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Environmental Health

Mouldy Homes and Buildings: Causes and Prevention of Indoor Mould

​​​​​What are some common causes of indoor mould?

Common causes 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 back-up

Flood and sewer back-ups need special attention as they likely contain bacteria, viruses, or chemical hazards.

What are some signs of possible indoor mould?

Mould growth may be black, brown, or green. It may have a fuzzy or patchy texture. Watch for signs like:

  • stains or speckled areas on walls, ceilings, floors, or furniture
  • water condensation or frost on inside walls
  • musty or earthy smell in the building
  • symptoms seen with exposure to mould
  • water leaks that cause water to pool on floors, drip from pipes, or stain surfaces like drywall or cement
  • mould that looks like it is spreading or comes back after cleaning

What can I do to prevent mould from growing indoors?

To prevent mould from growing indoors, you need to control indoor moisture. This is why it’s important for a building to be properly constructed and maintained.

  • Keep the roof, exterior walls, windows, and foundations in good repair and weatherproof.
  • Make sure surface water runs away from the ​house with proper grading, eaves troughs, downspout extensions, and sump pump drainage lines.
  • Use moisture barriers, weeping tile systems, and sump pumps to prevent ground water 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 exterior walls and ceilings to prevent condensation.

Keep your home well ventilated and control humidity. Keep the relative humidity below 50% to keep mould from growing. You can buy a hygrometer at hardware stores to measure indoor relative humidity. Below are some ways you can control humidity.

  • Make sure there is good general ventilation throughout the building, including exhaust ventilation (like ceiling fans or kitchen hoods) in damp areas such as crawlspaces, bathrooms, and kitchens.
  • Make sure that exhaust systems are properly installed 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 (like bathrooms or entries).
  • Dry areas damaged by water or moisture well to prevent mould from growing.
  • Keep humidity settings low.

Current as of: November 3, 2016

Author: Environmental Public Health, Alberta Health Services