What Does Mold Smell Like?

What does mold smell like? Musty, earthy and damp. People describe it as wet cardboard, old books, a pile of damp leaves, sweaty socks or the air in an old cellar. The smell is stale and stays put, and it is usually stronger in one area of the house, after rain, or when a closed room is first opened.

What does mold smell like in a house?

The exact smell changes with the kind of mold, what it is growing on and how wet it is. A few versions come up again and again.

• Earthy, like soil or a forest floor. Common in basements and crawlspaces.
• Stale and papery, like wet cardboard or old newspapers. Common where drywall or stored boxes have been damp.
• Sour or tangy, like damp laundry left in the washer. Common in bathrooms, closets and around washing machines.
• Sharp and a bit sweet, like rotting wood. Common where framing or subfloor has been wet for a long time.

Musty is the word most people land on. If you walk into a room and think of a cottage that has been closed up all winter, that is the smell.

Why mold smells

The odour is not the spores. Growing mold gives off gases as it digests whatever it is living on. These are called microbial volatile organic compounds. Bacteria in damp materials produce similar gases, which is why a wet house smells musty even before any mold is visible.

Two things follow from that. First, the smell means active growth on something damp. Mold that has dried out and gone dormant produces much less odour. Second, the smell travels differently than spores do. Gases pass through drywall, flooring and gaps around pipes, so you can smell growth that is sealed inside a wall or under a floor. A musty odour with nothing visible is a real finding and worth following up. If odours or damp air at home seem to bother your breathing, talk to your doctor.

Where a musty smell usually comes from

• Basement perimeter walls, especially behind finished drywall and stored items.
• Under carpet and underpad on a concrete slab.
• The cabinet under a kitchen or bathroom sink.
• Behind and under the washing machine, and in its door gasket.
• Around the sump pit and floor drain.
• Closets on outside walls, where air does not circulate.
• The furnace humidifier pad and the air conditioner drain pan.
• Crawlspaces with bare soil floors.
• Attics, where the smell is often noticed at the hatch.

In older London homes with stone, brick or block foundations, a faint cellar smell in the basement is common and comes from moisture moving through the wall. It becomes a concern when it gets stronger, spreads upstairs, or comes with visible growth on joists and stored belongings.

Smells that get mistaken for mold

• Sewer gas. A rotten egg or sewage smell near a floor drain or an unused basement bathroom usually means the water in the drain trap has evaporated. Pour a litre of water down the drain and see if it clears.
• Natural gas. Gas utilities add a rotten egg odour so leaks get noticed. If you suspect gas, leave the house and call your gas utility's emergency line or 911 from outside.
• A dead mouse or other animal. This is a sharp, sickly smell that peaks over a week or two and then fades.
• Wet dog, old carpet and smoke residue. These are stale smells that do not change with the weather.
• A fishy or burning plastic smell. This can point to overheating electrical parts and should be checked by an electrician promptly.

The tell for mold is the link to moisture. It gets stronger in humid weather, after rain, or when the air conditioning or furnace fan first starts.

How to track down a musty smell

1. Reset your nose. Step outside for a few minutes, then come back in and note where the smell hits first.
2. Close interior doors for a few hours, then check room by room to find where it is strongest.
3. Get low. Smell along baseboards, at floor drains, inside lower cabinets and at the bottom of closets.
4. Open things. Check under sinks, behind the access panel for the tub, inside the cold room and behind stored boxes.
5. Look for supporting signs: staining, bubbling paint, swollen baseboards, rusty carpet tack strip, white deposits on foundation walls.
6. Measure the humidity with a hygrometer. A basement sitting well above 60% in summer is damp enough to grow mold on contents and framing.
7. Note when it is worst. After rain suggests water getting in from outside. All the time suggests a plumbing leak or steady humidity.

A musty smell but no visible mold

This situation is common. The growth is usually behind something: finished basement walls, under flooring, inside a bathroom wall or above a ceiling. Air fresheners, candles and odour-absorbing gels only cover it up, and the material keeps getting wetter. The practical next step is a moisture inspection. A thermal camera shows cool, damp areas behind surfaces and a moisture meter confirms them, so one small opening can be made in the right place and the wall is not cut up on a guess. Our inspections run $249 to $549. Before you call anyone, write down where the smell is strongest and when it is worst. That narrows the search more than anything else.

Common questions