Condensation Risk Calculator — Dew Point & Surface Condensation Check

This calculator estimates whether surface condensation will occur on your walls by comparing the wall’s internal surface temperature with the dew point temperature of the room air.

Dew point calculation

The dew point is calculated using a simplified version of the Magnus formula:

Td ≈ T − (100 − RH) ÷ 5

Where Td = dew point (°C), T = room temperature (°C), RH = relative humidity (%)

Surface temperature estimation

The internal wall surface temperature is estimated from the room temperature, external temperature and wall U-value:

Tsi = Ti − (Rsi × U × (Ti − Te))

Where Tsi = internal surface temperature, Ti = internal air temperature, Te = external temperature, Rsi = internal surface resistance (0.13 m²K/W), U = wall U-value

U-values used

• Solid wall (uninsulated): 2.1 W/m²K — typical pre-1920s construction
• Cavity wall (unfilled): 1.5 W/m²K — typical 1930s–1980s construction
• Insulated cavity wall: 0.3 W/m²K — meets current Building Regulations Part L

Risk assessment

• Low risk: Surface temp more than 5°C above dew point
• Medium risk: Surface temp 0–5°C above dew point
• High risk: Surface temp at or below dew point — condensation likely

Standards

Condensation risk assessment should follow BS 5250:2021 (Management of moisture in buildings) and Approved Document Part C. For detailed interstitial condensation analysis, the Glaser method or dynamic simulation tools (WUFI) are used. This calculator provides a simplified surface condensation check only.