How to look for moisture and mold in the basement and attic
To check for mold, inspect the basement, attic and every plumbing spot for stains, musty smells, condensation and visible growth, then measure humidity. EPA says to keep it below 60%, ideally 30 to 50%. You can usually clean mold covering less than about 10 square feet yourself, but fix the water source first or it will come back.
How to check for mold where it hides
Mold is the symptom. Water is the problem you're hunting. EPA says that getting rid of all indoor mold is virtually impossible, because spores are everywhere. They only grow on damp material, so your scan is really a search for places that stay wet.
Visible spots are the easy part. EPA's training course lists the hiding places to think about: behind drywall, paneling and wallpaper, under carpet pads, in wall cavities and pipe chases, in cold corners behind furniture, in crawl spaces with bare earth floors, and in poorly draining air handler pans.
A moldy smell with nothing to see can mean mold is hidden. Condensation on windows, walls or pipes points to high humidity. Downstairs, look at wall bases and around the water heater and washer. Up in the attic, look at the underside of the roof boards and check where the bathroom fan ducts end. EPA says to run a fan or open a window when showering, and to vent dryers outdoors, so that moisture needs a path outside.
How to check for mold, room by room
- Start with your nose
Walk the empty house with the doors closed for a while first. Musty smells are strongest when the air is still.
- Follow the plumbing
With a flashlight, check every sink cabinet, around toilets, behind the washer and below the water heater.
- Work the basement walls
Look for stains, white mineral deposits and damp spots, especially low on the walls and behind stored items.
- Climb into the attic
Stay on the joists. Look for dark or stained boards and confirm the bath fan ducts lead outside. Don't disturb loose vermiculite insulation, which EPA says may contain asbestos.
- Place a humidity meter
Put one in the basement and one on the main floor, then note the numbers after a day.
- Walk the outside
Confirm the ground slopes away from the foundation and that gutters aren't blocked.
- Sort what you found
Mark each spot as humidity, active leak or old stain, and deal with the sources first.
Signs of mold in a basement or attic and what they mean
| What you find | What it means | What to do |
|---|---|---|
| Humidity 30 to 50% | EPA's ideal range | Nothing. Check again next season |
| Humidity above 60% | Enough moisture to feed mold | Run the AC or a dehumidifier and look for the source |
| Condensation on pipes or windows | High humidity or cold surfaces | Dry it, cut the moisture source, and insulate cold pipes |
| Musty smell, nothing visible | Possible hidden mold | Look for past leaks. Call a pro before opening walls |
| Mold under about 10 sq ft (roughly 3 ft by 3 ft) | Most people can clean this | Scrub hard surfaces with detergent and water, then dry fully |
| Mold over 10 sq ft, or heavy water damage | May need a contractor | Get a professional's opinion |
| Damage from sewage or other dirty water | Needs an experienced pro | Don't clean it yourself |
| Mold in your HVAC system, or suspected there | Running it may spread spores | Turn it off and call a pro |
What to do if you find mold
Stop the water first. Cleaning before the leak or humidity is fixed means you'll do it again. If a leak or spill just happened, EPA says drying wet materials within 24 to 48 hours will usually prevent mold growth.
For a small cleanup, EPA recommends an N-95 respirator that fits properly, long gloves and goggles without vent holes. Carpet and ceiling tiles may need to go in the trash, because absorbent materials can't always be saved. Don't paint or caulk over mold, since paint over mold is likely to peel. EPA doesn't recommend bleach as a routine cleanup step, and you should never mix it with ammonia.
You usually don't need a mold test if you can see mold. EPA and other federal agencies have set no mold limits, so a sample can't show that a home meets a federal standard.
Call a structural engineer or other qualified pro if the mold may have damaged framing. If someone in the home has health symptoms, talk to a doctor.
What homeowners say about how to find hidden mold
Treat the first heavy rain as a second inspection. Owners walk the house, attic and yard during the storm to see where gutters dump and where water pools. Inspections usually happen in dry weather and miss this. A plumber in another thread said to fix even small leaks right away, before they become rot or mold. (Reddit)
A cheap pin-type moisture meter finds hidden leaks before stains show. One owner tests the drywall around toilets, showers and sinks and treats readings above about 10% as suspect. (Reddit)
Put a cheap humidity monitor in the basement and keep it under about 50 to 60%. Owners who track it say it caught problems early. A dehumidifier set around 50 to 55% runs only when needed. (Hacker News, DIY Stack Exchange)
Basement moisture and mold questions
Should I pay for a mold test?
Usually not if mold is visible. EPA says testing is generally unnecessary in that case, and there are no federal limits to compare results against. Spend the money on fixing the moisture.
Will bleach kill basement mold?
EPA doesn't recommend bleach as a routine cleanup method. Detergent and water work on hard surfaces, and the area must then be dried fully. Without a moisture fix, the mold will likely return.
How much mold is too much to clean myself?
EPA's line is about 10 square feet, or a patch roughly 3 feet by 3 feet. Beyond that, or with sewage or heavy water damage, bring in a professional.
Sources: EPA
General guidance for U.S. homes, not professional advice. For gas, electrical and structural work, call a licensed pro.