In order to prevent dampness of walls, that part of the wall below the level of the ground should be constructed with particular care. Nothing will better prevent dampness in walls than a " damp course" of some impervious material. Asphaltum is probably best for this purpose, though layers of slate in concrete or damp proof tiles are very efficient. If at all practicable there should be a dry area all around the foundation walls in order to prevent any dampness in the walls originating from the earth surrounding it at the sides. If such an area cannot be provided a double wall with an air-space between inner and outer walls should be used.