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How to Work Out Dew Point and Wet Bulb
Manesh Jayawardhana
CIO & Co-founder
Relative humidity is the humidity figure everyone quotes and the least useful one for predicting anything, because it depends on temperature as much as on moisture.
Air at 24°C and 65% relative humidity, cooled to 17°C, is at 100% relative humidity — with exactly the same amount of water in it. Nothing was added. The air’s capacity changed.
Dew point is the number that predicts condensation
Dew point is the temperature at which air becomes saturated with the moisture it currently holds. Cool anything below it and water condenses on it.
That makes it directly actionable in a way relative humidity isn’t. Air at 24°C and 65% RH has a dew point of about 17.1°C, which means:
- A window whose inner surface is below 17.1°C will run with condensation.
- A cold water pipe below 17.1°C will sweat.
- A thermal bridge in a wall — a lintel, a junction, an uninsulated section — below 17.1°C will collect moisture, and eventually mould.
Unlike relative humidity, dew point doesn’t change when air moves between rooms at different temperatures. It only changes when moisture is added or removed. That makes it the stable, portable measure of how much water is actually present.
Absolute humidity for comparing conditions
Absolute humidity — grams of water per kilogram of dry air — is the direct measure of moisture content.
It’s what you need when comparing air at different temperatures. Outdoor air at 5°C and 90% RH sounds damp and contains far less moisture than indoor air at 21°C and 50% RH. Which is why ventilating with cold outdoor air dries a house, even though the outdoor air has higher relative humidity.
That result surprises people every winter and it’s straightforward once you look at the absolute figures.
| Air | RH | Absolute humidity |
|---|---|---|
| 5°C outdoor | 90% | ~4.9 g/kg |
| 21°C indoor | 50% | ~7.8 g/kg |
| 24°C indoor | 65% | ~12.2 g/kg |
Wet bulb temperature
The temperature a thermometer reads with a wet wick around its bulb, air moving past it. Evaporation cools the bulb, and how much depends on how dry the air is.
Two practical uses.
Evaporative cooling. Wet bulb is the theoretical floor for evaporative cooling — a swamp cooler cannot cool air below it, which is why they work in dry climates and not in humid ones.
Human heat tolerance. The body cools by sweating, which is evaporation. When wet bulb temperature approaches body temperature, sweat stops being able to shed heat, and no amount of shade or water helps. Sustained wet bulb temperatures above about 35°C are regarded as beyond the limit of human survivability, which is why wet bulb rather than air temperature is the measure used in extreme heat warnings.
Common mistakes to avoid
- Using relative humidity to compare air at different temperatures.
- Measuring room humidity and concluding a wall is safe — the surface temperature at a cold bridge is what matters, not the room average.
- Assuming ventilating with humid-feeling cold air makes a house damper.
- Reading a dew point at one location and applying it after the air has been heated, which changes RH but not dew point.
- Ignoring altitude in a psychrometric calculation, where pressure affects the relationships.
How to do it with Psychrometric Calculator
The Psychrometric Calculator derives the full set from any two properties.
- Enter two known properties — dry bulb plus relative humidity is the usual pair.
- Read the dew point, which tells you the surface temperature at which condensation starts.
- Use absolute humidity when comparing air at different temperatures.
- Check wet bulb if evaporative cooling or heat stress is the question.
The CIBSE and ASHRAE both publish psychrometric guidance for building applications. Other engineering calculators are in the tools directory.
Frequently asked questions
Why does relative humidity mislead?
Because it depends on temperature as well as moisture. Air moved from a cold room to a warm one has the same moisture and a much lower relative humidity, which is why heated winter air feels dry.
What is dew point actually useful for?
Predicting condensation. Any surface colder than the dew point collects moisture — the basis of insulation design, cold bridge analysis, and why a glass of iced water sweats.
Why does wet bulb temperature matter?
It sets the limit for evaporative cooling and, in extreme heat, for human survivability — sweat cannot cool you below the wet bulb temperature no matter how much you produce.
Final thought
Measure dew point rather than relative humidity when you’re chasing damp. It’s the number that tells you which surfaces are at risk, and it doesn’t change as the air moves around the building.