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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 (mm) | Vapour Resistance (MNs/g) |
|---|---|---|
| 250 mu polythene VCL | 0.25 | ~25–50 |
| 12.5 mm plasterboard | 12.5 | ~0.06–0.10 |
| 9 mm OSB/3 | 9 | ~0.27–0.72 |
| 100 mm mineral wool | 100 | ~0.50 |
| Breather membrane | ~0.5 | ~0.03–0.06 |
Related Calculators
See the Insulation Lambda Values for thermal conductivity, or the Insulation U-Values for Part L compliance targets. See also the Condensation Risk Calculator.
Next up: related pages
Frequently Asked Questions
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.
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."
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.
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Last updated: April 2026
Verified against UK standards · estimates only, confirm with your supplier.