Causes of Indoor Mould Growth

Factors Affecting Mould Growth

Three combined factors cause mould growth in buildings:

  1. Availability of water or moisture
  2. Presence of a food source, building materials such as drywall, ceiling tiles, and wood for examples
  3. Time, as little as 24 hours

Moisture (water) is the most significant factor causing mould growth indoors. A simple explanation is that water provides and transports nutrients needed for growth. Building materials (food source) and time cannot be eliminated from the indoor environment.


Water in Buildings

Moisture issues that cause mould contamination in buildings are broken down into two categories:

  1. Wetting Incidents
  2. Moisture Issues that occur over time (chronic moisture issues)


Wetting Incidents – Mould Growth

  1. Water Infiltration from surface or storm water. During heavy wetting from flooding or rain, soils can become saturated and water may enter a building.
  2. Water Leaks from roofs, flashing, or walls that are damaged during a storm.
  3. Water from fire fighting activities.
  4. Powerful storm driven rain that can pass through doors, walls, or windows.
  5. Water overflows from washing machines, bathtubs, sinks, and toilets.
  6. Ice dams that form on the edges of roofs. As ice or snow melts, these dams may direct water into a building.
  7. Broken or cracked plumbing that results in flows or sprays of water.
  8. Sewer back-ups from excessive rain or other causes.
  9. Air Leaks allowing humid air to enter. Moisture laden air can travel through most building assemblies.


Chronic Moisture Issues – Mould Growth

  1. Improper design or construction of exterior materials, rain shedding elements, or HVAC equipment.
  2. Wetting of wall cavities from air or roof leaks.
  3. Condensation:
    1. Hidden in floors or walls from air leaking through the building envelope.
    2. On doors or windows that allows water to travel into walls, floors, or ceilings.
    3. Attics in cold climates (Ottawa) that cause water to drip onto ceilings and into walls. One common issue is the improper installation of washroom exhaust ducts. Warm air travels through the ducts but is directed onto the underside of the roof resulting in condensation and mould growth.


Moud Growth in Buildings

Once water or moisture enters a building, susceptible materials become wet and little time is needed for contamination to start. In closing, controlling water or moisture infiltration is the key to preventing mould contamination. 

Buildings in mixed climates such as Ottawa and Gatineau are problematic. During the summer moist air travels from outside to inside a building potentially wetting materials. During the winter moist air travels from inside to outside a building potentially wetting materials. This is due to air pressure and temperature differences.


Mould Resources

  1. Fungal Contamination in Public Buildings: Health Affects and Investigation Methods (Health Canada)
  2. Mould Guidelines for the Canadian Construction Industry (Canadian Construction Association)
  3. Ottawa Contaminant Solutions Mould Services Page

Reference: Hung, L., D.J. Millar, K.H. Dillon: Field Guide for the Determination of Biological Contaminants in Environmental Samples, 2nd Edition. American Industrial Hygiene Association:39-40 (2005).

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Ottawa Contaminant Solutions 

1-125 Pretoria Avenue
Ottawa, ON  K1S 1W8