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The Water You Breathe

We have lived in the North and the South and have indoor humidity problems year-round in both climates. What is the proper target humidity level, and what can be done to maintain it efficiently?—Randy W.

Humidity-related problems are generally worse during the winter in the North and during the summer in the South. But there are year-round problems anywhere.

There is not just one ideal indoor humidity level. For personal comfort, a target of 40 to 45 percent relative humidity is good. Most people are comfortable with a relative humidity from 30 to 50 percent and can tolerate 20 to 60 percent. When the relative humidity is in the proper range, you can set your furnace or central air conditioner thermostat down or up to save energy.

When the relative humidity level gets too high, there can be health problems related to allergies, dust mites, mold and mildew growth, and other harmful microbes. With the relative humidity too low, a person’s mucus membranes may dry out, increasing susceptibility to colds and respiratory illnesses. Also, some nasty microbes prefer excessively dry air.

To understand how to control indoor humidity, it is important to understand the term “relative humidity,” or RH. Warmer air can hold more water vapor (moisture) than colder air. If the air at 75 degrees has an RH of 50 percent, it means the air is holding 50 percent of the maximum amount of water vapor it can hold at that temperature.

If that same air drops to 50 degrees, the same amount of water vapor may now be 70 percent RH of the maximum amount the cooler air can hold. When the air gets cool enough, next to window glass during the winter or the refrigerator door seal during the summer, it reaches a point when it can no longer hold that much water vapor. This is called the dew point. This is when the windows or the refrigerator door sweats or it starts to rain outdoors.

You can purchase an inexpensive hygrometer at most hardware stores to measure indoor relative humidity.

The keys to maintaining a comfortable and efficient indoor humidity level are to control the sources of moisture and ventilate them efficiently. The average person gives off one-quarter cup of moisture an hour from breathing. If you exercise at home, it can be much higher. Cooking for a family of four produces five cups of moisture a day. A shower contributes one-half pint and a bath contributes one-eighth pint.

Exterior sources are leaky roofs, plumbing, windows, doors, etc. Once you have taken care of these problem areas, check the slope of the ground around your home. It should slope downward slightly, away from your house walls. Soggy soil around your home allows excess moisture to migrate indoors.

Installing new efficient replacement windows or exterior storm windows is the best method to control window condensation. This also saves energy during the summer. With more efficient glass, you should be able to close insulating window shades at night to save energy.

Install new bathroom vent fans with humidity sensors. These come on automatically and run until the humidity level drops. Check the seal around the clothes dryer duct leading to the outdoor vent.

Install a new furnace/heat pump with a variable-speed blower and compatible thermostat to allow it to run in an efficient dehumidification mode during the summer. Make sure the damper handle on the central humidifier is set for the proper season. Use electric countertop cookers and vegetable steamers in the garage instead of in the kitchen during the summer. I use an outdoor solar-powered steamer on sunny days.

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