Moisture transport is the collection of mechanisms by which water vapour moves through a building element: mainly vapour diffusion and convection (air movement). It determines whether a structure becomes damp or stays dry and is therefore a fundamental concept in building physics.
Two mechanisms
Moisture transport happens in two ways:
- Vapour diffusion — molecular movement through materials driven by vapour pressure differences. Described by the Sd-value of a material.
- Convective transport — water vapour carried along by air streaming through gaps, seams and poorly sealed joints. Most damp damage in poorly detailed buildings starts here.
In a well-detailed assembly, diffusion dominates and convection is negligible. In practice, unfinished joints, cable penetrations and untaped seams cause the largest share of moisture problems.
Application
Understanding moisture transport matters for:
- Façade detailing — choosing the vapour control layer and breather membrane
- Renovation and retrofit insulation — preventing interstitial condensation
- Timber-frame construction — critical to avoid timber rot
- Damp surveys in existing buildings — tracing the source of mould or thermal-bridge condensation
Why airtightness matters
Studies show that a leaky structure can transport hundreds of times more moisture at the same vapour pressure than an air-tight wall. Air-tight construction is therefore just as important as choosing the right membrane. A blower-door test makes leaks visible.
Related terms
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