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.
A worked example — reading an sd-value profile
Consider a solid-wall room being lined internally with wood-fibre insulation. Working from the warm (inside) face outward, a typical vapour-open build-up might be: a lime or clay plaster finish (sd ~0.1 m), an intelligent vapour control membrane (sd variable, roughly 0.25–10 m depending on humidity), 80 mm of wood-fibre board (μ ≈ 5, so sd ≈ 0.4 m), a wood-fibre adhesive/parge coat, and then the original solid brick (sd a few metres) finished externally with lime render. The design principle is visible in the numbers: keep the warm-side resistance modest and broadly graded so that any vapour entering can continue outward and dry, rather than meeting a high-resistance layer on the cold side that would trap it. Reverse that order — a vapour-tight foil on the cold face — and you build a moisture trap.
| Layer | μ (approx.) | Thickness | sd (m) |
|---|---|---|---|
| Lime/clay plaster | ~8 | 15 mm | ~0.12 |
| 80 mm wood-fibre board | ~5 | 80 mm | ~0.4 |
| Intelligent AVCL (humid) | variable | — | ~0.25 |
| Intelligent AVCL (dry) | variable | — | ~10 |
| Polythene VCL (1000 ga) | very high | 0.2 mm | ~50–100 |
| Foil-faced PIR (per 100 mm) | very high | 100 mm | >100 |
The 5:1 rule and where resistance belongs
A classic detailing heuristic for vapour-closed (sealed) build-ups is that the vapour resistance on the warm side of the insulation should be several times — often quoted as at least five times — that on the cold side, so vapour is held back before it reaches the cold zone where it could condense. This is why a vapour control layer belongs on the warm (internal) side of the insulation, never the cold side. For vapour-open ('breathable') build-ups the philosophy is different: rather than holding vapour back hard, the assembly is kept open throughout so moisture passes through and dries out, and an intelligent membrane manages the seasonal direction. Both are valid; mixing them — a high cold-side resistance with an open warm side — is the failure mode.
Diffusion vs air leakage — a question of scale
It is essential to keep diffusion in proportion. Designers can spend enormous effort perfecting the vapour profile of a build-up while a far larger moisture flow walks straight past it through gaps. Air leakage — warm, moist indoor air carried bodily into the construction by pressure differences — typically transports an order of magnitude or two more water into a wall or roof than diffusion through the same area of intact material. The practical consequence is that a continuous air barrier on the warm side is usually a more important moisture-safety measure than the vapour control layer alone, and that a beautifully specified vapour profile is undone by a single unsealed service penetration. Vapour design and airtightness are two halves of one job.
Intelligent (humidity-variable) membranes
The modern refinement that makes much retrofit possible is the intelligent, or humidity-variable, air-and-vapour control layer. Its sd-value changes with the ambient relative humidity: high (vapour-tight, perhaps sd ~10 m) in dry winter conditions, so it limits vapour entering the construction during the heating season; and low (vapour-open, perhaps sd <0.5 m) in humid summer conditions, so the wall can dry inward when the vapour gradient reverses. This gives a build-up the protection of a vapour barrier in winter and the drying potential of a breathable layer in summer — particularly valuable for internal wall insulation on solid masonry, where summer inward drying is often the wall's main route back to a safe moisture content.
