I am remodeling a 1905 Folk Victorian (code for farmhouse). The majority of the house is 2x4, with the exception of an addition which is 2x6. I looked into foam exterior but it would involve removing a lot of original molding and architectural details that add to the house. With spray foam at $7000, it is a $6000 premium over fiberglass. Not worth it. This is where I decided to go with XPS on the interior (or even polyiso).
I have read that the interior foam approach is somewhat accepted. I have a different question though: inside of my walls is a layer of plaster and lath (presumably for air sealing). I want to be sure if I leave it be that it would not cause issues in regards to vapor retarder location. The old wall system would be as follows: siding>tyvek>3/4 t&g planks>plaster>fiberglass (or cellulose)>xps>fir strips>drywall.
On the first floor I had to tear out much of the insulation for structural repairs. However the rest of the existing structure has blown in cellulose in excellent shape, so I'd like to leave it in.
The addition has 2x6 with R-19 fiberglass. I would probably add foam here too. I'm not looking for superinsulation, I'm simply trying to outperform the bare minimum.
I live in South Dakota, zone 6a. Interior vapor retarder is required.
Any advice appreciated!