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posted: 2/5/2010 12:01 AM

Humidity level in a home is a matter of personal comfort

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Q. My kids have allergies, so I am not sure how humid the air should be inside my house during winter and summer. What is a good target humidity level and what is the best and most efficient way to maintain it?

A. There is not just one ideal year-round indoor humidity level. People are typically comfortable with a RH (relative humidity) in the 30- to 50-percent range and can tolerate 20 to 60 percent. The ideal RH depends whether you live in the North or South and it changes with the seasons. High RH is often a summertime problem in the South and excessively low or high RH may create wintertime problems in the North.

Controlling humidity lowers your utility bills by allowing you to set the furnace or central air conditioner thermostat down or up respectively. It also creates a healthier indoor environment. Although overly humid air creates the most health problems, some harmful microbes thrive better in very dry air.

It is important to understand what RH actually means. Warm air can hold more water vapor (moisture) than cold air. If the air at 75 degrees has a RH of 50 percent, the air is holding 50 percent of the maximum amount of water vapor it can hold at that temperature.

If the airdrops to 50 degrees with the same moisture level, it may now have a RH of 70 percent. When air gets cool enough, next to window glass during winter or the refrigerator door during summer, it no longer can hold all the water vapor. This is called the dew point and condensation forms on the cool surface.

To maintain a comfortable level efficiently, you must control the sources of moisture and ventilate excess away. Indoor sources are many. For example, each person gives off one-quarter cup of moisture per hour just breathing. Cooking for a family of four produces five cups of moisture per day. Using an exterior vented range hood, particularly during summer, can exhaust both this moisture and heat from the range.

Exterior sources are leaky roofs, plumbing, windows, doors, etc. Once you take care of these problem areas, check the slope of the ground around your home. The ground should slope downward slightly away from your house walls.

Installing replacement windows with efficient glass usually helps reduce window condensation problems during winter and heat gain during summer. On the other hand, if your family generates a lot of indoor moisture, installing new windows may cause excessive indoor RH because of less air infiltration.

Install a new bathroom vent fan with a humidity sensor. This comes on automatically and runs until the humidity level drops. With a manual switch, you have to either turn it off prematurely when you leave for work or let it run all day. Check the seal around the clothes dryer duct leading to the outdoor vent. During summer, hang your wet clothes outdoors to dry.

Install a new furnace or central air conditioner with a variable-speed blower and compatible thermostat to allow it to run in an efficient dehumidification cooling mode during summer. Make sure the duct 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 summer. I use an outdoor solar-powered oven on sunny days.

Q. There is fog between the glass panes of my double-pane windows. The foggy area gets smaller on sunny days. I assume the seal between the panes is bad. How can I repair the leaky seal?

A. I think you have diagnosed the foggy problem correctly. The foggy area gets smaller during the daytime because the sun warms the glass and the air in the gap.

Unfortunately, there is no good method to reseal the glass panes. Even if you could, the desiccant material inside the gap is already saturated with moisture. The pane will have to be replaced. If the sash frame has sealed corners, you will have to replace the entire sash.

• Write to James Dulley at 6906 Royalgreen Drive, Cincinnati, OH 45244 or visit