MoldHelpUSA
Mold in a house is caused by moisture that does not dry. The spores are already there: the EPA notes that mold spores are found indoors and outdoors, drifting in through doors, windows, vents, and on clothing. They do nothing until they land on a surface that stays damp and has something to feed on. Wood, drywall paper, carpet, fabric, and even household dust all count as food. Add moisture that lasts more than a day or two, and growth starts.
That is why the EPA puts it simply: the key to mold control is moisture control. Find the water, and you have found the cause.
The three things mold needs
- Moisture. The one you can control. Leaks, condensation, humidity, and flooding.
- Food. Almost any organic material in a home.
- Time and mild temperatures. The EPA advises drying wet materials within 24 to 48 hours to prevent mold growth. Most homes are kept at temperatures mold tolerates well.
Because you cannot remove spores or food from a house, controlling moisture is the only reliable fix.
The most common causes
- Plumbing leaks: supply lines, drains, toilet seals, dishwasher and washing machine hoses, and pipes inside walls.
- Roof and window leaks: water that runs down inside a wall or ceiling and stays out of sight.
- High indoor humidity: the EPA recommends keeping it below 60 percent, ideally 30 to 50 percent. Cooking, showers, drying laundry indoors, and humid outdoor air all raise it.
- Condensation: warm, moist air meeting a cool surface, such as exterior walls, windows, cold water pipes, and air conditioning ducts.
- Air conditioning problems: a clogged condensate line, an overflowing drip pan, or ducts sweating in an unconditioned attic.
- Poor ventilation: bathrooms without exhaust fans, dryers vented indoors, and attics with blocked vents.
- Ground moisture and flooding: water through foundations, basements, and crawl spaces, or floodwater that soaked walls and floors.
What causes mold, room by room
| Room | Usual causes | What to check first |
|---|---|---|
| Bathroom | Shower steam, no exhaust fan, failed caulk, toilet or sink leaks | The ceiling above the shower, caulk lines, under the vanity |
| Kitchen | Sink and dishwasher leaks, refrigerator water line, cooking steam | Under the sink, behind the dishwasher and refrigerator |
| Bedroom | Condensation on exterior walls and windows, furniture against walls, closets | Corners, behind the headboard, window frames, closet walls |
| Basement | Ground moisture, foundation leaks, summer condensation | Wall bases, framing, stored boxes, carpet |
| Attic | Roof leaks, blocked vents, bath fans venting into the attic | Roof sheathing near vents and chimneys, insulation |
| Laundry | Washer leaks, dryer venting indoors | Behind the washer, hose connections, the dryer duct |
| AC closet | Clogged condensate line, overflowing pan, sweating ducts | The drip pan, the condensate line, the air handler base |
Mold in the bedroom
Bedrooms come up again and again because people spend a third of the day there and notice the smell. The usual pattern is condensation. Warm, moist air from breathing, showers, and laundry reaches a cool exterior wall or window, especially behind a bed or dresser where air does not move, and the wall stays damp enough for mold.
- Leave a few inches between furniture and exterior walls.
- Keep closet doors open part of the day and avoid overfilling closets on exterior walls.
- Run the bathroom fan during and after showers, and keep humidity below 60 percent.
- Wipe condensation off windows and check the sills.
- If one spot keeps coming back, look for a leak in the wall, window, or roof above.
Mold on a pillow or mattress means it has been staying damp. Launder what can be washed and dried completely, and replace pillows or mattresses with mold growth. If anyone has symptoms, talk to a doctor.
Does turning off the AC cause mold?
In humid places, yes, it can. Air conditioning dehumidifies as it cools. A house left closed with the AC off can climb well above 60 percent humidity, and cabinets, closets, furniture, and walls can grow mold within weeks. Keep the AC running at a moderate setting, or use a dehumidifier, in a closed-up home.
Does heating stop mold?
No. Heat does not kill mold, and in winter cold exterior walls and windows can still collect condensation. Warm air holds more moisture, so running heat can lower relative humidity, which helps, but the real fixes are ventilation, humidity control, and keeping cold surfaces dry.
Why mold causes differ by climate
The causes are the same everywhere, but which one dominates depends on where you live:
- Humid South Florida: humidity and air conditioning. NOAA data for Miami put average relative humidity around 73 percent, so AC closets, closed-up seasonal homes, and condensation on cool interior surfaces are frequent sources. Orlando sees the same pattern inland.
- Gulf Texas: humid summers plus heavy storms. In Houston, flooding and roof leaks after storms are common causes; in San Antonio, plumbing leaks and AC condensation are common causes.
- Dry Colorado: mold in Denver almost always points to a specific water source, such as a frozen pipe, a hail-damaged roof, a swamp cooler, or a damp basement, because the air rarely stays humid enough on its own.
- Older cities: in Washington, DC, about 69 percent of homes were built before 1980, according to the Census Bureau, and aging plumbing, roofs, and basements in rowhouses are common causes.
How to find the cause in your home
- Look at the pattern. Spots in corners and behind furniture suggest condensation; a spreading stain suggests a leak.
- Check what is above and behind it. A bathroom, roof, or plumbing wall is often the source.
- Measure humidity. An inexpensive hygrometer tells you if indoor air is above 60 percent.
- Use a moisture meter on walls and floors around the growth.
- Watch whether it returns after cleaning. Growth that comes back means the moisture is still there.
See signs of mold in your house and how to test for mold.
Not sure where the water is coming from? A local inspector can trace it with a moisture meter and thermal camera, and the call to find one is free.
Health
According to the CDC, mold can cause a stuffy nose, sore throat, coughing or wheezing, burning eyes, or skin rash in some people, with stronger reactions in people with asthma or mold allergies. If anyone in the home has symptoms, talk to a doctor. See black mold symptoms.
Next step
Fixing the cause comes first, but mold that covers more than about 10 square feet, grows inside walls, or keeps returning needs professional removal. If you own your home, call the number on this page to be connected with an independent local mold inspection or mold remediation company.