Probably quite a simple question. I have managed to make a 225 x 140mm hole by removing some bricks on a cavity wall. I now need to cement a liner and an air brick on the outer leaf of the cavity wall, all above head height close to the ceiling. I am concerned about rain going in. If I were to leave a gap between the liner and air brick, water from the rain could find it's way through the cavity wall and I've got an electrical cable going through that wall a few inches from there. If I were to cement the liner and the air brick together, would rain water go as far as the wallpaper (or will it just eveporate)? What's the correct way of doing it? Thank you.