Basement humidity too high?
A relative humidity above 65% in the basement is a warning sign. Above 70%, mold risk increases dramatically. Here you'll learn what values are normal and what you can do.
What humidity is normal in a basement?
| Range | Humidity | Rating |
|---|---|---|
| Below 40% | Too dry | Rare in basements, can occur with heavy heating |
| 40–60% | Ideal | No mold risk, optimal climate |
| 60–70% | Elevated | Action recommended, especially on cold surfaces |
| Above 70% | Critical | Mold risk, immediate action needed |
Why is the basement humidity so high?
- Rising moisture: Without a horizontal barrier, water is drawn from the ground through the masonry.
- Wrong ventilation: In summer, many basement owners ventilate with open windows – warm outdoor air condenses on the cold basement walls and increases humidity.
- Drying laundry: Each wash cycle releases 2–5 liters of water into the air.
- Leaky pipes: Dripping water pipes or defective seals.
- Poor ventilation: Basements without windows or exhaust have no natural air exchange.
How to reduce humidity
- Ventilate based on dew point: Only ventilate when outdoor air is drier. Use the calculator below to check.
- Use a dehumidifier: For persistent levels above 70%, an electric dehumidifier helps as an immediate measure (note power costs: 200–600 kWh/year).
- Fix the cause: Stop rising moisture with a horizontal barrier or exterior waterproofing.
- Heating helps: Warm air can hold more moisture. Even light heating reduces relative humidity.
- Automate: Sensors measure 24/7 and control ventilation automatically via relay.
Calculator: does ventilation help right now?
Indoor
20 °C
-10 °C40 °C
65 %
10 %100 %
Outdoor
5 °C
-20 °C40 °C
80 %
10 %100 %