Single vs Double Skin Blockwork — UK

Single skin blockwork (100mm or 140mm) is suitable for: internal non-load-bearing partitions, garden and boundary walls up to 2m (with piers), single-storey outbuildings and garages, retaining walls (with engineering design), and below-ground foundation walls. Single skin is not suitable for external walls of habitable buildings due to thermal and weather resistance requirements.

A standard UK cavity wall consists of an outer leaf (typically 100mm dense or facing blockwork/brick), a cavity (typically 50-150mm, partially or fully filled with insulation), and an inner leaf (typically 100mm aerated blockwork). The two leaves are connected with wall ties at specified centres. Total wall thickness is typically 275-375mm depending on cavity width.

A single skin 100mm block wall costs approximately £40-60/m² for materials and labour. A full cavity wall with insulation costs approximately £100-160/m². The cavity wall is roughly 2-3 times more expensive but provides weather protection, thermal insulation (meeting Part L), structural stability for multi-storey buildings, and sound insulation.

Single skin walls need a foundation width approximately 3 times the wall thickness — so a 100mm wall needs a 300mm wide strip foundation. Double skin cavity walls need 450-600mm wide strip foundations depending on soil conditions and building height. Cavity wall foundations are wider and use more concrete, adding to the cost difference. Approved Document A Table 10 gives minimum foundation widths.

Not for external walls in England and Wales. Building Regulations require external walls of habitable buildings to meet Part L thermal requirements (typically U-value 0.26 W/m²K for new build), Part C weather resistance, Part E sound insulation (party walls), and structural requirements. A 100mm single skin wall cannot practically achieve these. Single skin is only used for external walls in certain agricultural and non-habitable buildings.