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 (%). This approximation is reliable to within about ±0.5°C for RH ≥ 50%; below that it over-estimates the dew point (so it errs on the side of caution and never hides real condensation).

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 — typical retrofit cavity-fill (note: the current Part L limit is 0.26 W/m²K for a new-build wall and 0.18 W/m²K for an extension, so a brand-new wall must do better than 0.3)

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 — liquid condensation likely

The “high risk” band uses the dew point (100% surface saturation), i.e. the point at which visible liquid water forms. The 5°C low-risk buffer is a practical convention rather than a code figure. For mould growth specifically, BS EN ISO 13788 uses a stricter test — a critical surface relative humidity of 80% (factor φ_si,cr= 0.8), meaning mould can develop on a surface that is warmer than the dew point but still cold enough to sit above 80% RH. Treat a “medium” result as a prompt to improve ventilation and insulation, not an all-clear.

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.