RW writes from Edmonton, Alberta: "What should the humidity level be in a full basement and then in an adjacent crawl space. We are trying to determine what level setting to set our dehumidifier. Full basement is about 900 sq. feet, and the crawl space is about 300 sq. feet. The crawl space is a conditioned space. Both basement and crawl space are below ground (well, not so much the crawl space). Also, our outside climate experiences all 4 seasons experiencing temps from -20 celcius (winter) to +30 celcius (summer). The water table in our area is very high. We have dehumidifier - but would like a better idea of what the level should be set for our basement and our crawl space."

Answer: One of my Canadian friends recommends 30 to 35 percent relative humidity. He says it will feel comfortable and help reduce the risk of condensation in the winter.

North Dakota State University's Web site addresses the issue on their page titled "Keep Your Home Healthy." Author Dr. Kenneth Hellevang recommends humidity level in the winter of 30 to 40 percent in cold climates. A relative humidity above about 40 percent increases the potential for condensation on windows and other cool surfaces. Humidity levels below about 30 percent lead to dry skin and nasal passages, increasing the potential for respiratory illnesses.

You can find other perspectives on the Web as well, such as from the Canada Mortgage and Housing Corporation (CMHC). I also recommend checking with the manufacturer of your dehumidifier.