External Wall Insulation is a system that wraps the exterior of a building in a layer of insulation boards bonded or mechanically fixed to the wall, with a protective finish layer applied over the top. It is one of the most effective ways to improve the thermal performance of older, uninsulated solid-walled buildings. EWI with a render finish is one type of external wall cladding system, while EWI with a panel or board finish is another.