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Mouldy Homes and Buildings

What causes indoor mould and how to prevent it

​​What are some common causes of indoor mould?

Common causes of indoor mould include:

  • leaky foundations, walls, roofs, or windows
  • leaking water pipes or wastewater lines
  • poor window or wall construction and insulation
  • not enough weather stripping around doors or windows
  • high indoor humidity
  • poor exhaust ventilation in kitchens or bathrooms, like kitchen hoods and bathroom fans
  • damp basements or crawl spaces
  • flood or sewer backup (These need special attention. They likely contain harmful bacteria, viruses, or chemicals.)

What are some signs that there might be mould indoors?

Mould may be 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
  • water condensation or frost on inside walls
  • musty or earthy smell in the building
  • water leaks that cause water to pool on floors, drip from pipes, or stain surfaces like drywall or cement
  • mould that looks like it’s spreading or comes back after cleaning
  • you or someone in your home has the symptoms of being exposed to mould

How can I stop mould from growing indoors?

To stop mould from growing indoors, you need to control indoor moisture. This is why it’s important for homes and other buildings to be properly built and maintained. Here’s what you can do:

  • Keep the roof, outside walls, windows, and foundations in good repair and weatherproof.
  • Make sure surface water runs away from your home with proper grading, eavestroughs, downspout extensions, and sump pump drainage lines.
  • Use moisture barriers, weeping tile systems, and sump pumps to prevent groundwater from seeping into basements or crawlspaces.
  • Make sure any dirt crawlspaces are covered, sealed, and made watertight.
  • Make sure walls and ceilings have enough insulation to keep cold air out.
  • Install vapour barrier on the warm side of outside walls and ceilings to prevent condensation.

Keep your home well ventilated (allow air to move around) and control humidity. Below are some ways you can control humidity:

  • Make sure there is good general ventilation throughout the home, including exhaust ventilation (like ceiling fans or kitchen hoods) in damp areas such as crawlspaces, bathrooms, and kitchens.
  • Make sure that exhaust systems are installed correctly and vent to the outside, not into the attic.
  • Vent dryers to the outside, not into the attic or basement.
  • Make sure heating vents aren’t blocked.
  • Don’t have too many plants in the room.
  • Clean the bathroom and kitchen exhaust systems regularly.
  • Regularly wipe clean bathroom walls and ceilings that are often wet.
  • Regularly de-scale, clean, and sanitize humidifiers.
  • Don’t use wall-to-wall carpets in areas that are often damp or wet, such as bathrooms or entries.
  • Dry water-damaged areas well to stop mould from growing.
  • Keep the relative humidity below 50%. You can buy a hygrometer at hardware stores to measure indoor relative humidity.
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Current as of: February 18, 2021

Author: Environmental Public Health, Alberta Health Services