Drying and site conditions
Surface condensation risk (surface temperature against dew point)
- where
- intermediate Magnus term (dimensionless)
- relative humidity (%)
- air temperature (°C)
- dew point temperature (°C)
- surface temperature above dew point (°C)
- surface temperature (°C)
- margin over the required clearance (°C)
- required margin above dew point, 3 °C by BS EN ISO 8502-4 (°C)
Explain this equation
Checks whether a background is warm enough to stay dry, by comparing its surface temperature with the dew point of the room or outside air. Use it before applying primers, bonding agents, render or decoration to cold walls.
The first two lines work out the dew point Td from air temperature T and relative humidity RH (Magnus formula). Ts is the surface temperature from an infrared thermometer. S is how far the surface is above the dew point. Mr is the safety margin you require, and M is what is left after that margin.
If a surface is at or below the dew point, water forms on it and materials will not bond or dry properly. The 3 °C margin comes from the coatings standard BS EN ISO 8502-4 and allows for meter error and for the surface cooling further.
Sense check: on a damp morning at 12 °C and 85% RH the dew point is about 9.6 °C, so a wall at 9 °C is already wet. At 20 °C and 50% RH, a wall at 15 °C clears the dew point by nearly 6 °C.
Checks whether a wall or ceiling is warm enough above the dew point to stay dry, using an infrared surface reading and a hygrometer.
Enter your values to see the result.
Good to know
The 3 °C margin is the coatings industry rule from BS EN ISO 8502-4 (and ASTM D3276): it allows for meter error and for the surface cooling further. It is a sensible check before applying primers, bonding agents, render or decoration to cold external walls and in unheated new builds. Condensation on the background also means plaster or render may not bond and will dry much more slowly.
Common mistake
Measuring the air but not the surface. Solid walls, steel lintels and concrete soffits are often several degrees below the air temperature in the morning.
Limits
Point check only: conditions change through the day. Dew point by the Magnus formula (−40 to 50 °C). The 3 °C margin is from paint and coating practice; plaster and render manufacturers state temperature limits rather than a dew point margin.
Sources
- BS EN ISO 8502-4:2017 Preparation of steel substrates before application of paints: guidance on estimation of the probability of condensation (surface at least 3 °C above dew point)
- DeFelsko: Measuring environmental conditions (restates the ISO 8502-4 / ASTM D3276 3 °C above dew point rule)
- M. G. Lawrence, The relationship between relative humidity and the dewpoint temperature in moist air, Bulletin of the American Meteorological Society 86(2), 2005, pp. 225–233 (Magnus form with Alduchov–Eskridge constants 17.625 and 243.04 °C)
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the manufacturers’ system and product data, BS 8212, BS EN 13914 and Building Regulations. Coverage rates vary by product, background and thickness. Check prices, VAT and CIS rules with your own records and HMRC.