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Dew point calculator
Is it safe to coat today? Enter the air temperature, relative humidity and surface temperature. You get the dew point, the margin and a clear yes or no, with the working shown.
How the calculation works
The dew point is the temperature at which the moisture in the air starts to condense. If the surface you are coating is at or near it, a thin film of water forms on it, and the coating loses adhesion, blushes or blisters.
Dew point = 243.12 × γ ÷ (17.62 − γ)
Safe to apply when surface temperature − dew point ≥ 3 °C (5.4 °F)
T is the air temperature in °C and RH the relative humidity in %. This is the Magnus formula used by most dew point meters.
Site tips
- Measure the surface, not the air. Slabs on grade are often several degrees colder than the room.
- Check again as conditions change: evening cooling is when most condensation problems start.
- Record the readings with the time. If a floor fails later, your records are your defence.
Questions
Why 3 °C above the dew point?
It is the usual rule for coatings and resin floors, and it appears in ISO 8502-4. A concrete or steel surface can cool quickly, so a margin of 3 °C (5.4 °F) keeps moisture from forming under the coating. Always follow the manufacturer's data sheet if it sets a different limit.
How do I measure the surface temperature?
Use a contact or infrared surface thermometer on the actual floor, wall or steel, not the air. Check the coldest spots: near doors, slabs on grade, shaded areas and early morning.
What humidity is too high?
Many coating data sheets set a maximum relative humidity of 85% during application and curing. Check yours: some products allow more, some less.
How accurate is the calculation?
It uses the Magnus formula, which is accurate to a fraction of a degree in normal site conditions. Your thermometer and hygrometer are the bigger source of error, so leave a margin.
Does this replace moisture testing of the slab?
No. Dew point is about condensation on the surface during application. Moisture inside the slab needs its own test, such as in-situ relative humidity or the method on your data sheet.