Musty Odour: Moisture, Mold or Another Cause?
Updated: 18 hours ago
Does This Sound Like Your Odour?
Health Canada and other public-health sources describe possible moisture- and mould-related odours as musty, earthy, stale or mouldy. [1][2][3]
The smell may remind you of:
A closed, damp basement or cellar
Damp soil
Stale air in an attic, closet or storage room
Wet cardboard or damp paper
A persistent mouldy smell that seems trapped in the room
This odour profile is usually more earthy and enclosed than sharp, sweet, sour or burnt.
What Can Cause a Musty, Mouldy or Damp Smell?
Moisture does not have one specific smell. A mouldy damp or musty odour in house can come from damp materials, mould growth, bacterial growth or a combination of these factors.
Damp materials: Dampness can cause building materials to release compounds or odours, even when microbial growth has not been confirmed. The World Health Organization notes that dampness can cause chemical degradation of building materials and odour not linked to microbial growth. [4]
Mould growth: If materials remain damp, mould may grow. Some moulds produce microbial volatile organic compounds, or mVOCs, which can create a musty or mouldy smell. [2]
Bacterial growth: Damp conditions can also support bacteria. Both fungi and bacteria can produce volatile organic compounds that contribute to odour problems in moisture-affected buildings. [5]
Even when no visible mould is present, a material that remains damp can later support microbial growth. Health Canada recommends identifying and correcting the moisture source because mould can grow in areas with excessive moisture. [1]
Why a Persistent Musty Odour in House Matters
A persistent musty odour is a reason to investigate moisture, including possible hidden leaks, water infiltration or condensation. Health Canada notes that hidden mould may sometimes present only as a musty odour and that significant mould growth can contribute to poor indoor air quality. [7]
The source may not be visible. A slow plumbing leak, water entering through the foundation or building envelope, condensation inside a wall or ceiling, or moisture trapped beneath flooring can create conditions that support mould growth and persistent odours.
Possible Health Effects
People exposed to damp or mouldy conditions are more likely to experience:
Eye, nose or throat irritation
Coughing or phlegm
Wheezing or shortness of breath
Worsening asthma symptoms
Other allergic reactions [1][7]
The level of concern depends on the extent of the moisture or mould problem, how long it has been present and each person’s health and sensitivity. Children, seniors and people with asthma, severe allergies or severely weakened immune systems may be more susceptible. [1][7]
An odour cannot diagnose the cause of a person’s symptoms. The U.S. EPA notes that symptoms associated with mould exposure can also result from other exposures or health conditions. [2]
The practical priority is to identify and correct the moisture source. If anyone has health concerns they believe may be related to indoor air quality, they should speak with a health care professional. [1][2]
How to Investigate Safely
Start with accessible areas and the odour pattern. Health Canada recommends a visual walk-through for signs of moisture, water damage and possible mould. [1]
Note When and Where the Odour Is Strongest
Record whether the smell is strongest:
After rain or snowmelt
After showering, cooking, laundry or running water
When heating or air conditioning starts
In one room, near one wall or throughout the home
In the morning, after the home has been closed up or during humid weather
Check for Water Entering from Outside
Look inside around windows, exterior doors, basement walls and ceilings for:
Water stains or discolouration
Peeling paint or wrinkled wallpaper
Cracked plaster or warped wood
Condensation near windows or pipes
A smell that becomes stronger after rain or snowmelt
From ground level, look for damaged exterior materials, missing downspouts or drainage that directs water toward the home. [1]
Check Plumbing and Water-Using Areas
Inspect accessible areas below sinks and around toilets, tubs, showers, dishwashers, washing machines, refrigerators and water heaters.
Look for:
Active drips or leaking connections
Damp cabinet materials or flooring
Water stains on nearby walls or ceilings
An odour that appears after showering, flushing, doing laundry or running water
Check for Humidity and Condensation
Look for:
Condensation on windows, pipes or exterior walls
Dampness in bathrooms, kitchens, laundry rooms, closets or basements
Exhaust fans that do not work properly or do not vent outdoors
Furniture, boxes or stored belongings pressed against a cool exterior wall
Check Damp Materials and HVAC Patterns
Inspect accessible carpeting, rugs, cardboard boxes, fabrics and stored belongings for dampness, staining or a concentrated odour. [1]
If the smell begins when heating or air conditioning starts:
Check accessible filters and humidifiers
Note whether the odour occurs throughout the home or only near specific vents
Record whether it returns after accessible components have been cleaned or changed
Health Canada recommends inspecting accessible filters and humidifiers when an odour is associated with the start-up of a fan-driven heating or cooling system. The U.S. EPA also identifies ventilation systems as areas where moisture may collect and require investigation. [1][6]
What Not to Do
Avoid quick fixes that hide the odour while the moisture problem continues.
Do not cover the smell with air fresheners, fragrance or scented cleaning products.
Do not paint over mould or water stains. Paint can hide the problem without correcting the moisture source. [1]
Do not use bleach to clean mould. [1]
Do not open walls, lift carpeting or dismantle HVAC equipment to search for a concealed source. Moisture and mould can be hidden behind walls, above ceilings and beneath carpeting. [6]
Do not assume a dehumidifier has solved the problem if water is still entering the home or materials remain damp.
Do not begin with broad mould air testing. Health Canada does not recommend routine air testing for mould in most situations. [1]
Disturbing or removing moisture- or mould-affected materials without appropriate precautions can spread contamination, increase exposure and make the underlying problem more difficult to resolve. [1]
When a Musty Smell Needs Professional Help
Call a qualified professional if:
The odour is strong, persistent or unexplained.
You suspect moisture or mould inside a wall, ceiling, floor or HVAC system.
There has been a significant leak, flood, sewage backup or contaminated water event.
Mould returns after cleaning, appears in several locations or covers a large area.
The odour continues after accessible filters and humidifiers have been addressed.
Health Canada recommends professional involvement when hidden contamination is suspected, mould returns, building cavities have been affected by flooding or contaminated water was involved. [1] Its indoor-air-quality guidance also notes that mould, moisture and ventilation issues may require qualified professionals with appropriate training and equipment. [7]
Questions about a persistent musty odour?If the source is unclear or may be concealed, an assessment can help identify a safe, practical next step. |
References
[1] Health Canada. Guide to Addressing Moisture and Mould Indoors. Government of Canada, 2023. https://www.canada.ca/en/health-canada/services/publications/healthy-living/addressing-moisture-mould-your-home.html
[2] U.S. Environmental Protection Agency. What Does Mold Smell Like? https://www.epa.gov/mold/what-does-mold-smell
[3] New York City Department of Health and Mental Hygiene. Mold. https://www.nyc.gov/site/doh/health/health-topics/mold.page
[4] World Health Organization. WHO Guidelines for Indoor Air Quality: Dampness and Mould. “Building Dampness and Its Effect on Indoor Exposure to Biological and Non-Biological Pollutants.” 2009. https://www.ncbi.nlm.nih.gov/books/NBK143945/
[5] Claeson, A.-S., Levin, J.-O., Blomquist, G., & Sunesson, A.-L. “Volatile Metabolites from Microorganisms Grown on Humid Building Materials and Synthetic Media.” Journal of Environmental Monitoring, 2002. https://pubs.rsc.org/en/content/articlelanding/2002/em/b202571j
[6] U.S. Environmental Protection Agency. Mold Course Chapter 3: Investigating Hidden Mold. https://www.epa.gov/mold/mold-course-chapter-3
[7] Health Canada. Guidance for Indoor Air Quality Professionals. September 2025. https://www.canada.ca/content/dam/hc-sc/documents/services/publications/healthy-living/guidance-indoor-air-quality-professionals/guidance-indoor-air-quality-professionals.pdf
About the authorJohn Slavich is the founder of Puritas, an indoor air quality investigation service in Greater Montréal. He holds a B.Sc. in Physics, a master’s degree in engineering and several IICRC certifications. His mold investigations draw on the BNQ 3009-600 standard. He holds RBQ general contractor licence 5795-5080-01. |
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