Spacers and a spacing strip for use in an outer wall of a building between the outer vertical surface of its underlying structure and the inner surface of siding through which the siding is nailed to the underlying structure to provide a ventilation space between the rear surface of the siding and the outer surface of the underlying structure. Ventilation channels can be provided at both the lower and upper ends of the ventilation space to facilitate movement of air to the atmosphere from such a ventilation space.