Mold on Walls: Why It Grows and How To Get Rid of It

Mould on walls is one of the most common things GTA homeowners and tenants call us about once the weather turns cold. It tends to show up in the same places every winter: the top corner of a bedroom, the wall behind a bed or dresser, the back of a closet, and bathroom walls and ceilings. It usually starts as a scatter of small black or grey dots and spreads slowly from there.

The spots are the symptom. Mould only grows where a surface stays damp, so a wall with mould on it is telling you one of two things: water is getting into it from a leak, or the wall is cold enough for moisture in the air to condense on it. Cleaning the spots without fixing that is why the same patch comes back every year.

This guide covers why mould grows on walls, how to tell it apart from dirt and dust marks, how to clean a small patch safely, and the point where it becomes a job for professional mould removal.

Warm indoor air carries moisture from showers, cooking, laundry, plants and simply breathing. When that air touches a surface colder than its dew point, the moisture condenses into a thin film of water, the same way a cold glass sweats in summer. On a window you can see the droplets. On a painted wall you usually cannot, but the surface stays damp enough for mould to grow on the paint, on the paper facing of the drywall and on the dust that settles on both.

Exterior walls run coldest in a few predictable places:

Where the mould is usually tells you why it is there. Use this table as a starting point.

If the mould is mainly around windows, our guide to condensation on the inside of windows covers that case in detail, including the maximum indoor humidity for each outdoor temperature.

Not every dark mark on a wall is mould. The two common lookalikes are ordinary dirt and what painters call ghosting: grey shadows or stripes that follow the studs, ceiling joists or the edge of the ceiling. Ghosting happens when dust and soot from candles, fireplaces or cooking settle more heavily on the colder strips of a wall. It points to the same cold spots that grow mould, but it is not growth itself.

A few quick checks will usually tell you which one you have:

Health Canada notes that mould in the home can cause eye, nose and throat irritation, coughing, wheezing and shortness of breath, and can make asthma and allergies worse. Infants and young children, older adults, and people with asthma, allergies, other breathing conditions or a weakened immune system are more likely to be affected.

Mould on a bedroom wall deserves extra attention simply because of the hours spent next to it. A small patch caught early is a quick fix; a wall that has been growing mould behind a bed for several winters is a different job. If anyone in the home has symptoms you think may be linked to mould, speak with a doctor.

Health Canada says you can generally clean small and medium areas of mould yourself. It describes small as one to three patches, each smaller than one square metre, and medium as more than three patches, or patches between one and three square metres. Larger growth, and mould that keeps coming back, should be handled by a professional. Toronto Public Health puts the professional threshold at about four square metres or more.

The surface matters as much as the size. Mould on a hard, sealed surface can be cleaned. Mould that has grown into something porous usually cannot, because the growth is inside the material.

For a small patch on a painted wall or another hard surface, the process is simple. The care is in keeping the drywall dry and the spores contained.

Have the wall assessed if any of these apply:

Once the wall is clean and dry, a few habits do most of the work of keeping it that way.

Under Ontario's Residential Tenancies Act, landlords are responsible for keeping a rental unit in a good state of repair and fit to live in, which includes fixing leaks and building problems that cause mould. Tenants are responsible for ordinary cleanliness, so everyday habits like running the bathroom fan and not blocking cold walls matter too.

If you rent, report mould to your landlord or property manager in writing, with photos and the date you first noticed it, and ask what they plan to do about the cause. If nothing happens, Toronto tenants can call 311 to request a property standards inspection, and tenants anywhere in Ontario can apply to the Landlord and Tenant Board about maintenance problems.

Usually not when the mould comes from condensation, humidity or a slow, long-term leak, which most home insurance policies treat as maintenance issues. Mould that results directly from a sudden, insured water event, such as a burst pipe, may be considered as part of that claim. Policies vary, so check your wording and speak with your broker. Our guide to whether home insurance covers water damage in Ontario explains how insurers usually draw the line.

If the patch is bigger than you are comfortable cleaning, it keeps coming back, or the drywall underneath is stained or soft, it is worth having it looked at before another winter of growth. Rosehill Restoration Services provides mould inspection and removal across Toronto, North York, Scarborough, Etobicoke, Mississauga, Markham and the wider GTA.

For a sense of budget, our guide to how much mould removal costs in Toronto breaks down typical pricing by area size. When you are ready, book a free mould estimate and we will find what is growing, what is causing it and what it will take to fix.