Why does mold grow in basements?
Mold needs moisture, and basements have more of it than any other part of the house. No type of mold grows without water, so every basement mold problem traces back to a moisture source.
- Humid air. Warm, humid summer air meets the cooler basement and condenses on walls, pipes and floors.
- Foundation seepage. Groundwater pushes through block or poured walls and the joint where the wall meets the floor, especially after long rains.
- Leaks that were never fully dried. A water heater leak, a burst pipe or a past flood leaves water in carpet pad, drywall and framing. The EPA says drying wet materials within 24 to 48 hours prevents mold in most cases.
- Cold surfaces. Cold water pipes, ducts and exterior walls sweat. The EPA suggests insulating cold pipes to stop condensation.
- Finished basements. Carpet, drywall and paneling over a damp slab or wall hide moisture until the mold shows through or the smell gives it away.
- Stored cardboard and fabric. Boxes on the floor soak up moisture and feed mold.
What does basement mold look like?
Mold shows up as fuzzy or slimy spots in black, green, gray or white, often with a musty smell. On concrete and block, it is easy to confuse with efflorescence, a white mineral deposit left when water evaporates through masonry.
| Check | Mold | Efflorescence |
|---|---|---|
| Looks like | Fuzzy or slimy spots, many colors | White, chalky or crystal-like powder |
| Grows on | Wood, drywall, carpet, boxes, dust on any surface | Concrete, block and brick only |
| Smell | Musty, earthy | None |
| What it tells you | There is a moisture problem to fix and mold to remove | Water is moving through the masonry |
Both point to water getting into the basement. Efflorescence is a warning sign; mold is the problem itself.
Why does my basement smell musty if I cannot see mold?
A musty smell usually means mold is growing somewhere you cannot see. Common hiding spots in a basement are behind drywall or paneling on exterior walls, under carpet and pad, inside ceiling cavities under bathrooms, and around the sump pit or water heater.
Check for staining low on walls, peeling paint, warped baseboards and damp carpet. If you find nothing, a moisture meter or a professional inspection can show where the water is getting in before you start opening walls.
Is mold in the basement dangerous?
It can be a health problem, especially for sensitive people. The EPA says molds produce allergens and irritants, and in some cases potentially toxic substances, and that touching or breathing mold can cause allergic reactions in sensitive individuals.
That is a reason to deal with it promptly, not to panic. Keep people with allergies, asthma or weakened immune systems away from the cleanup, and if anyone has health concerns, the EPA suggests talking to a health professional before starting. Basement air also moves through the rest of the house, so mold downstairs does not always stay downstairs.
Can you get rid of basement mold yourself?
For a small area on a hard surface, usually yes. The EPA says that if the moldy area is less than about 10 square feet, roughly a 3 foot by 3 foot patch, in most cases you can do the job yourself.
The EPA suggests bringing in a professional when:
- There has been a lot of water damage, or mold covers more than about 10 square feet
- The water or mold came from sewage or other contaminated water
- You suspect mold in the heating and cooling system. Do not run it if so, since it can spread mold through the house
- Anyone in the house has health concerns. Talk to a health professional before starting
Mold behind finished walls or under carpet is almost always bigger than what you can see, so treat it as a larger job.
How do you get rid of mold in a basement?
Fix the moisture first, then clean, then dry. Cleaning without fixing the water source means the mold comes back.
- Find and stop the water. A leak, seepage, a failed sump pump, or humidity.
- Protect yourself. The EPA suggests gloves, goggles and an N-95 respirator.
- Scrub hard surfaces with detergent and water, then dry them completely.
- Remove porous materials that are moldy. The EPA notes that carpet, ceiling tiles and other absorbent materials may need to be thrown away, since mold can grow into them and be impossible to remove.
- Bag and remove debris so you do not carry spores through the house.
- Dry the area with fans and a dehumidifier, and keep it dry.
What should you not do?
- Do not paint or caulk over mold. The EPA says to clean and dry the surface first, because paint over mold is likely to peel.
- Do not count on bleach. The EPA says biocides such as chlorine bleach are not recommended as a routine practice during mold cleanup.
- Never mix bleach with ammonia or cleaners that contain it. The fumes are toxic.
- Do not run the furnace or air conditioner if you think the system has mold in it.
- Do not spray and walk away. The EPA notes that dead mold can still cause allergic reactions in some people, so it needs to be removed.
Do you need to test basement mold?
Usually not if you can see it. The EPA says that in most cases, if visible mold growth is present, sampling is unnecessary, and there are no federal limits to compare results against.
Testing helps when you smell mold but cannot find it, when you want to know how far it spread behind finished walls, or when you are selling a home. Read How to test for mold in your house, or see our mold inspection and testing. Selling soon? See Mold disclosure when selling a home in Virginia.
What does professional basement mold remediation involve?
A professional job contains the area, removes what cannot be cleaned, cleans what can, and dries the structure so mold does not return.
- Find the moisture source and the full extent of the mold
- Contain the work area so spores do not spread upstairs
- Remove moldy drywall, insulation, carpet and other porous materials
- Clean framing, block and concrete
- Dry everything and confirm it with moisture readings
- Rebuild the walls and floors that came out
Our mold remediation crew does every step, and the same crew rebuilds the finished basement afterward. More detail: What is mold remediation, and how long does it take?. Wondering about coverage? Does homeowners insurance cover mold?
How do you keep mold out of your basement?
Control the moisture and mold has nothing to grow on.
- Keep humidity below 60 percent, ideally 30 to 50 percent, as the EPA recommends. Run a dehumidifier through the humid months and empty or drain it.
- Keep gutters clean and downspouts pointed away from the house.
- Grade the ground so it slopes away from the foundation.
- Insulate cold water pipes so they do not sweat.
- Vent the clothes dryer outside.
- Dry any leak within 24 to 48 hours. If it is bigger than a towel can handle, see Basement flooded? What to do first.
- Store things in plastic bins off the floor, not in cardboard.
In Northern Virginia, humid summers and, in parts of Fairfax County, marine clay soils that hold water against foundations make basement moisture a year-round job.