What is the best way to insulation an old house with lath and plaster?
Many houses have walls made of lath and plaster and some don't have insulation between the interior wall and the sheathing. Then the cladding is installed outside the sheath. While replacing the cladding for those houses, is it a good idea to remove all the old cladding and sheath, spray foam between the studs and then install new sheath and cladding allowing for proper space between the sheath and the clad for air flow and water drainage.
If you are removing the siding of an old house, the best way to add insulation is to the exterior. See this page here first for more information - The best way to insulate the exterior of old houses
Before you gut the entire wall from the outside, easier would be leaving it as is and adding insulation if you have the space. So a first question – do you have property lines in close proximity on either side preventing you from expanding the exterior dimension of the house? in tight urban areas that’s often the case is the reason I ask. If you don’t have space restrictions then leaving the sheathing on and not disturbing the entire wall would be the easiest way to do it.
It is pretty safe to assume that a house old enough to have lath and plaster does not have a poly vapor barrier, which is a bit of an advantage when it comes to installing additional insulation on the outside of walls.
So the short story on how to do it is as follows – remove the siding and take the wall back to the sheathing or sheathing boards if that’s what you have. Add an exterior air barrier membrane, using a peel and stick exterior air barrier will likely be your best chance to make the house airtight.
From there add rigid insulation, and you can see here our page about choosing the best rigid insulation for exterior walls, and the short story on THAT is… the safest is Rockwool / mineral wool insulation as it does not trap moisture, and other than that, if you’re doing foam then the more you do the better, it will be a vapor barrier, but if there is enough of it that’s not a problem. see this page about choosing the right rigid insulation and when they act as a vapor barrier.
After that, be sure to install siding so walls can stay dry (see here), and then install the new cladding.
If you choose to strip wall from the outside all the way to the lath and plaster, or if you have to do that to avoid encroaching on neighbouring properties then spray foam will work fine. Spray foam is not cheap, and it has a pretty high ecological impact so we don’t love it, but it will work as the insulation, the air barrier and the vapor barrier.