What is the Ideal Humidity for a Finished Basement?

The ideal moisture level for a basement, whether finished or unfinished, is 30 to 50 percent relative humidity. During summertime, outside moisture pours in through basement walls and windows, bumping this figure up to 65 to 70 percent in much of the United States and Canada.

Even if you don't have yet have a moisture meter, also called a hygrometer, you'll know you have a problem by the damp, earthy and chilly sensation you get as you descend the stairs into your basement.

Winter Targets

In winter, a heated finished basement should dry out considerably and naturally fall below 50 percent relative humidity -- as long as you keep the heat at 65 degrees Fahrenheit or greater.

If the outdoor temperature is exceptionally cold, such as below 0 degrees Fahrenheit, aim for even lower than the 30 to 50 percent range. At -10 to 0 degrees, for example, the humidity should be around 25 percent to avoid ice on the inside of basement windows.

Steps to Take

You can adjust your basement humidity so that it becomes a conditioned, usable, comfortable space.

  • Add a dehumidifier to the finished basement and drain it into the existing main stack or a feeder drain, or into a utility sink. If the dehumidifier doesn't have a built-in display of the relative humidity, hang a hygrometer nearby.
  • Lower the moisture level of the space by running an exhaust fan when showering and venting your clothes drier outside.
  • Extend downspouts away from the foundation and avoid overwatering foundation plantings.
  • Identify and repair any sources of outdoor leaks through, for example, cracks in the basement walls and repair them.

Even unfinished basements used for storage benefit from controlled humidity, so that mold and mildew don't become a problem on stored items.