We have a garage that has been stuck on to the side of the house. The original house has feature towards the bottom of the wall where it steps out 63mm, as shown in my attached picture. I'm planning on battening and plasterboarding the wall between the house and the garage that is the old external wall of the house, in order to have a straight vertical wall for putting shelves etc up against in the garage. Whilst I'm at it, I thought I may as well stick some insualtion in the gap - probably mineral wool. The garage has it's own cavity wall insulation, so doesn't get super cold, but it's a garage, so obviously gets quite cold. My plan is to batten the wall, put in some rockwool, and then plasterboard over the top. But my question is should I include a vapour barrier, and where should it go? Between the battens and original wall, as the living room will be the warm room? Or between the plasterboard and battens on the garage side of the wall?
The garage has insulated cavity walls on the outside, and a wooden garage door. But the wall between the garage and house is the original three skin solid brick wall. My worry is if I put a vapour barrier somewhere I'll end up trapping moisture in to the insulation.
The above might help understand options and how it works. Foil back insulation acts as a vapour barrier and would be fine. I also think baton and rock wool is fine for you and skip membrane... Not needed as no wet rain ect on wall. I'm no expert though