How does condensation form on walls?

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.

What relative humidity causes condensation in UK homes?

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.

How do I calculate the dew point?

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.

What U-value prevents condensation?

A lower U-value means better insulation and higher internal surface temperatures. Modern Building Regulations require walls with U-values of 0.3 W/m²K or better, which generally prevents surface condensation in normal conditions. Older uninsulated solid walls (U=2.1) and single glazing (U=5.0) are most at risk. Improving insulation is the most effective long-term solution.

Can ventilation prevent condensation?

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.

TradeCalculator

Condensation Prevention Calculator — Dew Point & Surface Temperature

Calculate surface temperatures and dew points to assess condensation risk. Uses U-value and internal surface resistance per BS EN ISO 13788.

Wall / Surface Type

Determines U-value for surface temp calculation

Indoor Temperature

Outdoor Temperature

UK winter design: -2 to 4°C typical

Indoor Relative Humidity

How We Calculate This

This calculator determines whether condensation will form on internal surfaces by comparing the surface temperature (calculated from the U-value) with the dew point (calculated from indoor temperature and humidity).

Surface temperature calculation

Tsi = Ti - (Rsi x U x (Ti - Te)), where Rsi is the internal surface resistance (0.13 m²K/W for walls per BS EN ISO 6946), U is the thermal transmittance, Ti is the indoor temperature, and Te is the outdoor temperature.

Risk assessment

High risk: Surface temp below dew point — condensation will form
Moderate risk: Within 2°C margin — likely in cold spells
Low risk: 2-5°C margin — generally safe
Minimal risk: 5°C+ margin — very unlikely

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.