Basements often feel cool and a little damp, which is no surprise. But humidity that is too high can lead to moisture and mould. Here is the level you should aim for in the basement and how to keep it.
Short answer: In a basement the relative humidity should ideally stay below 60 percent. Basements are often cooler and damper than the rest of the house, especially in summer. Measure with a hygrometer and lower the moisture with ventilation and a dehumidifier if it is too high.
What humidity should you have in the basement?
Aim for below 60 percent relative humidity in the basement, ideally around 50 to 60 percent. It is the same upper guideline as for the rest of the home, but basements reach it more easily because they are cooler. If the level stays above 60 to 70 percent the risk of moisture and mould is raised.
Why does the basement get damp?
Basements are damper for several reasons. Walls and floors against the ground are cool, and moisture can enter from outside. A common cause is so-called summer condensation: when warm, humid summer air enters the cool basement it cools down, and the moisture settles on cold surfaces. That is why the basement can actually be damper in summer than in winter.
How to measure the humidity in the basement
Use a hygrometer and place it in the middle of the space, not right against a cold exterior wall. Read it at different times and seasons, since the level varies a lot between summer and winter. A more detailed walkthrough is in the guide on measuring moisture at home.
How to lower high humidity in the basement
- Air out at the right time. Air out when the outdoor air is drier and cooler, often in the morning or evening, rather than on warm, humid summer days.
- Review the ventilation so air can circulate in the space.
- Lower the moisture with a dehumidifier. If the humidity stays high a dehumidifier can lower the relative humidity. In cool basements a desiccant dehumidifier often works better than a compressor one. We explain the difference in the guide on compressor and desiccant dehumidifiers.
When moisture has already caused problems
If the basement has already been affected by moisture or mould, lowering the humidity is not enough on its own; the cause needs to be addressed. Read more about moisture in the basement, how to get rid of moisture in the basement and about mould to know how to proceed.
Frequently asked questions
What humidity should you have in a basement?
Ideally below 60 percent relative humidity, around 50 to 60 percent. If the level stays above 60 to 70 percent the risk of moisture and mould is raised.
Why is the basement damper in summer?
Because of summer condensation. When warm, humid summer air enters the cool basement it cools down and the moisture settles on cold surfaces. That is why the basement can be damper in summer than in winter.
How do I lower the humidity in the basement?
Air out when the outdoor air is dry and cool, review the ventilation, and use a dehumidifier if the level stays high. In cool basements a desiccant dehumidifier often works best.
Which dehumidifier suits a basement?
In cool spaces a desiccant dehumidifier often works better than a compressor one, because it handles low temperatures well. Which one suits depends on the basement's temperature and size.
