I have an old 1940’s house with 2×4 exterior walls, sawdust insulation and 3/4” shiplap on the inside. Exterior is also shiplap covered with stucco. I’m looking to replace the insulation. I am thinking of using rock wool batts, covered with a 3/4” foam board on the inside, so that the final wall is the same thickness. I know it may be more challenging to attach the drywall to the foam board, but I was going to try long drywall screws.

The inside foam board would also act as a vapor barrier but do you think I might have a problem with condensation on the outside of the foam?

Alternatively I was thinking of doing a thin coat of DIY spray foam to the outside wall, then rock wool batts. 

thanks!