A: To insulate your cavity walls, the installer drills small holes around 22mm in size at intervals of around 1m in the outside wall of your home. The installer then blows insulation into the cavity using special equipment. Once all the insulation is in, the installer fills the holes in the brickwork so you’ll barely notice them. Filling cavity walls is not a job you can do yourself. A professional can do the job in around two hours for an average house with easily accessible walls. It shouldn’t make any mess.