We have an old Victorian house with an extremely cold kitchen. I’m planning to insulate the suspended floor and also the walls internally. I have 100mm Rockwool slabs available, I was wondering if I can use these on the walls (obviously they will take up more space) or better to buy Celotex boards and use those ( extra expense but less space loss). Do I need to leave an air gap behind the Celotex if I decide to use that? Can I apply the Rockwool directly onto the brick wall? Thanks