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Keeping indoor humidity at a pleasant level

  • Article by: Alan J. Heavens
  • Philadelphia Inquirer
  • February 25, 2014 - 3:36 PM

Q: My daughter and her 5-year-old sleep in a bedroom with a hardwood floor, filled with clothing and stuffed animals. She recently added a small tabletop air filter and a humidifier that she activates every night, putting a half-gallon of water into the air in a 12-hour period.

I already have a whole-house humidifier on the furnace, and I am concerned that she is going to cause a mold or mildew problem in the room, in addition to the unfinished attic and possibly adjoining bedrooms.

A: What motivated your daughter to add the humidifier and filter if you already had a whole-house humidifier, which, if properly maintained, does a fine job adding moisture in the driest of seasons indoors?

Perhaps the whole-house humidifier is not doing its job properly, and your daughter and grandchild are suffering because of it. You may need to keep adjusting it as the outdoor temperature rises and falls.

Relative humidity indoors in winter should be between 40 and 60 percent, depending on the outdoor temperature. The indoor temperature, in this reckoning, is 70 degrees.

The lower the outdoor temperature, the lower the humidity indoors. For example, if the outside temperature is 20 to 40 degrees, humidity indoors should not be more than 40 percent. If the outdoor temperature is lower than 20 below zero, inside humidity should not be more than 15 percent.

I doubt that what your daughter is doing, for whatever reason — typically dry air and the sinus congestion that can result — will cause mold and mildew to form at this time of year, especially if the house is properly ventilated.

The Environmental Protection Agency says that maintaining relative humidity between 30 and 60 percent will help control mold. Houses in northern climates just don’t get more humid than that during the winter.

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