Water vapour moves through materials by diffusion, driven by a difference in vapour pressure between the two sides — vapour migrates from the warm, humid side (usually indoors in a UK winter) toward the cold, drier side (outdoors). This is a slow, distributed process, quite different from the bulk movement of moisture by air leakage (covered in the Airtightness guide), but it determines whether a closed-up wall build-up can dry or whether moisture accumulates within it.
The driving force: vapour pressure
Just as heat flows from hot to cold, vapour flows from high vapour pressure to low. In winter, warm indoor air holds more moisture and exerts a higher vapour pressure than the cold outdoor air, so the net diffusion is outward, into and through the construction. In summer the gradient can reverse. The materials in the build-up either let this vapour pass (vapour-open) or hold it back (vapour-closed), and where you place the resistance determines where — if anywhere — moisture risks condensing inside the wall.
The μ-value (vapour resistance factor)
The μ-value (mu, the water-vapour resistance factor) is a dimensionless material property: it tells you how many times more resistant a material is to vapour diffusion than an equal thickness of still air. Air has μ = 1 by definition. The higher the μ, the more the material resists vapour:
| Material | Approx. μ | Character |
|---|---|---|
| Still air | 1 | Reference |
| Mineral wool insulation | ~1 | Very vapour-open |
| Wood-fibre / many natural insulants | ~3–5 | Vapour-open ('breathable') |
| Brick / lime plaster | ~10–20 | Moderately open |
| Plywood / OSB | ~50–200+ | Fairly closed (varies with grade) |
| Closed-cell PIR / XPS foam | ~50–150+ | Vapour-closed (foil facings far higher) |
| Polythene VCL | very high | Effectively a vapour barrier |
μ is a property of the material itself, independent of thickness. To know how much a given layer actually resists vapour, you have to account for how thick it is — which is what the sd-value does.
The sd-value — the number that actually matters
The sd-value (in metres) is the practical metric for a real layer: it is the μ-value multiplied by the material's thickness. It expresses the vapour resistance of that layer as the thickness of still air that would offer the same resistance — the 'equivalent air layer thickness'.
- sd < 0.5 m: vapour-open ('breathable') — vapour passes readily.
- sd 0.5–10 m: vapour-retarding — slows but doesn't stop vapour.
- sd > 10 m (some standards use higher): vapour-closed / vapour barrier — strongly resists vapour.
Vapour control layers and the 'fifth principle'
A vapour control layer (VCL), sometimes an air-and-vapour control layer (AVCL), is a high-sd membrane placed on the warm side of insulation to limit how much vapour enters the construction in the first place. The classic detailing rule for a vapour-closed build-up is to make the construction more vapour-open as you move outward (warm-side resistance higher than cold-side), so any vapour that does get in can escape to the outside rather than being trapped. A modern refinement is the 'intelligent' (humidity-variable) membrane, whose sd-value drops when humidity rises, allowing the wall to dry inward in summer — particularly valuable in retrofit.
Vapour-open vs vapour-closed strategies
There are two coherent philosophies, and the danger lies in muddling them:
- Vapour-closed (sealed): keep vapour out with a robust warm-side VCL and rely on it being continuous and undamaged. Effective in new build with good workmanship, but unforgiving — a puncture or a reversed gradient can trap moisture.
- Vapour-open ('breathable'): use vapour-open materials throughout (e.g. lime, wood-fibre, mineral wool) so the construction can buffer and release moisture and dry in multiple directions. More robust and forgiving, and often the safer choice for older, solid-wall and heritage buildings.