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Condensation Prevention Calculator — Dew Point & Surface Temperature
Calculate surface temperatures, dew points and surface humidity to screen condensation and mould risk. Uses the BS EN ISO 13788 surface-resistance (Rsi = 0.25 m²K/W) and 80% surface-humidity mould criterion.
UK winter design: -2 to 4°C typical
Optional — gives an indicative 0.5 ACH vent rate (rule of thumb, not an Approved Document F figure)
Overrides wall type selection
BS EN ISO 13788 uses 0.25 for surface condensation/mould (allows for corners, furniture). 0.13 is the BS EN ISO 6946 value for U-value/heat-loss calcs only.
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How We Calculate This
This calculator screens for condensation and mould risk on an internal surface by comparing its surface temperature (calculated from the U-value) with both the dew point (liquid condensation) and the 80% surface-humidity mould limit (calculated from indoor temperature and humidity per BS EN ISO 13788).
Surface temperature calculation
Tsi = Ti − (Rsi × U × (Ti − Te)), where Rsi is the internal surface resistance, U is the thermal transmittance, Ti is the indoor temperature and Te is the outdoor temperature. For surface condensation and mould assessment BS EN ISO 13788 specifies Rsi = 0.25 m²K/W (allowing for corners, furniture and curtains) — not the 0.13 m²K/W value, which BS EN ISO 6946 reserves for U-value and heat-loss calculations.
Mould criterion
Mould grows long before liquid water appears. The tool also computes the surface relative humidity and the temperature factor fRsi = (Tsi − Te)/(Ti − Te). Per BS EN ISO 13788 the surface relative humidity should stay below 80%; BRE IP 1/06 recommends fRsi ≥ 0.75 for dwellings to avoid mould (fRsi ≥ 0.70 avoids liquid condensation).
Risk assessment
- High risk: Surface temp below dew point, or surface RH above the 80% mould limit
- Moderate risk: Within 2°C of the dew point, or surface RH above 70%
- Low risk: 2–5°C margin and surface humidity comfortably below the mould limit
- Minimal risk: 5°C+ margin — very unlikely
This is an instantaneous screening check using the temperatures you enter (e.g. UK winter design conditions). A full BS EN ISO 13788 mould assessment instead uses monthly-mean climate data (mean monthly temperatures to a 1-in-10-year return, or the design year minus 2 K).
Frequently Asked Questions
Condensation forms when the surface temperature of a wall or window drops below the dew point of the indoor air. Warm moist air contacts the cold surface and the moisture condenses into water droplets. This is most common on single-glazed windows, uninsulated solid walls, and cold bridges. The dew point depends on both temperature and relative humidity.
At typical UK indoor temperatures (18-21°C), condensation risk increases significantly above 60% relative humidity. At 70% RH and 20°C, the dew point is 14.4°C — any surface below this temperature will see condensation. Aim to keep indoor RH between 40-55% through adequate ventilation and heating.
The dew point is calculated using the Magnus formula: Td = (237.7 × alpha) / (17.27 - alpha), where alpha = (17.27 × T)/(237.7 + T) + ln(RH/100). This calculator performs this calculation automatically from your room temperature and relative humidity inputs.
A lower U-value means better insulation and higher internal surface temperatures, which keeps the surface above the dew point and below the 80% surface-humidity mould limit. Approved Document L 2021 (England) limits new-build walls to 0.26 W/m²K and new walls in extensions or existing dwellings to 0.18 W/m²K — both far better than older uninsulated solid walls (U≈2.1) or single glazing (U≈5.0), which are most at risk. Improving insulation is the most effective long-term solution.
Yes. Mould can germinate when the relative humidity at a surface stays above about 80%, which happens at surface temperatures several degrees above the dew point. BS EN ISO 13788 therefore uses an 80% surface-humidity limit, and BRE IP 1/06 recommends a temperature factor fRsi of at least 0.75 for dwellings (fRsi above 0.70 avoids liquid condensation). Surfaces can pass a simple dew-point check yet still grow mould, so this calculator reports both the dew-point margin and the surface relative humidity.
For surface condensation and mould risk, BS EN ISO 13788 specifies an internal surface resistance of 0.25 m²K/W to represent the worst case in corners and behind furniture or curtains, where air movement is restricted and the surface runs colder. The familiar 0.13 m²K/W (and 0.10 for ceilings) belongs to BS EN ISO 6946 and is used only for U-value and heat-loss calculations. Using 0.13 for a risk check warms the predicted surface by roughly 3°C and understates condensation and mould risk.
Yes, adequate ventilation removes moisture-laden air and replaces it with drier outside air. Even cold outside air at 90% RH contains far less moisture than warm indoor air at 60% RH. Approved Document F requires background ventilation to control moisture. Trickle vents, extract fans in kitchens/bathrooms, and MVHR systems all help reduce indoor humidity.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.