Vapour flow is the amount of water vapour moving through a surface of a building element per unit time, expressed in grams or kilograms per square metre per hour (or second). It measures actual moisture transport through walls, roofs and floors and plays a central role in assessing condensation risk and how a structure dries out.
How does vapour flow arise?
A vapour flow starts as soon as there is a vapour pressure difference between two sides of a material. The direction follows the pressure gradient: from high to low. The size of the flow depends on:
- The pressure difference (Δp) between inside and outside
- The vapour resistance (Sd-value) of every layer
- The thickness and build-up of the structure
- The temperature on both sides
Vapour flow by diffusion alone is usually small. Air leaks cause a much larger flow: a single hole of a few square centimetres can transport many times more moisture than a square metre of vapour-open wall.
Application
Vapour flow is calculated for:
- Building physics design — determining how much moisture a structure can handle without damage
- Damp remediation — locating where the flow condenses
- Retrofit insulation reviews — checking whether existing structures can manage the added moisture load
- Energy performance — wet insulation loses much of its thermal resistance
Vapour flow versus air flow
Both transport moisture but are physically different. Vapour flow is molecular diffusion through an (air-tight) material. Air flow is convective transport through gaps and cracks. For a healthy structure, airtightness matters more than the Sd-value of the individual layers.
Related terms
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