Q We have noticed a mold odor in our house recently, but we see no evidence of mold. How can we find out where it's coming from?

A It's difficult. But some background information and dogged sleuthing using your nose can often lead you to the culprit. For mold to grow in your house it needs a moisture source, and that source might be outdoors, indoors or both.

Outdoor sources include:

• Leakage caused by clogged gutters might not be visible, but leaks can wet walls and attic spaces so that mold grows unseen.

• Leaks from improperly flashed windows, doors, decks and roofs. These defects also allow water to seep into wall cavities.

• If the ground around the house isn't sloped enough to drain surface water away, leakage can make the basement damp and increase humidity levels in the house.

Indoor moisture sources include:

• Plumbing leaks behind walls or under sinks.

• Condensation. Over winter, moist indoor air can cause condensation and ice to form on cold window panes and can do the same, unseen, inside cold wall cavities. Moist air makes its way from the indoors into the wall cavity through a variety of routes, including electrical outlets and light fixtures.

Look for evidence of moisture or possible moisture sources in the areas where you smell mold.

• Put your nose next to an outlet and see if the odor is stronger. (One can get used to an odor. Reset your nose by going outdoors for 10 minutes.)

• Gently remove window trim and baseboards to look for evidence of leaking.

• Lift carpet and move furniture and drapes to look for signs of moisture or mold staining.

• Check attic and basement for signs of leaks or mold.

• Outdoors, look for warped, bulging siding, premature peeling of paint or stained and cracked stucco or brick. Pay particular attention to areas around windows, doors, and decks and under roof valleys.

If you find evidence of water damage or leaks, determine the cause and rectify it so the areas can dry out. Then treat the mold if possible.

Keeping track, getting help

• A useful tool for dealing with an intermittent odor problem is a journal. When the odor is pronounced, note the time and what you are doing, what is happening or operating indoors (water heater, dryer, bathroom fan, etc.) and weather conditions. Sometimes connections can be made that will help identify the source of the odor.

• If your efforts are unsuccessful, you can hire a professional mold testing firm or house diagnostician to help you solve the problem. But be careful. It's an unregulated industry with wide variations in quality and cost. Get three bids, check out a company with the Better Business Bureau, look over the contract carefully and ask for names of customers and contact them.

• Before proceeding, it's a good idea to educate yourself about mold in homes. Free information is available from the Minnesota Department of Health at www.health.state.mn.us., or the Minnesota Extension Service at www.extension.mn.edu. Type "mold" in the search box.

Send your questions to Fixit in care of the Star Tribune, 425 Portland Av., Minneapolis, MN 55488, or call 612-673-7032, or e-mail fixit@startribune.com. Sorry, Fixit cannot supply individual replies.