few places I’d start your research and renovation design, this page here
How to Insulate Exterior Walls from the Outside
If you have a total of 3 inches before you risk encroaching on a neighbouring property, you can still make that work, and it seems you have a pretty good plan already. Myself I would probably go with a peel and stick exterior air barrier membrane on the boards, followed by 2 inches of insulation, strapping and siding.
That will only ad up to about R8, which is a lot better than you have, if you want to take that up a notch, you coule possibly blow dense packed cellulose insulation in empty wall cavities after you've stripped the walls and before you install a new weather barrier and insulation.
Be sure to install exterior home siding so it can dry properly (read more here), and Choosing the right exterior rigid insulation (see here) is very important. It is best to install a vapour open exterior insulation so you don’t trap moisture inside the walls, which is why we most often recommend installing some sort of Mineral Wool insulation like Rockwool (formerly Roxul) but there are other manufacturers as well now.
My house is 110 years old and did this almost 20 years ago. I recommend insulating from the inside by filling the 2x4 cavity with two-component polyurethane foam. It is foam yes bet it does not off-gas and it flows in so as to form a vapour barrier and an R-16 wall. A few days of drilling the holes, then one day for the applicator to fill the voids, then another few day patching the holes--presto.
The added extra is the that the ridge foam adds to the structural strength of the walls. From a structural engineer – me.