Musty smell in the house: how to find the source

What the smell is

The earthy, stale odour people call musty comes from compounds released by mold and bacteria as they grow. You can often smell it before any growth is visible, which makes it a useful early warning.

It is not caused by a room simply being old or closed up. Something in that space is damp, or was until recently.

Follow your nose

The smell is strongest when you first enter a space that has been closed, before your nose adjusts. Use that.

1. Go outside for a few minutes, then come back in and note where the smell hits.
2. Check each room with the door closed for a while first.
3. Get close to walls, floors, closets and cabinets.
4. Open cupboards under sinks and smell inside.
5. Notice whether it is worse when the furnace or air conditioning runs, which points to the ducts or the unit.
6. Notice whether it is worse in humid weather or after rain.

The usual hiding places

Basement

• Behind finished walls, especially the lower section
• Under carpet and along its edges
• In the cold room and under the stairs
• Cardboard boxes and fabric stored on the floor
• Around the laundry, water heater and floor drain

Bathrooms and kitchen

• Under sinks and behind toilets
• Behind the tub or shower surround
• Under the dishwasher and behind the fridge
• Inside cabinets against outside walls

Elsewhere

• Closets on exterior walls
• Around windows
• Carpet near patio doors
• The attic, particularly around bathroom fan ducts
• A crawlspace with an exposed dirt floor
• The air conditioner drip pan and the furnace humidifier

Tools that help

• A moisture meter shows whether a wall or floor is damp inside. Basic ones are inexpensive.
• A flashlight held flat against a wall shows bubbling paint and swelling.
• A humidity gauge tells you whether the air itself is too damp.

Smells that are not mold

• Sewer gas smells of rotten eggs and usually comes from a dry floor drain trap. Pour water into the drain.
• A dead animal in a wall or attic has a sharp, sweet smell that fades over weeks.
• Gas smells of rotten eggs or skunk. Leave and call the gas utility from outside.

Getting rid of it properly

1. Find the damp material.
2. Stop the moisture: fix the leak, improve ventilation or run a dehumidifier.
3. Remove what cannot be dried or cleaned: wet cardboard, underpad, moldy drywall.
4. Clean hard surfaces with detergent and water.
5. Dry the space and keep it dry.

Air fresheners, scented candles and odour sprays only cover the smell while the cause continues. Ozone machines do not remove damp material and should not be run in occupied rooms.

When to bring someone in

• You cannot find the source
• The smell is coming from inside a wall or under a floor
• It followed a leak or flood
• You find more than a small patch of mold
• Someone in the home has asthma, allergies or other breathing problems

An inspection with a moisture meter and thermal camera usually locates the source without tearing things apart.

Seasonal patterns that point to the cause

• Worse in humid summer weather: condensation in the basement or crawlspace. A dehumidifier is the first step.
• Worse after rain: water is getting in from outside, through the foundation, a window or the roof.
• Worse in winter: condensation on cold surfaces, such as closets on outside walls, window surrounds and the attic.
• Worse when the air conditioning runs: the drip pan, the condensate line or the ductwork.
• Constant, in one spot: a slow plumbing leak.

Keeping a short note of when the smell is strongest often identifies the source faster than searching at random.

More: mold removal and remediation.