Cavity wall insulation
Houses from the 1920s were built with a cavity between an outer wall and an inner wall. The original aim was to reduce water penetration. Prior to this, houses had a single solid wall. It is often possible to tell whether a house has a cavity or solid wall from the pattern of the brickwork on the outer wall. Houses with a cavity wall will have the Stretcher Bond pattern with exposed brickwork.
Houses built in the last decade will have insulation built into the wall. If your house has unfilled cavity walls which are at least 50mm wide and the walls are not exposed to driving rain, the building could benefit from cavity wall insulation. Installation is a straightforward process which may take only a couple houses and is relatively low cost.
During the retrofitting of cavity wall insulation, the installer will drill small holes every metre into the wall and insulating material is blown into the cavity through these. The insulating material used might be mineral wool, beads or insulating foam. If there are filled in drill holes between the brickwork on the external walls, it is likely that cavity wall insulation has already been retrofitted.