Both are costly. Even though surfaces seem dry to the touch, it's hard to tell if your house is really dry, that is, dry enough to stop mold and mildew growth. Allowing high humidity is risky for human health and risky for the integrity of the structure especially over prolonged periods of time. There are 3 conditions that promote growth of mold and mildew, moisture, warmth, and darkness. The first step you need to take when you suspect above normal moisture conditions in your home is to clean all surfaces with a disinfectant and then allow everything to dry out.
Mold growth is very damaging to the house structure, especially wood surfaces.
Molds existing in the air of the home when inhaled can potentially cause allergies, asthma and upper respiratory illnesses. More and more individuals are experiencing hypersensitivity to mold spores. These are good indicators of moisture build-up, lack of ventilation, poor air circulation, and lack of a fresh air supply. Often you can see mold growth, but even if you can't, mold can still be present. You can measure the amount of moisture and humidity in your home by using a humidistat or sling psycometer.