airspace

An airspace is the empty cavity deliberately left between the inner and outer leaves of a cavity wall, typically 25 to 100 millimetres wide depending on the wall design and location. This gap traps still air, which is a poor conductor of heat, and creates a thermal barrier that reduces heat loss from the building interior. The airspace also breaks the path for water penetration, allowing any moisture that enters the outer leaf to drain down and out rather than being drawn into the inner masonry by capillary action.

The effectiveness of an airspace depends critically on keeping it actually empty and dry. If mortar droppings, debris, or insulation material bridge the gap during construction, the thermal value is compromised. The cavity must remain ventilated at the base to allow accumulated moisture to evaporate; blocked cavities can trap water and cause damp problems inside the building. In older two-leaf walls with no insulation, the airspace alone provides modest thermal resistance, around 0.18 K.m²/W for a 50 mm gap. Modern constructions often fill part or all of the cavity with rigid foam, mineral fibre, or spray foam insulation while maintaining a small ventilated air gap, typically 25 mm, between the insulation and the outer leaf.

Variations and specification

Cavity width varies with building type and era. Traditional cavity walls, common from the 1920s onward, are typically 50 to 75 mm. Modern external walls may have wider cavities to accommodate thicker insulation, and the airspace may be only the residual gap after insulation is fitted. Exposed or high-wind locations sometimes use narrower cavities and sealed cavities without ventilation, which requires careful detailing to prevent water ingress.

Cavity wall ties or straps bridge the gap to tie the two leaves together structurally, spaced at regular intervals (typically 900 mm apart horizontally and 450 mm vertically). These ties must be sloped downward toward the outer leaf to direct water outward rather than inward. Corroded ties are a common problem in older buildings and allow leaves to separate, destroying the structural bond.

The term 'airspace' distinguishes the empty cavity from one that has been filled with insulation material. Specifying cavity width, ventilation, tie type, and any insulation fill is essential in construction documents because performance and water management depend on these details.