Abstract: A method of insulating a building that includes one or more existing external walls and an existing roof structure supported by the walls, comprises constructing a first external shell structure that covers an outer surface of at least one external wall, said shell structure being spaced from the outer surface of the wall to provide a wall void between the external wall and the shell structure. A second shell structure is constructed that extends around or through the existing roof structure and provides an enclosed roof void that extends around or through the roof structure. The roof void is interconnected with the wall void. The wall void and the roof void are filled with an insulating material to provide an insulating layer that extends substantially continuously through the roof void and the wall void.