Module 2 · Buildings & Moisture Basics
2.5 Temperature, Humidity & Condensation
8 min read
Condensation is the most common damp mechanism in occupied housing and the only one you can predict arithmetically. Three readings — air temperature, relative humidity and surface temperature — tell you whether a given surface will condense. This lesson is the physics behind that, in the form you actually use it.
Learning objectives
By the end of this lesson you will be able to:
- Explain relative humidity and why it changes with temperature alone.
- Define dew point and calculate condensation risk from it.
- Identify the surface temperature margin at which mould becomes likely.
- Explain why cold rooms get damp even when no extra moisture is added.
Key terms
- Relative humidity (RH)
- How much water vapour the air holds as a percentage of the maximum it could hold at that temperature.
- Absolute humidity
- The actual mass of water vapour in the air, independent of temperature. Ventilation changes it; heating does not.
- Dew point
- The temperature at which air becomes saturated. Any surface at or below it collects condensation.
- Surface temperature
- The temperature of the wall or ceiling face itself — usually several degrees below air temperature.
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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.