Question 1

When is single skin blockwork acceptable?

Accepted Answer

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.

Question 2

What does a double skin cavity wall consist of?

Accepted Answer

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.

Question 3

How much more does double skin cost?

Accepted Answer

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.

Question 4

What foundation does each wall type need?

Accepted Answer

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.

Question 5

Can I build a habitable room with single skin walls?

Accepted Answer

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.

Feature	Single Skin (100mm)	Double Skin Cavity
Wall thickness	100mm	275-375mm
Material + labour cost	£40-60/m²	£100-160/m²
Thermal performance	Poor (no insulation cavity)	Good (meets Part L)
Weather resistance	Poor — rain penetration	Good — cavity drains water
Structural height	Limited — single storey	Multi-storey capable
Foundation width	300mm minimum	450-600mm
Sound insulation	Limited	Good (meets Part E)
Building Regs habitable	No (external walls)	Yes

Single vs Double Skin Blockwork — UK

Side-by-Side Comparison

When Single Skin Is Appropriate

When Double Skin Cavity Is Required

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