Mould in the house or basement: causes and cleanup

With the heating back on, condensation returns to the windows, and with it the risk of mould. The Government of Quebec explains that mould needs water and something to feed on, such as drywall or cardboard, to grow. So the answer is always the same: find the water, then get rid of it.

1. The signs

• Spots of various colours, often greenish or black, on walls, ceilings, carpets, around windows or in closets.
• A musty or earthy smell, especially in the basement and storage areas.
• Stains, or paint that buckles or peels: water may be seeping into a wall or ceiling, and the problem may be hidden behind the materials.

According to Québec.ca, there is usually no need to have the air or surfaces tested: when mould is visible, the house should be inspected to find the cause instead. Air sampling is often hard to interpret anyway.

2. Keeping humidity low in winter

• Aim for humidity low enough to prevent condensation, usually between 30% and 55%. Hygrometers are sold cheaply in most hardware stores.
• Limit long showers, run the fan when bathing, showering and cooking, and make sure the dryer hose vents outside.
• Limit humidifier use, do not store firewood indoors and run the air exchanger as the manufacturer directs.
• Empty and clean the dehumidifier tray often; in summer, it usually belongs in the basement.
• Frequent condensation on windows or walls means the air is too humid: air the house out when needed, even for a few minutes.

3. After water damage: 24 to 48 hours

Clean up and fully dry any water damage within 24 to 48 hours: after that, mould can grow. Throw out badly soaked porous materials, such as upholstered furniture. Then fix the cause: a broken pipe, a leak, a sewer backup (see water leak: what to do).

4. Cleaning it yourself or calling a company

For a small area, such as a windowsill, wipe with a clean cloth and an all-purpose cleaner, then dry well. Bleach is not necessary; Québec.ca keeps it for disinfecting things that may go in the mouth, such as toys. Wear safety goggles, rubber gloves and a dust mask, and clean when nobody else is in the room, especially children and people with asthma or allergies; air the room before they come back.

If the affected areas are larger than 1 m², numerous, hard to clean or come back after cleaning, call a specialized company. Ask how it will find the cause, location and extent of the problem, and what experience it has. The repairs that follow (roof, foundation, plumbing, drain) require an RBQ licence: check the contractor’s.

5. Mould in a rental unit: Montréal and Longueuil

Do you rent out a unit in your plex, or are you a tenant? In Montréal, the page Locataire : vous avez droit à un logement en bon état (updated December 19, 2025, in French) says a dwelling must meet By-law 03-096 and must be free of mould. The tenant first reports the problem to the owner, preferably in writing; if nothing is done within a reasonable time, City teams may inspect the unit, order corrections and impose fines. The City also asks occupants to use the range hood when cooking and the bathroom fan during and after showers.

In Longueuil, the page Salubrité des logements (in French) states that a dwelling must be free of mould and vermin. Anyone who believes the by-law is not being followed first informs the owner in writing, ideally by registered mail, then, if the problem continues, makes a request through 311; this does not limit the right to go to the Tribunal administratif du logement.

6. Finding a company

To fix a leak or redo a damaged wall, see renovation contractors in Montréal, companies in Longueuil or in Rosemont–La Petite-Patrie, or describe the moisture problem to get quotes. See also: attic insulation and French drains.