Most houses built after the 1920s have cavity walls which consist of a small gap or cavity between the brick work, which, when insulated, significantly reduces the amount of heat escaping through the walls.
Find out more about cavity wall insulation
Most houses built before the 1920s have solid walls - they have no gap or cavity between the brick work and the only way to insulate them is from the inside with internal insulation or from the outside with external insulation. This significantly reduces the amount of heat escaping through the walls.
Find out more about solid wall insulation