We are paid when readers call a provider through this site. That never changes our advice. How we make money

Quick answer

A musty smell with no visible mold often means mold is growing on something damp you cannot see. EPA says to suspect hidden mold when a building smells moldy but you cannot see the source. [3] Find the moisture first: check under sinks, the laundry area, the basement or crawlspace, the HVAC drain pan, the attic and under carpets. Keep indoor humidity below 60 percent, ideally 30 to 50 percent, and call a mold remediation company if the smell is inside walls or ducts or you cannot find the source. [3]

Do this first: find the water, not the mold

A musty smell is a moisture problem before it is a mold problem. EPA puts it simply: moisture control is the key to mold control. [1] CDC says that if you see or smell mold, you should remove it and fix the moisture problem. [2] If you clean up mold but do not fix the water problem, EPA says the mold will most likely come back. [3]

  1. Follow your nose. Keep doors closed for a few hours, then walk room by room and note where the smell is strongest. Start there.
  2. Look for active leaks under sinks and around tubs, showers, refrigerators, dishwashers and the washing machine water lines. [4] Fix plumbing leaks as soon as possible. [3]
  3. Measure humidity. EPA says a humidity meter costs about $10 to $50 at many hardware stores. [3] Check the room with the smell, the basement and a main living area.
  4. Dry anything wet within 24 to 48 hours. EPA says that in most cases mold will not grow if wet materials are dried in that window. [3]
  5. Ventilate. Run bath and kitchen exhaust fans, and open the house when the air outside is less humid than inside. [3] [5]
  6. If you suspect the heating and cooling system, turn it off. EPA says not to run the HVAC system if you know or suspect it is contaminated with mold, because it could spread mold through the building. [3]
  7. Skip fans aimed at an area where mold may already be growing. FEMA says fans can spread existing mold. [5]

Water near power

If you find standing water near outlets, cords or the electrical panel, do not walk into it. Turn power off at the panel only if you can reach it from a dry spot; otherwise call your utility or an electrician.

Renters: EPA says to report all plumbing leaks and moisture problems right away to your building owner or manager. [3] Report the smell to your landlord in writing so there is a dated record.

What does a musty smell mean if you cannot see mold?

Molds give off compounds with strong smells, called microbial volatile organic compounds (mVOCs). EPA says these can be the source of the musty smell people link with mold, and that a moldy odor suggests mold is growing in the building and should be investigated. [1] CDC also notes mold can smell musty. [2]

Mold does not need much water. EPA says a little condensation in a bathroom or around a window sill can be enough. Common sources include roof leaks, condensation from high humidity or cold spots, slow leaks in plumbing fixtures, humidification systems and floods. [1]

So the smell often comes first. EPA's hidden spots include the back side of drywall, the underside of carpets and pads, inside walls around leaking or condensing pipes, walls behind furniture, and inside ductwork. [3] Its mold course adds poorly draining HVAC drain pans and humid crawl spaces, especially with a bare earth floor. [6]

If it smells like rotten eggs, that is a different problem. Virginia's health department says a rotten egg smell can be caused by sewer gas, or by a natural gas leak in homes that use gas to heat or cook. Sewer gas can contain hydrogen sulfide, which has a rotten egg smell, and dry traps in sewer drains or cracks in the waste line under or next to the home might let it in. The department advises adding water to floor drains and sink drains, especially those used less often, so the traps do not dry out. If you detect a rotten egg odor in your home, it says to go outside and call the fire department for assistance. [7]

How to track the smell room by room

FEMA suggests routinely checking problem spots such as the bathroom, laundry and mechanical room for moldy odors. [5] Work through the house with a flashlight and this table.

Where to look for the source of a musty smell, and the moisture clue in each area
AreaWhere to lookMoisture clue
Kitchen and under sinksCabinet bases, the wall behind the sink, around the dishwasher and refrigerator, the refrigerator drip panPlumbing leaks; a musty smell in cabinets, drywall or wood
BathroomsCaulk, grout and tile, around the tub and shower, the exhaust fanDamaged caulk or grout; a fan that does not vent outdoors
LaundryWashing machine water lines, the dryer ventLeaking lines; a dryer venting indoors
BasementWalls, ceiling and joists, foundation cracks, sump pump, floor drain, dehumidifier drip panPeeling paint, discoloration, cracks
CrawlspaceThe soil and its vapor barrier, walls and cracksBare earth floor, high humidity, no plastic vapor barrier over the soil
HVAC systemCondensate drain pan, cooling coil, insulation near the coil, ductsStanding water or debris in the pan, wet insulation near the coil
AtticWalls, joists, insulation, exhaust duct ends, soffitsLeaks, exhaust ducts that do not reach outside, blocked soffits
Living areas and closetsCarpet underside, walls behind furniture, window frames, closetsCondensation on windows or walls, peeling drywall around window frames, poor closet airflow

Table sources: NC State Extension's mold and moisture checklist [4] and EPA. [3] [6] [8]

Crawlspace and basement. EPA says soil in a crawl space wicks moisture that evaporates into the air, and that moisture can pass into the house through cracks in walls, floors and ceilings. [6] NC State Extension says a crawl space should be sealed with at least a 6 to 8 mil vapor barrier covering the soil, the sump pump should work, and downspouts should extend 6 to 10 feet from the foundation. [4]

HVAC drain pan and ducts. EPA says moisture should not be present in ducts. Make sure the condensate pan drains properly, since substantial standing water or debris there indicates a problem needing immediate attention, and check insulation near the cooling coils for wet spots. Wet or moldy insulation inside ducts cannot be effectively cleaned and should be removed and replaced. [8]

Carpet. If you are concerned about mold under carpet, NC State Extension says to lift a corner and inspect the underside for visible mold. [4]

Attic. Inspect the walls, joists and insulation for any musty smell or visible mold. [4]

Do not tear into walls to find it

EPA warns that investigating hidden mold requires caution. Removing wallpaper, for example, can release a massive amount of spores if mold is growing underneath. If you believe you have a hidden mold problem, EPA says to consider hiring an experienced professional. [3]

What humidity should your house be?

EPA's guidance, in its own words: "If possible, keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity." [3] EPA's mold course explains why: humidity in the air alone can sometimes supply enough moisture for mold to grow. [6] CDC sets a tighter line, saying to keep humidity as low as you can, no higher than 50 percent, all day long, and that an air conditioner or dehumidifier will help. [2]

How to read your humidity number against EPA and CDC guidance. The bands are this site's reading of the two agencies' numbers, not agency categories.
ReadingHow it compares with EPA and CDC numbersWhat to do
30 to 50 percentEPA's ideal range. Also within CDC's limit of no higher than 50 percent.Keep it there. If the smell continues, the moisture is likely local: a leak, a wet spot or a drain pan.
Above 50, below 60 percentUnder EPA's 60 percent line, but above CDC's limit of no higher than 50 percent.Run the air conditioner or a dehumidifier and use exhaust fans.
60 percent or higherAt or above EPA's 60 percent line. EPA lists high humidity (60 percent relative humidity) as a common moisture problem.Dehumidify and look for the source, such as a damp crawlspace or a dryer venting indoors.
Condensation on windows, walls or pipesEPA says condensation can be a sign of high humidity.Dry the surface quickly, insulate cold pipes and increase ventilation.

Table sources: EPA [3] [6] and CDC. [2] Empty and clean the dehumidifier drip pan regularly. Other humidity sources include many plants, large aquariums, drying laundry indoors, and unvented heaters or gas logs. [4]

How to dry it out and keep it dry

  1. Fix plumbing leaks and other water problems as soon as possible, and use air conditioners or dehumidifiers when needed. [3]
  2. Run the bathroom fan or open a window when showering, and use exhaust fans when cooking. [3] If a single sheet of toilet paper stays against the running fan, it is working. [4]
  3. Make sure the dryer and the kitchen and bath fans vent outdoors, not into the attic, crawlspace or garage. [4]
  4. Keep air conditioning drip pans clean and the drain lines unobstructed and flowing properly. [3]
  5. Insulate cold water pipes that sweat. [3]
  6. Clean and repair gutters, and make sure the ground slopes away from the foundation. [3]

Safe to do yourself

If you find the mold and it covers less than about 10 square feet, roughly a 3 foot by 3 foot patch, EPA says in most cases you can handle it yourself. EPA suggests an N-95 respirator and says not to paint or caulk moldy surfaces. [3] NC State Extension says to remove contaminated items, clean with soap and water, and dry the area quickly. [4] If you have health concerns, EPA says to consult a health professional before you start. [3]

When to call a mold remediation company

Call a mold remediation company that serves your area when any of these apply:

  • You checked the likely spots and still cannot find the source. EPA says to consider an experienced professional for a hidden mold problem. [3]
  • The smell seems to come from inside a wall, a ceiling or the HVAC system. Do not run a system you suspect is contaminated. [3]
  • Mold covers more than about 10 square feet. For that size, EPA points readers to its remediation guide for schools and commercial buildings, which it says applies to other buildings too. [3] [9]
  • The house flooded or had a lot of water damage. EPA points to the same remediation guide in that case. [3] FEMA says forced air ducts that came in contact with water or mold should be professionally checked. [5]
  • Sewage or other contaminated water was involved. EPA says to call in a professional with experience cleaning buildings damaged by contaminated water. [3]
  • Insulation inside your ducts is wet or moldy and needs to be removed and replaced. [8]

Ask about experience before you hire. EPA says to make sure a contractor has experience cleaning up mold, and its duct guidance says to ask the service provider to show you any mold they say exists. [3] [8]

Do you need a mold test, and what about health?

Usually not. CDC does not recommend mold testing and says that whatever type of mold is present, you need to remove it. [2] EPA notes there are no federal limits for mold, so sampling cannot show whether a home meets a standard. [3] Your effort is better spent finding the moisture.

On health, CDC says damp and moldy environments may cause a variety of health effects, or none at all. People with asthma or mold allergies may have severe reactions, and people with weakened immune systems or chronic lung disease may get lung infections from mold. [2] If anyone in your home has symptoms or health concerns, talk to a doctor.

Common questions

Why does my house smell musty but I cannot find any mold?

Mold often grows where you cannot see it, such as the back side of drywall, the underside of carpet pads, inside ductwork or above ceiling tiles. [3] EPA says a moldy odor suggests mold is growing and should be investigated. [1] Trace the moisture room by room, starting where the smell is strongest.

Can high humidity alone cause a musty smell?

It can. EPA says humidity in the air can sometimes supply enough moisture for mold to grow, and recommends keeping indoor relative humidity below 60 percent, ideally 30 to 50 percent. [6] CDC says no higher than 50 percent. [2]

Can a musty smell come from my air conditioner?

Yes. EPA lists poorly draining condensate drain pans inside air handling units as a site of hidden mold. [6] Standing water or debris in the pan indicates a problem needing immediate attention. [8] If you suspect the system is contaminated, EPA says not to run it. [3]

Should I get a mold test if I smell mold but cannot see it?

CDC does not recommend mold testing, and says all mold should be removed regardless of type. [2] EPA says there are no federal limits for mold, and sampling should be done by experienced professionals. [3] Finding and fixing the moisture source matters more than a lab result.

Does homeowners insurance cover hidden mold?

It depends on the cause. Washington's insurance commissioner says homeowner insurance usually covers sudden leaks but may not cover gradual leaks, and usually does not cover mold unless it comes from water damage the policy covers. Some insurers provide limited mold coverage. [10] Check your own policy and ask your insurer.

Sources

  1. Mold Course Chapter 1 U.S. Environmental Protection Agency, accessed October 12, 2026
  2. About Mold and Your Health Centers for Disease Control and Prevention, accessed October 12, 2026
  3. A Brief Guide to Mold, Moisture and Your Home U.S. Environmental Protection Agency, accessed October 12, 2026
  4. Mold and Moisture Checklist (April 4, 2022; adapted from University of Georgia Extension) NC State Extension (N.C. Cooperative Extension), accessed October 12, 2026
  5. Dealing With Mold and Mildew in Your Flood Damaged Home FEMA, accessed October 12, 2026
  6. Mold Course Chapter 2 U.S. Environmental Protection Agency, accessed October 12, 2026
  7. Frequently Asked Questions About Sewer Gas (last updated September 10, 2021) Virginia Department of Health, Division of Environmental Epidemiology, accessed October 12, 2026
  8. Should You Have the Air Ducts in Your Home Cleaned? U.S. Environmental Protection Agency, accessed October 12, 2026
  9. Mold Cleanup in Your Home U.S. Environmental Protection Agency, accessed October 12, 2026
  10. Leaks, water damage and mold Washington State Office of the Insurance Commissioner, accessed October 12, 2026