We get asked this question a lot.
The ideal humidity is between 40 and 60%. A comfortable target would be 50%.
Note: Above 60% mold growth may occur and below 40% materials such as paper, wood, etc. may be damaged. When thinking about preserving materials the constancy of the humidity is a factor. Extreme changes in humidity can cause damage. (Note that extreme changes in heat can also have this effect… something people find out when trying to kill bedbugs by opening their windows in winter.)
Basements can also be tricky as they’re often cooler than the rest of the house. Cooler air means that the relative humidity will be higher (typically) and people often need a basement dehumidifier as a result.
If you’re interested in relative humidity you can find a bit more information in the video below