Module 2 · Buildings & Moisture Basics
2.6 Vapour, Capillary Action & Moisture Pathways
7 min read
The single most disorientating thing about damp is that the symptom and the cause are frequently in different places, sometimes on different floors. This lesson covers the three transport mechanisms, so that when you find damp you can reason backwards along the path instead of assuming the defect must be behind it.
Learning objectives
By the end of this lesson you will be able to:
- Describe the three ways moisture moves through a building.
- Explain capillary action and what limits how high water can rise.
- Trace a plausible pathway backwards from a symptom to candidate sources.
- Explain why moisture emerges where it does rather than where it entered.
Key terms
- Vapour diffusion
- Water vapour spreading through air and porous materials from higher to lower concentration.
- Capillary action
- Liquid water drawn through fine pores by surface tension, able to move in any direction including upward.
- Interstitial condensation
- Condensation forming inside a construction rather than on its surface, where warm vapour reaches a cold layer within the build-up.
- Path of least resistance
- The route water actually takes — along a joist, down a cavity, behind a render — rather than the shortest one.
Free — enrol to continue
Read the rest of Buildings & Moisture Basics
Module 1 is open to everyone. The remaining six modules — building science, each damp mechanism in depth, meters and thermal imaging, the full inspection method, and report writing — are free too. We just ask for an email so you can pick up where you left off, and so we can send your certificate when you finish.