Site conditions
Minimum application temperature and humidity check
- where
- lowest relevant temperature (°C)
- air temperature now (°C)
- surface temperature now (°C)
- lowest forecast temperature until touch dry (°C)
- margin above the product minimum (°C)
- product minimum application temperature (°C)
- margin below the product maximum humidity (%)
- product maximum relative humidity (%)
- relative humidity now (%)
Explain this equation
Checks that conditions meet the paint data sheet before you start: air, surface and forecast temperature above the minimum, and humidity below the maximum.
Ta, Ts and Tn are the air now, the surface now and the coldest the forecast expects before the coat is touch dry. Tlow is the worst of the three. Tmin and RHmax come from the data sheet. M and R are the margins; both must be zero or more.
Paint has to stay above its minimum temperature until it has formed a film, so the lowest of the three readings is the one that counts. Humidity is a simple upper limit because damp air slows drying and can spoil the finish.
Sense check: an 8 °C minimum emulsion on a morning with a 9 °C room but a 7 °C forecast fails by 1 °C. A 7 °C masonry paint with everything at 9 °C or above passes with 2 °C to spare.
Watch out: also check the surface is at least 3 °C above the dew point.
Checks air, surface and forecast temperature against the paint's minimum, and humidity against its maximum, before you open the tin.
Enter your values to see the result.
Good to know
Typical published minimums: Dulux Trade Diamond Matt 8 °C (citing BS 6150), Weathershield Smooth Masonry 7 °C with no application above 80% RH, Tikkurila masonry paints 5 °C air or substrate. Water-based paints below their minimum may not form a proper film and can crack or chalk; cold also lengthens drying and recoat times. Exterior paint usually needs protection from rain for the first day or two.
Common mistake
Starting on a cold morning because the air has warmed up while the wall is still cold, or ignoring an overnight dip before the coat is dry.
Limits
Simple comparison against the data sheet figures you enter. Does not cover direct sun or hot surfaces (many data sheets set an upper limit too) or wind.
Sources
- Dulux Trade Diamond Matt product data sheet (volume solids 49% white; minimum application temperature 8 °C per BS 6150; recoat 4–6 h; VOC max 1 g/L against 30 g/L limit, cat. A/a; thin up to 1 part water to 10 parts paint)
- 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)
- Tikkurila: Does your external masonry paint have a minimum temperature for use? (not below 5 °C air or substrate; protect from rain for 48 h)
- BS 6150:2019 Painting of buildings – Code of practice
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.