Musty Smell but No Visible Mold, Where to Look

Why You Can Smell It but Not See It

The musty smell comes from gases that mold and bacteria give off as they grow. Your nose can pick up these gases at very low levels, long before growth is big enough to see or in places you cannot see at all.

A musty smell usually means one of three things.

• Hidden growth behind a wall, under flooring, inside a cabinet, in the HVAC system or in a crawlspace.
• A damp material that has not grown visible mold yet, like a slowly wet carpet pad or a sweating cold water pipe.
• Air from a damp space, such as a basement or crawlspace, being pulled into the living area.

Smell is a clue about where, not how much. A strong smell can come from a small spot in a closed space. A faint smell can come from a large area behind drywall.

Treat a lasting musty smell as a moisture problem to find, not an odor to cover. Air fresheners, candles and odor sprays hide the warning while the damage continues. Once you find the wet spot and fix it, the smell usually fades within days to weeks after cleanup.

Use Your Nose Like a Tool

Your nose gets used to a smell within minutes. That is why the smell hits you when you come home and fades after you have been inside a while.

Use that to your advantage.

1. Go outside for 5 minutes to reset your nose, then walk back in slowly.
2. Stop in each room and note where the smell is strongest. Write it down.
3. Close the doors and turn off the heat or air conditioning for a few hours, then check again. Smell that builds up in one closed room points to that room.
4. Get down low. Many sources are at floor level or inside base cabinets. Sniff at the toe kick under cabinets and along baseboards.
5. Open closets, cabinets and vanity doors one at a time. A burst of smell when a door opens is a strong clue.
6. Notice timing. A smell that gets worse when the air conditioning starts points to the HVAC system. One that is worse after rain points to a roof, window or foundation leak. One that is worse in summer humidity points to a basement or crawlspace.

Ask someone who does not live in the home to walk through with you. A fresh nose often finds the source faster.

The Usual Hiding Places

These spots cause most musty smells with no visible mold. Check them in this order.

• Under sinks. Look at the cabinet floor for swelling, stains or soft spots. Run the water and feel the drain and supply fittings. Check the dishwasher supply line too.
• Behind and under the fridge. Many fridges have a drip pan under the front grille that collects defrost water. A dirty pan can smell bad. Ice maker lines also leak slowly.
• The washing machine. Front-loading washers often grow mold in the rubber door gasket and dispenser drawer. Pull back the gasket and look.
• The HVAC system. A wet drain pan or dirty coil smells musty when the system starts. Check the filter and condensate drain.
• Around toilets. A failing wax seal leaks under the floor. Look for a soft floor or stains around the base.
• Closets on outside walls. Cold walls behind stored clothes and boxes collect condensation with no airflow.
• Behind furniture against outside walls, especially beds and dressers in cool bedrooms.
• Windows. Condensation runs down the glass into the sill and the wall below.
• Carpet over concrete in basements or on ground floors, where moisture from the slab soaks the pad.
• The basement or crawlspace. Air from these spaces rises into the home.

Measure Moisture to Find the Source

A moisture meter costs about $25 to $40 and is the single most useful tool for this search. It tells you when a wall, floor or cabinet is wet even when it looks and feels dry.

How to use it. Take a reading in a known dry spot of the same material, like a wall in the middle of the house. Then take readings in the suspect area every foot or so. A reading clearly higher than your dry baseline means water is present. Follow the higher readings until they peak. That is usually close to the source.

Focus on these spots.

• The bottom 12 inches of walls behind toilets, tubs, showers, sinks and appliances.
• Walls and ceilings below bathrooms and laundry rooms.
• Walls under windows.
• Cabinet floors and the subfloor near the fridge and dishwasher.

Use a hygrometer to check humidity in each room. The EPA recommends keeping indoor humidity below 60 percent, ideally between 30 and 50 percent. A basement at 70 percent will smell musty even if nothing is leaking.

An infrared thermometer can help too. Damp areas are often a few degrees cooler than the surrounding wall because water evaporating cools the surface. It is a clue to check with the moisture meter, not proof by itself.

You can also check for a hidden supply leak. Turn off every faucet and water-using appliance, then watch your water meter for 1 to 2 hours. If it moves, water is leaking somewhere.

Smells That Are Not Mold

Not every bad smell is mold. Knowing the difference saves time and points you to the right fix.

A dry drain trap is a very common false alarm. Floor drains, guest bathroom tubs and basement sinks that are rarely used can dry out and let sewer gas in. Pour a quart of water in each one and see if the smell fades within a day.

When to Bring in a Professional

If you have searched carefully and still cannot find the source, a professional inspection is worth the money. Look for an inspector who uses moisture meters and a thermal camera, not just air samples. Moisture mapping finds hidden water that air tests only hint at.

A good inspection covers the whole home, including the attic, crawlspace, HVAC system and the areas around plumbing. Expect a written report with photos, moisture readings and specific recommendations.

Air testing can help with a hidden problem. It compares indoor and outdoor spore counts and types to see whether there is an indoor source. It does not tell you where the source is. Always make sure an outdoor control sample is taken.

Hire an assessor who does not also do mold removal. That removes the incentive to find a problem.

You should call sooner, not later, if any of these apply.

• A moisture meter shows a wet wall or ceiling and you cannot find the leak.
• The smell is strongest near a shared wall or ceiling in a condo or apartment.
• Someone in the home has asthma, allergies or a weak immune system and symptoms are worse at home.
• There was a past flood, sewage backup or long leak in the home.

Step by Step

1. Reset Your Nose

   Step outside for 5 minutes, then walk back in slowly. Note which rooms smell strongest and write them down.

2. Close Off and Recheck

   Close room doors and turn off the HVAC for a few hours. Check again to see which room the smell builds up in.

3. Check the Top Hiding Places

   Look under every sink, behind the fridge and its drip pan, in the washer gasket, around toilets and in closets on outside walls.

4. Take Moisture Readings

   Get a baseline reading on dry drywall, then check the bottom 12 inches of walls near plumbing and under windows. Follow higher readings to their peak.

5. Check Humidity Room by Room

   Leave a hygrometer in each suspect room for 20 minutes. Anything above 60 percent needs a dehumidifier or better ventilation.

6. Test for a Hidden Supply Leak

   Turn off all faucets and appliances and watch the water meter for 1 to 2 hours. Movement means a leak somewhere in the supply lines.

7. Rule Out Other Smells

   Refill unused drain traps with water, check the toilet seal and consider a dead animal if the smell is sweet and in one spot.

8. Inspect the HVAC and Lower Spaces

   Check the air filter and condensate drain, then look in the basement or crawlspace for standing water, damp insulation or growth.

9. Fix, Clean and Recheck

   Once you find the source, fix the water, clean or remove small growth safely and dry the area. If the smell remains after two weeks, call a professional.

Mistakes That Make It Worse

• Covering the smell with air fresheners, candles or an ozone machine. The damp source keeps growing behind the scent.
• Buying a petri dish test kit. It almost always grows mold and does not tell you where the source is.
• Cutting holes in walls to follow the smell. That spreads spores and, in older homes, can disturb lead or asbestos.
• Ignoring a rotten egg smell as just another odor. That can be a gas leak.
• Forgetting the washer gasket and fridge drip pan, which cause many smells with no visible wall mold.

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