Mold rarely announces itself with a big patch on the wall. More often it starts with a smell, a stain, or a bit of paint that will not stay flat. Knowing the early signs helps you catch a moisture problem while the fix is still small.
This guide covers the most common signs of mold in a house, where hidden mold tends to grow, and what to do when you notice them.
The most common signs of mold
- A musty smell. An earthy, damp odor that stays after you air the room out is one of the most reliable signs, especially in closets, bathrooms, basements, and near the air conditioning system.
- Visible growth. Spots or patches of any color: black, green, gray, white, brown, or orange. Mold comes in many colors, not just black.
- Water stains. Brown or yellow rings on ceilings and walls mean water has been there. Where there was water, there may be mold.
- Bubbling, peeling, or cracking paint and wallpaper. Moisture behind the surface lifts paint and loosens wallpaper.
- Soft, warped, or crumbling materials. Drywall that gives when pressed, baseboards that swell, and floors that cup or buckle all point to long-term moisture.
- A recent leak or flood. Materials that stay wet for more than 24 to 48 hours can start growing mold, according to the EPA.
- Condensation and high humidity. Fogged windows, sweating pipes, and damp-feeling rooms suggest indoor humidity above the 60 percent the EPA recommends staying under.
- A musty smell when the air conditioning starts. It can point to growth in the air handler, drain pan, or ducts.
- Mold that keeps coming back. Growth that returns in the same spot after cleaning means the moisture source is still there, or more growth is hidden nearby.
- Symptoms that seem to follow the house. Some people notice a stuffy nose, coughing, or itchy eyes that improve when they are away. Many things cause these symptoms, so this is a reason to check the house and talk to a doctor, not proof of mold.
Where hidden mold grows
Mold does not need light, and a lot of it grows out of sight. The EPA lists hidden surfaces such as:
- The back side of drywall
- Wallpaper and paneling
- The top side of ceiling tiles
- The underside of carpets and pads
And hidden places such as:
- Damp areas behind walls and in crawl spaces
- Pipe chases, where water pipes run inside walls
- Acoustic liners inside ventilation ducts
- Roof materials above ceilings
In a typical home, also check under kitchen and bathroom sinks, around toilets, behind the refrigerator and washing machine, inside the air conditioning closet, around windows, in the attic, and behind furniture pushed against exterior walls, where condensation can form.
Signs of black mold in a house
The signs are the same as for any mold. Black mold, the common name for Stachybotrys chartarum, grows on materials that stay wet, such as the paper face of drywall and ceiling tiles, so it usually appears with water stains, soft drywall, or peeling paint near a leak. You cannot identify it by color alone, and the CDC says it is not necessary to identify the species, because all molds are removed the same way. See what black mold looks like and our black mold guide.
When the house smells like mold but you cannot see it
A persistent musty smell with no visible growth usually means mold is hidden. Start with the likely places:
- The air conditioning closet, drain pan, and drain line
- Under sinks and around the base of toilets and tubs
- Behind appliances with water connections
- Closets on exterior walls and rooms below bathrooms
- Carpet and pad in rooms that have ever had a leak
Do not start cutting into walls or pulling up flooring to look. The EPA warns that disturbing hidden mold can release large numbers of spores. A professional inspection with a moisture meter and a thermal camera can find damp areas without opening everything up. See what a mold inspection involves.
What to do when you notice the signs
- Find the moisture. Look for leaks, condensation, drainage problems, and high humidity, and stop the source.
- Document it. Take dated photos of the growth, stains, and any water.
- Estimate the size. Under about 10 square feet on a hard surface can often be cleaned by the homeowner, according to the EPA. Growth on drywall is often larger on the back than on the front.
- Decide who handles it. Larger areas, hidden growth, and mold in the air conditioning system call for a professional. See DIY vs. professional mold removal.
- Check your insurance if a leak was sudden. Mold that follows a burst pipe or storm damage may be covered up to a limit. See does homeowners insurance cover mold.
If you rent, tell your landlord in writing; see renters insurance and mold.
Signs to watch for in Florida homes
In Florida's humid climate, a few situations come up again and again: homes closed up with the air conditioning off for weeks, air conditioning drain lines that clog and overflow, roof and window leaks after storms, and condos where water travels from one unit to another. A musty smell after returning from a trip, or after a storm, is worth investigating quickly. See local pages for Tampa, Orlando, and the rest of Florida.
Next step
If you own your home and see these signs, especially a smell you cannot trace or stains that keep spreading, call the number on this page to be connected with an independent local mold inspection company.