Why do my double-glazed windows get condensation?

Condensation forms when the internal surface temperature of the glass drops below the dew point of the room air. This is most common in high-humidity rooms (kitchens, bathrooms) during cold weather. Even modern double glazing can suffer condensation if indoor humidity is excessive or ventilation is poor. Keeping relative humidity below 55-60% significantly reduces the risk.

What is the dew point?

The dew point is the temperature at which air becomes saturated and water vapour condenses into liquid. It depends on both air temperature and relative humidity. For example, at 21°C and 60% relative humidity, the dew point is approximately 12.9°C. If any surface is at or below this temperature, condensation will form on it.

Is condensation between the panes a fault?

Yes. Condensation between the panes of a sealed double glazed unit indicates the perimeter seal has failed, allowing moist air into the cavity. This cannot be fixed — the sealed unit must be replaced. This is different from condensation on the inside surface of the inner pane, which is caused by room conditions.

Will triple glazing eliminate condensation?

Triple glazing significantly reduces condensation risk by keeping the internal surface temperature much higher. However, it cannot eliminate condensation entirely if indoor humidity is very high and ventilation is inadequate. Controlling moisture at source (extractor fans, trickle vents) is equally important as glazing specification.

What relative humidity should I maintain indoors?

The recommended indoor relative humidity for UK homes is between 40% and 60%. Below 40% can cause dry skin and respiratory irritation. Above 60% increases condensation and mould risk, particularly in cold weather. In kitchens and bathrooms, humidity can spike above 80% during cooking and bathing — mechanical extract ventilation is essential in these rooms.

TradeCalculator.co.uk

Window Condensation Calculator — Dew Point & Risk Assessment

Assess window condensation risk from room temperature, humidity, external temperature and window U-value. Uses the Magnus-Tetens dew point formula.

Room Temperature

Typical UK living room: 18-21\u00B0C

Relative Humidity

Measured or estimated indoor RH

External Temperature

Overnight outside temperature

Window U-Value

Whole-window U-value

Room TypeAffects humidity recommendations

VentilationCurrent ventilation provision

How We Calculate This

This calculator uses two key calculations to assess condensation risk on window surfaces.

Dew point (Magnus-Tetens formula)

Td = (b × α) ÷ (a - α)

Where α = (a × T) ÷ (b + T) + ln(RH/100), with constants a=17.27, b=237.7

Internal surface temperature

Tsi = Ti - Uw × Rsi × (Ti - Te)

Tsi = internal surface temperature of window (°C)
Ti = indoor air temperature, Te = outdoor temperature
Uw = whole-window U-value (W/m²K)
Rsi = internal surface resistance (0.13 m²K/W for vertical glazing)

Condensation assessment

If Tsi < Td, condensation will form on the glass surface.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.