Will opening a window reduce condensation?

The forecast says 90% humidity, your room says 70%, so opening a window must make it worse — right? Usually not. Enter both and see what actually happens to the moisture.

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Why "90% outside" is usually drier than "70% inside"

Relative humidity is a percentage of how much water the air could hold at its current temperature. Cold air holds very little. Air at 5 °C and 90% humidity contains about 6.1 grams of water per cubic metre. Air at 18 °C and 70% contains about 10.7 g/m³ — nearly twice as much.

When that cold outdoor air comes inside and warms to 18 °C, its relative humidity falls to around 38%. So swapping room air for outdoor air removes water, even on a misty day. This is the principle behind trickle vents, extract fans and positive-input ventilation.

When opening a window doesn't help much

  • Warm, muggy summer weather, when outdoor air genuinely holds as much water as indoor air. The calculator will show this.
  • When it chills the room a lot. If the room temperature drops and walls cool, cold surfaces can end up closer to dew point. Short, wide openings (5–10 minutes) exchange air with less cooling than a window left ajar all day.
  • When the problem isn't humidity at all — rain getting in, a leak or a failed double-glazed unit. See damp or condensation?

Ventilation costs heat — how much?

Every cubic metre of air you swap needs heating back up. As a rough guide, one complete air change in a 25 m³ bedroom with a 14 °C indoor–outdoor difference takes about 0.12 kWh of heat — around 1p with a gas boiler or 3p with direct electric heating. A window on the latch overnight might add several air changes an hour. The full assessment calculates this trade-off for your room alongside a dehumidifier and other options.

Best habits for ventilating in winter

  1. Keep trickle vents open all the time — they're designed for exactly this.
  2. Run bathroom and kitchen extract fans during and for 15–20 minutes after showers and cooking, with the door closed so moisture doesn't spread.
  3. Open bedroom windows wide for a few minutes in the morning rather than slightly all night, unless you can keep the room warm.
  4. Close internal doors to wet rooms; leave a small gap under the door so fans can draw air in.