Mold Smell in House: What It Means and How to Find It

The CDC says mold can smell musty. New York City's health department describes a moisture problem's odor as a musty, earthy or stale odor. People often compare it to a damp basement, wet cardboard, old books or dirty socks.

What does mold in a house smell like compared to other odors?

Table: what does mold in house smell like, and what the musty smell usually points to.

Not every unpleasant smell is mold, and a smell alone can't tell you the size of the problem. But a musty odor is a moisture warning worth taking seriously.

Mold smells because of gases it releases while it grows. The EPA calls them microbial volatile organic compounds, or mVOCs. In its words, some compounds produced by molds have strong smells and are volatile and quickly released into the air, and because mVOCs often have strong or unpleasant odors, they can be the source of the "moldy odor".

Two things follow from this:

1. The smell means active growth is likely. Gases are released as mold grows, so a fresh musty odor suggests something is growing now, usually because something is wet.
2. The smell travels. Gases move through wall cavities, ducts and gaps, so the strongest smell isn't always right on top of the mold.

What about health? The EPA says the health effects of inhaling mVOCs are largely unknown. Mold itself can cause symptoms in some people, such as a stuffy nose, sore throat, coughing or wheezing, burning eyes, or skin rash, and those with asthma or mold allergies may have severe reactions. If you have symptoms that don't go away, see a doctor. A mold test of your house can't diagnose illness. Our black mold symptoms guide explains what the evidence says.

This is very common. The EPA says you may suspect hidden mold if a building smells moldy, but you cannot see the source. Hidden mold grows in places you don't look every day.

Common sources of a mold smell

Air conditioning and HVAC. Your air conditioner pulls water out of the air and drains it away, so a clogged drain or dirty pan can stay wet for weeks. The EPA says to keep air conditioning drip pans clean and the drain lines unobstructed and flowing properly. It also notes that mold growing near the intake to an HVAC system indicates potential ventilation humidity problems and that ductwork is one place hidden mold can grow.

Under sinks and inside walls near pipes. Slow drips are easy to miss. The EPA lists inside walls around pipes among hidden mold spots and says the inside of walls where pipes and wires are run are common sites of mold growth.

Crawl spaces. If your home is raised with a crawl space underneath, check it. The EPA says crawl spaces where relative humidity is high are common sites of hidden mold growth, especially over a bare earth floor.

Bathrooms and laundry rooms. Excessive moisture can cause mold to grow in the bathroom, and improperly vented dryers can increase room humidity.

Behind furniture, wallpaper, carpet and ceiling tiles. The EPA lists the back side of drywall, wallpaper or paneling, the top side of ceiling tiles and the underside of carpets and pads as hidden mold spots. It also finds mold on walls in cold corners behind furniture where condensation forms.

After a roof leak or flood. Mold will grow where there is moisture, such as around leaks in roofs, windows, or pipes, or where there has been a flood. If your home took on water during a hurricane or heavy storm, see our mold after a flood in Texas guide.

Work through the house the way a mold assessor would: follow the moisture.

1. Use your nose, room by room. Note where the smell is strongest and when. A smell that's worse when the AC kicks on points to the HVAC system. One that's worse after rain points to a roof, window or foundation leak.
2. Check the obvious wet spots. Open sink cabinets, look behind the toilet, around the water heater, under the fridge and at the AC air handler and drip pan.
3. Look for water signs. The EPA says don't ignore wet spots or water stains. Peeling, bubbling paint and warped floors are also signs of a moisture problem. Our signs of mold in house checklist covers more.
4. Measure humidity. The EPA says relative humidity can be measured with a moisture or humidity meter. Aim for below 60 percent, ideally between 30 and 50 percent. Watch for window condensation too, which can be a sign of high indoor humidity.
5. Stop before you tear things open. The EPA warns that investigating hidden mold requires caution since disturbing moldy areas may spread mold throughout the building, and that removal of wallpaper can lead to a massive release of spores if mold is growing underneath.

If you find the source and it's small, you can usually clean it yourself. The EPA says that's reasonable when the moldy area is less than about 10 square feet. Fix the water first, scrub mold off hard surfaces with detergent and water, and dry completely, and wear at least a NIOSH Approved N95 respirator, gloves and goggles.

Air fresheners and candles only cover the odor. The EPA's goal for a finished cleanup is that visible mold and moldy odors should not be present. If the smell returns, moisture is still getting in somewhere.

You often don't need a test. The EPA says in most cases, if visible mold growth is present, sampling is unnecessary, and the CDC does not recommend mold testing in general.

A smell with no visible source is different. That's exactly the hidden-mold situation where the EPA says to consider hiring an experienced professional. In Texas, that's a licensed mold testing company. TDLR defines mold assessment as inspecting or surveying a structure to provide information about the presence of mold. Ask any assessor you call how they plan to look for the moisture source, not just which samples they'll take.

Testing also helps after cleanup. The EPA says surface sampling may be useful to determine if an area has been adequately cleaned or remediated. Read more in how to test for mold, mold testing cost and mold clearance testing.

Testing and removal: two different companies

If the assessor finds a larger problem, a separate licensed mold removal company does the cleanup. Texas law says a license holder may not perform both mold assessment and mold remediation on the same project. Small areas are exempt: TDLR says areas of visible mold of less than 25 contiguous square feet can be cleaned up by people who are not licensed.

Find licensed companies in your city:

• Houston mold testing and Houston mold removal
• Corpus Christi mold testing
• San Antonio mold testing
• Dallas mold testing
• Every city: Texas mold directory

Check any company with our license check tool, and see how we verify every listing against TDLR records.