What causes mold in a house?
Mold in a house is caused by moisture that stays on a surface for more than a day or two. The usual sources are leaks from roofs and plumbing, flooding, high indoor humidity, condensation on cold surfaces and poor ventilation. Remove the moisture and the mold loses what it needs to grow.
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Mold needs three things
Mold needs moisture, a food source and a suitable temperature. Houses have plenty of food in the form of wood, drywall paper, carpet and dust, and the temperature is rarely a barrier. So moisture is the only one you can control, and nearly every case traces back to water that was where it should not be.
The time factor matters. A surface that is wetted and dries quickly rarely grows mould. A surface that stays damp for days is the place where it takes hold.
Common moisture sources in Toronto homes
- Roof and flashing leaks that wet attic insulation and sheathing
- Plumbing leaks inside walls, under sinks and behind tubs
- Basement seepage and floods after heavy rain or a sewer backup
- Warm, humid air meeting cold surfaces, such as attic roof decks in winter
- Bathroom and kitchen fans that are missing, too small or vented into the attic
- Dryer vents that end inside the building
Why some houses are more at risk
Older homes can have drafty or poorly insulated envelopes that create cold surfaces for condensation. Newer, tighter homes can trap humidity if ventilation is not working. Finished basements add drywall and carpet in a place that is naturally damp. Attics collect warm air that rises from the living space and condense on the cold underside of the roof. Our attic mould removal page explains one of the most frequent patterns.
How to lower the risk
- Fix leaks fast.Dry anything wet within a day or two.
- Use your fans.Run bathroom and kitchen fans during and after use, and make sure they vent outside.
- Control humidity.A dehumidifier in the basement helps in summer.
- Watch for condensation.Wipe window sills and look for damp corners.
- Inspect the roof and eavestroughs.Keep water moving away from the house.
Toronto seasons
Winter brings condensation on windows and in attics as warm indoor air meets cold surfaces. Spring melts and heavy rain test basements and roofs. Summer humidity makes basements damp and air conditioning can sweat on ducts. Each season has its own trigger, so a yearly check of the roof, the eavestroughs, the fans and the basement is a good habit. If you notice a smell, act early.
Related questions
Can mould grow in a clean house?
Yes. Cleanliness does not stop it if moisture is present.
Should I have the house inspected?
If you smell mould or see staining, an inspection can find the cause. See our mould inspection page.
Is the assessment free?
Yes, and you receive a written scope and price.
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