To deter mildew.
I was once told by a home inspector that under 40% will prevent mold growth, but I am not sure if that is "the" number.
Temperature: Molds need an environment between 40 and 100 degrees Fahrenheit to grow.
The ideal for home furnishings and humans is 40 - 50%. If your windows sweat at the higher percentage you should reduce the humidity because that condensation will trigger mold growth.
There are three key components to healthy air. It must be fresh, clean, and have the proper humidity (less than 50% relative humidity). Fresh outside air is rich in oxygen and flushes your home of stale air, which is full of harmful airborne pollutants.
Yet, outside air is not necessarily clean air. In order to have clean air, it must be filtered to remove particles, airborne allergens, and other biologicals.
Maintaining relative humidity below 50% prevents dust mite infestations, mold and mildew growth, and inhibits bacteria. This lower relative humidity also reduces the out-gassing of VOCs.
In colder climates, wintertime humidity levels must be even lower — generally 30–40% — to prevent condensation on windows and other surfaces.
The most comfortable indoor humidity level will vary from one household to the next, depending on personal preferences. In general, this temperature guide will show you where to keep your indoor relative humidity levels to ensure comfort.
Outdoor temperature over 50˚F, indoor humidity levels shouldn’t exceed 50%
Outdoor temperature over 20˚F, indoor humidity levels shouldn’t exceed 40%
Outdoor temperature between 10˚F and 20˚F, indoor humidity levels shouldn’t exceed 35%
Outdoor temperature between 0˚F and 10˚F, indoor humidity levels shouldn’t exceed 30%
Outdoor temperature between -10˚F and 0˚F, indoor humidity levels shouldn’t exceed 25%
Outdoor temperature between -20˚F and -10˚F, indoor humidity levels shouldn’t exceed 20%
Outdoor temperature at -20˚F or lower, indoor humidity levels shouldn’t exceed 15%
Wow! Thanks! Looks like I have got to get a de-humidifier, now if I can just figure out what size...😕
William is right on the mark that when optimal humidity inside a home is concerned, one must also factor in the outside humidity.