Basement ventilation in winter
Sounds counterintuitive but is physically logical: winter is the best season to dehumidify your basement. Cold winter air contains significantly less moisture than warm summer air.
Why does ventilation work better in winter?
The physics is simple: cold air can hold less water vapor than warm air. Outdoor air at 0°C and 80% relative humidity contains absolutely less water than basement air at 15°C and 70%.
An example:
Basement
15°C, 70% RH
Dew point: 9.3°C
Outdoor air (winter)
2°C, 85% RH
Dew point: −0.2°C
Difference: 9.5°C → Ventilation is very beneficial!
In summer it would be the opposite: outdoor air at 28°C and 75% has a dew point of 23.2°C – much higher than the basement air. Ventilation would bring moisture in.
When NOT to ventilate in winter
Even in winter there are exceptions:
- During rain or fog: Outdoor air is nearly saturated (close to 100% RH). Ventilation brings no benefit.
- At mild winter temperatures (above 10°C): On warm winter days, outdoor air can be more humid than expected.
- At extreme cold (below −10°C): Water-carrying pipes could freeze. Brief burst ventilation instead of continuous.
Proper ventilation: how to do it
- Check dew point: Use the calculator below. Difference ≥ 3°C = ventilation makes sense.
- Burst ventilation instead of tilting: Opening windows wide for 10–15 minutes is more effective than tilted windows for hours.
- Several times daily: Ventilating 2–3 times a day is better than once for a long time.
- Or automate: A Shelly sensor measures continuously and switches a fan via relay when needed. No more manual checking.
Check now: is ventilation worthwhile right now?
Indoor
20 °C
-10 °C40 °C
65 %
10 %100 %
Outdoor
5 °C
-20 °C40 °C
80 %
10 %100 %