Ah you are about to face one of my favorite problems. A high-performing building is air-tight, and well-insulated, but old masonry/brick buildings rely on leaky

Ah you are about to face one of my favorite problems. A high-performing building is air-tight, and well-insulated, but old masonry/brick buildings rely on leaky envelopes with no insulation to maintain a dry enclosure. Basically, the lack of insulation in these old buildings means that the heat emitted through your walls is likely drying your walls during the winter. Once you add insulation on the inside, any vapour from the outside (rain-driven, or drawn through material pores through capillary action) is going to hit the cold/exterior side of the insulation and condense inside your wall (which then causes mold, conducts heat). You can calculate this by assuming steady-state heat flow across your wall during cold/moist conditions and see if the temperature drop across the wall hits the dewpoint (point where air vapour saturates into liquid). That's why it's better to have the insulation on the outside, with a vapour barrier underneath it to protect your structure. With these kind of old building retrofits, that's not possible. However, you can solve it, basically by calculating the moisture and heat flow through your wall, and identifying the maximum insulation that doesn't cause condensation long enough to grow mold (this is called hygrothermal analysis). There's also various coats you can add to the exterior that have some success in control moisture drive, but if they backfire, you are basically sealing up the moisture inside, so that's always a back-up solution. Source: I've worked on retrofitting the envelopes of a lot of old masonry/brick buildings.