What humidity level prevents mold?

Keep relative humidity under 60 percent. In a basement that has grown mold before, aim nearer 50 percent in summer. In an old Gettysburg house in January, the living space often needs to sit lower, around 30 to 40 percent, or the windows and cold plaster will sweat.

Buy two cheap gauges. Read them morning and evening for a week. A number you do not measure is a slogan.