Indoors, mould can grow and spread in places that are wet or humid. Common causes of mould include:

• leaky foundations, walls, roofs, or windows
• leaking water or wastewater pipes
  
• condensation or frost on interior walls
  
• high indoor humidity
• poor airflow in kitchens or bathrooms
  
• damp basements or crawl spaces
• not enough weather stripping around doors or windows
  
• flood or sewer backup (These need special attention. They likely contain harmful bacteria, viruses, or chemicals.)

What are some signs of mould indoors?

Mould may be different colours, including black, brown, or green. It may look fuzzy or patchy. 

Watch for signs of mould such as:

• stains or speckled areas on walls, ceilings, floors, or furniture, especially if it looks like it's spreading or comes back after cleaning
  
• a musty or earthy smell in the building
• you or someone in your home has symptoms of being exposed to mould

How can I prevent mould from growing indoors?

Control indoor moisture to prevent mould from growing. This is why it’s important for homes and other buildings to be properly built and maintained. 

Here are some steps you can take to control moisture and prevent mould:

Ventilate and control humidity

• Use exhaust fans in damp areas like bathrooms, kitchens, and crawlspaces.
  
• Make sure exhaust systems are installed correctly and vent to the outside, not into the attic. Clean the bathroom and kitchen exhaust systems regularly.
• Vent dryers to the outside, not into the attic or basement.
  
• Make sure heating vents aren't blocked so air can move around your home.
  
• Regularly clean bathroom walls and ceilings that are often wet.
  
• Don't use wall-to-wall carpets in areas that are often damp, such as bathrooms or entries.
  
• Dry water-damaged areas quickly and thoroughly.
• Keep the relative humidity below 50%. You can buy a hygrometer at a hardware store to measure indoor relative humidity.
  
• Use a dehumidifier to help lower humidity if needed. Follow the manufacturer's instructions for cleaning any appliance with a water reservoir.
  

Keep your home in good condition

• Keep the roof, outside walls, windows, and foundations in good repair and weatherproof.
• Make sure rain and water flow away from your home using eavestroughs, downspout extensions, sump pump drainage lines, and proper grading.
• Use moisture barriers, weeping tile systems, and sump pumps to keep groundwater out of basements or crawlspaces. Seal and waterproof dirt crawlspaces.
• ​Make sure walls and ceilings have enough insulation to keep cold air out.
  
• Install vapour barrier on the warm side of exterior walls and ceilings.