Site conditions
Surface temperature margin above dew point (3 °C rule)
- 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 (°C)
Explain this equation
A go/no-go check before painting: is the surface warm enough above the dew point that condensation will not form under the wet paint?
T and RH give the dew point Td, as in the dew point calculator. Ts is the measured surface temperature. S is how far the surface sits above the dew point, and M is what remains after subtracting the required margin Mr, normally 3 °C.
Condensation forms on any surface at or below the dew point. The 3 °C margin from BS EN ISO 8502-4 allows for instrument error and for the surface cooling while the paint dries, so the test is simply surface minus dew point minus margin.
Sense check: on a 12 °C, 85% RH morning the dew point is about 9.6 °C, so a 9 °C wall is already wet. At 20 °C and 50% RH a 15 °C surface clears the margin by about 2.7 °C. M must be zero or positive to paint.
Watch out: steel and north-facing walls lag behind the air temperature, so measure the actual surface.
A go/no-go check before painting: the surface must be at least 3 °C warmer than the dew point of the surrounding air.
Enter your values to see the result.
Good to know
The 3 °C margin comes from BS EN ISO 8502-4 and is applied in ISO 12944-7 for coating work; it allows for meter error and for the surface cooling further while the paint dries. Check it on exterior joinery, metalwork and render first thing in the morning and late afternoon, and in unheated new builds. Many masonry paints also give a maximum relative humidity (Weathershield Smooth: not above 80% RH).
Common mistake
Measuring the air but not the surface. Steel, glass-adjacent frames and north-facing walls are often several degrees below the air temperature.
Limits
Point check only; conditions change. Dew point by the Magnus formula (−40 to 50 °C). Always also meet the data sheet's minimum temperature.
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)
- ISO 12944-7:2017 Paints and varnishes – Corrosion protection of steel structures by protective paint systems – Part 7: Execution and supervision of paint work
- 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)
- Dulux Trade Weathershield Smooth Masonry Paint data sheet (volume solids 38% white; do not apply below 7 °C or above 80% RH; recoat 2–4 h; VOC max 10 g/L against 40 g/L limit, cat. A/c; airless thinning up to 1 part water to 5 parts paint)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Coverage depends on the product, surface and application. Always follow the paint and wallcovering manufacturers’ data sheets and BS 6150 / BS 8000-12. Check prices and VAT with your own records and HMRC.