External Wall Insulation
According to the National Insulation Association, about a third of UK homes were built with solid walls. They estimate that 45% of heat loss is escaping through the walls.
External wall insulation involves fixing an insulating material to the externals walls which is then finished with a protective render - so it will affect the appearance of your property.
External wall insulation is usually more expensive than internal wall insulation but it should lead to bigger savings on heating bills.