The recommended humidity level for all rooms including the attic and crawl space in the winter is 30 to 40 percent in cold climates. A relative humidity above about 40 percent increases the potential for condensation on cool surfaces increasing the opportunity for rust to form on metal gas lines and other non-copper surfaces (this includes the electrical panel). Humidity levels below about 30 percent lead to dry skin and nasal passages, increasing the potential for respiratory illnesses. Overly dry air can inflame the symptoms of sinusitis, lead to laryngitis and get in the way of a good night's sleep. Also, low humidity levels cause problems in furniture and pianos due to wood shrinkage and increase the incidence of static electricity shocks.
During the summer, indoor humidity levels should be kept below about 65 percent to minimize the potential for mold growth and below about 50 percent to minimize dust mites. Hygrometers (relative humidity gauges) may not be accurate. Mechanical hygrometers are frequently in error by more than 20 percent. Calibrate your hygrometer by sealing it in a plastic bag along with a cup containing ½ cup of water and ¼ cup of table salt (Figure 3). After at least 12 hours at room temperature, the meter should read 75 percent relative humidity. Electronic meters tend to be more accurate, but they should also be calibrated.
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