The dew point is the temperature at which water vapour in the air becomes saturated and starts to condense into liquid water. It is a core concept in building physics because condensation forms exactly where a surface or a layer in the structure is colder than the dew point of the adjacent air.
How is the dew point determined?
The dew point depends on two things: air temperature and relative humidity (RH). Examples:
| Air temperature | RH | Dew point |
|---|---|---|
| 20 °C | 50 % | 9.3 °C |
| 20 °C | 70 % | 14.4 °C |
| 20 °C | 90 % | 18.3 °C |
| 23 °C | 60 % | 14.9 °C |
The higher the humidity, the closer the dew point is to the air temperature and the faster condensation forms on a cold surface.
Application in construction
The dew point matters for:
- Thermal bridge condensation — on window frames, concrete beams and other cold patches of the envelope
- Interstitial condensation — inside a wrongly built up insulated wall or roof
- Mould growth — in cold corners behind furniture and in unheated rooms
- Misted windows — single glazing in older houses
A rule of thumb: keep every interior surface (including detail junctions) above roughly 13 °C under design conditions. That keeps them safely above the dew point of normal indoor air.
Lowering the dew point
The dew point of indoor air can be lowered by:
- Ventilating — extracting moist indoor air, supplying drier outdoor air
- Heating — warmer air can hold more moisture without saturating
- Limiting moisture sources — using extraction when cooking and showering, drying laundry outside
Related terms
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