What is the difference between vapour resistivity and vapour resistance?

Vapour resistivity is a material property measured in MNs/gm (mega Newton seconds per gram metre) — it tells you how resistant the material is to vapour flow per unit thickness. Vapour resistance is the actual resistance of a specific layer, calculated as resistivity x thickness. A thin layer of high-resistivity material may have less vapour resistance than a thick layer of low-resistivity material.

Why does vapour resistance matter in construction?

When warm, moist indoor air passes through a wall or roof, it can cool to the dew point within the structure and condense — causing rot, mould and structural damage (interstitial condensation). By placing high vapour resistance on the warm side (VCL) and low resistance on the cold side (breather membrane), you ensure moisture can escape outward rather than building up within the structure. This principle is called "the warm side should be 5 times more vapour-resistant than the cold side."

Do I always need a vapour control layer?

Most timber frame constructions, warm flat roofs and internally insulated walls require a VCL on the warm side of the insulation. Masonry cavity walls with full-fill insulation generally do not need a VCL as the masonry itself provides reasonable vapour resistance. The need for a VCL should be determined by a condensation risk analysis (BS EN ISO 13788 or BS 5250). Intelligent membranes that adjust their vapour permeability with humidity can be used where moisture needs to dry in both directions.

UK Vapour Resistance Values Reference

Vapour resistivity and vapour resistance values for common building materials including plasterboard, OSB, PIR, breather membranes and VCLs.

Vapour Resistivity of Common Building Materials

Higher values = more resistant to vapour flow. Vapour resistance of a layer = resistivity x thickness (m).

Material	Vapour Resistivity (MNs/gm)	Notes
Aluminium foil	Very high (effectively infinite)	Used as VCL when sealed at joints
Polythene sheet (250 mu)	~110,000	Common VCL/DPM material
PIR/PUR insulation (foil-faced)	~100–150	Foil facings add significant vapour resistance
XPS insulation	~150–600	High vapour resistance, used below ground
EPS insulation	~25–100	Moderate vapour resistance
OSB/3 (9-11 mm)	~30–80	Acts as partial vapour check in timber frame
Plywood (softwood)	~15–50	Moderate resistance
Plasterboard (standard)	~4–8	Low — does not serve as VCL
Plasterboard (foil-backed)	High (foil-dependent)	Foil backing acts as VCL if joints sealed
Mineral wool insulation	~5	Very low — vapour passes through readily
Brick (engineering)	~25–100	Dense, moderate resistance
Brick (facing, common)	~15–40	Variable by type and porosity
Dense concrete block	~15–30	Moderate resistance
Aerated concrete block	~8–15	Lower resistance due to air cells
Cement render	~25–50	Moderate resistance
Lime render / lime plaster	~8–15	Breathable — suitable for heritage buildings
Breather membrane (standard)	~0.3–0.6	Very low — allows vapour to escape outward

Typical Vapour Resistance of Specific Layers

Layer	Thickness	Vapour Resistance (MNs/g)
250 mu polythene VCL	0.25 mm	~25–50
12.5 mm plasterboard	12.5 mm	~0.06–0.10
9 mm OSB/3	9 mm	~0.27–0.72
100 mm mineral wool	100 mm	~0.50
Breather membrane	~0.5 mm	~0.03–0.06