Air always contains some water vapour. How much it can hold depends entirely on its temperature: warm air can hold a lot, cold air very little. The dew point is the temperature to which you would have to cool a given parcel of air for it to become saturated — to reach 100% relative humidity — at which point any further cooling forces the vapour to condense into liquid water. It is a property of the air's actual moisture content, not of any surface.
Why warm air holds more moisture
The amount of water vapour air can hold rises steeply with temperature — the saturation vapour pressure roughly doubles for every 10–11 °C of warming. This is why a bathroom mirror fogs the instant warm, moist air meets the cold glass: the air touching the glass is chilled below its dew point and dumps its excess moisture. It is also why winter condensation is worse: cold external walls and windows sit closer to (or below) the indoor air's dew point.
The relationship between temperature, humidity and moisture content is described by psychrometrics — and plotted on a psychrometric chart. You don't need the chart to use dew point diagnostically; you need just two measurements: the air's temperature and relative humidity (which together fix the dew point), and the temperature of the surface you're worried about.
Surface temperature vs dew point — the core diagnostic
Condensation forms on a surface when, and only when, that surface is colder than the dew point of the air touching it. This simple rule is the foundation of measured damp diagnosis:
| Condition | Result |
|---|---|
| Surface temperature well above dew point | Surface stays dry |
| Surface temperature near dew point (small margin) | At risk; elevated surface RH → mould risk |
| Surface temperature at or below dew point | Condensation forms (visible liquid water) |
On a survey we measure room air temperature and RH (giving the dew point), then use a calibrated thermal camera to read the surface temperature across walls, reveals, lintels and corners. Wherever a surface is at or below the dew point, the thermal image and the physics agree: that is where condensation and mould will appear. It removes the guesswork from a diagnosis.
The dewpoint margin — and why mould beats condensation to it
Here is the subtlety that trips people up: you do not need liquid condensation to grow mould. Mould germinates when the relative humidity at a surface stays above roughly 80% for sustained periods — and a surface reaches 80% surface RH while it is still several degrees above the dew point. In other words, mould appears on surfaces that are cold but not actually wet.
Using dew point to choose the right fix
Once you frame a damp problem in dew-point terms, the remedy becomes obvious. There are only two levers:
- Raise the surface temperature above the dewpoint margin — internal or external wall insulation, thermal-bridge correction at lintels and reveals, eliminating cold spots. This is usually the durable structural fix.
- Lower the air's dew point by reducing indoor humidity — controlled ventilation (extract or MVHR), source control (lids on pans, venting tumble dryers, drying washing outside) and avoiding unflued moisture sources.
Most successful remediations use both: lift the cold surfaces and control the humidity, so the surface temperature and the dew point move apart and never meet. A chemical 'damp treatment' addresses neither lever, which is why it so often fails on what is actually a condensation problem.