Q How much would it cost to run a heated birdbath? We have a 150-watt, 120-volt unit with a built-in thermostat.

A There are many variables that make it difficult to give a definitive answer, including temperature, which varies widely over winter, and whether the birdbath is in the sun. But state energy specialist Phil Smith did offer a guess: The added electricity usage would cost between $5 and $7 a month.

Window as witness

Q I was wondering if the presence of moisture on the inside of a window is a good indicator of a proper level of humidity inside a house?

A Condensation on an inside window pane can indicate excessive indoor moisture. A window mostly covered with condensation, or ice, on most days indicates the indoor humidity probably is too high. However, a small ribbon of condensation at the bottom of the window, especially on really frigid days, is not a concern.

But the answer to your question is not that simple, really.

Indoor moisture is necessary in the heating season to feel good in your home, to avoid static shocks and bloody noses. But the house itself, specifically the wall cavities, attic spaces and window frames, needs lower humidity levels to prevent excessive condensation and ice that can, over time, result in mold and rot. Generally, the colder is it outdoors, the lower the indoor humidity needs to be to avoid damage. However, it is when the weather is really cold that occupants want and need more moisture in the home.

It's a balancing act.

Indoor air quality experts and building scientists recommend using a hygrometer (measures relative humidity levels), to make sure indoor air humidity levels are kept below 50 percent. Make sure it's lower when outdoor temperatures dive.

An easy, quick way to give a rough idea of humidity levels is to watch the windows. Generally, condensation can indicate what's happening unseen in walls and attic spaces. Some condensation now and then isn't a problem. But lots of it -- say, every day over several winter seasons -- can be, especially in newer homes, where building materials are much more sensitive to moisture.

One last thought: It's not likely, but it is possible that excessive moisture on windows, especially if it occurs suddenly, is a warning sign of carbon monoxide in the home. Why? Because furnaces, boilers, water heaters, fireplaces and appliances that burn fuel can malfunction. When that happens, byproducts of combustion (including carbon monoxide and water vapor) enter the home instead of going up the chimney and outdoors. Water, a large part of this exhaust, can then be seen as condensation on windows, a visible sign of an invisible problem.

Send your questions to Fixit in care of the Star Tribune, 425 Portland Av. S., Minneapolis, MN 55488, or call 612-673-7032, or e-mail fixit@startribune.com. Past columns are available at www.startribune.com/fixit. Sorry, Fixit cannot supply individual replies. Fixit appears every day except Friday.