Moisture, mould and indoor air

Mould needs moisture to grow, so every mould problem is really a moisture problem. Find and fix the source of water, whether a leak, condensation or high humidity, then clean up or remove what has grown. Killing mould without fixing the moisture only means it comes back.

How moisture gets into buildings

Mould spores are everywhere, indoors and out. They only grow when they land on something they can feed on, such as drywall paper, wood, dust or ceiling tile, and that material stays damp for long enough. The moisture usually comes from one of four places:

• Liquid water from outside: roof leaks, failed flashings, poor drainage at windows, foundation leaks.
• Plumbing leaks: supply lines, drains, fixtures, roof drains, and condensate lines from cooling equipment.
• Condensation: warm moist air reaching a cold surface, such as window frames, exterior walls with missing insulation, cold water pipes and ducts carrying cooled air.
• High indoor humidity: from occupants, cooking, showers, poor ventilation, humidifiers set too high, or processes like pools and kitchens.

In a Canadian winter, condensation inside walls and attics is a frequent cause. Warm, moist indoor air leaks into the building envelope through gaps and condenses on cold sheathing. See building envelope basics for how air barriers prevent this.

Where to look

• Below and around windows, and at the corners of exterior walls.
• Ceilings below roofs, roof drains and mechanical rooms.
• Around plumbing fixtures, under sinks and behind toilets and washers.
• Basements, crawl spaces and below-grade rooms.
• Around HVAC equipment: drain pans, cooling coils, humidifiers and supply diffusers.
• Stained, sagging or crumbling ceiling tiles.
• Behind furniture and stored items pushed against exterior walls.
• Anywhere with a persistent musty smell, even if nothing is visible.

Moisture meters and thermal imaging help find wet or cold areas behind finished surfaces without opening walls. A thermal camera does not see mould, but it can point to the dampness that lets mould grow.

Controlling humidity and ventilation

Indoor relative humidity that stays high for long periods encourages mould and dust mites. In cold weather, humidity that is too high also causes condensation on windows. Practical steps:

1. Run bathroom and kitchen exhaust fans, and make sure they vent outdoors, not into the attic or ceiling space.
2. Keep ventilation systems running as designed. In commercial buildings, outdoor air rates are set by codes and standards such as ASHRAE 62.1. See ventilation and indoor air quality.
3. Maintain HVAC drain pans and condensate lines so they drain freely.
4. Lower winter humidifier settings when windows start to show condensation.
5. Use dehumidifiers in basements during humid summer months.
6. Dry any water leak within a day or two where possible. The longer materials stay wet, the more likely mould becomes.

Investigating a mould complaint

A typical case: occupants on one side of an office floor report a musty smell each winter, and a few ceiling tiles near the exterior wall have brown stains. A useful investigation works from the complaint back to the moisture source, recording what is found at each step.

1. Talk to the occupants and staff. When does the smell appear? After rain, in cold weather, or all year? Has there been a leak, a flood or a renovation? The timing is often the best clue to the source.
2. Map the visible signs. Mark stains, damaged materials, visible growth and condensation on a floor plan, with photos. Patterns along an exterior wall, below a roof drain or near a mechanical room each point in a different direction.
3. Take moisture readings. Use a moisture meter on drywall, wood and ceiling materials, comparing suspect areas with dry areas of the same material. Record the values and locations so they can be checked again after repairs.
4. Look behind the surface where needed. Thermal imaging, a ceiling tile lifted, or a small inspection opening can confirm what is happening inside the wall or ceiling. Check for asbestos-containing materials before opening anything in an older building.
5. Check the building systems. Look at roof drains, window and wall flashing, ventilation rates, humidifier settings, drain pans and condensate lines serving the area.
6. Confirm the cause and fix it. In this case, the stained tiles sit below a perimeter area where warm air leaks into a cold ceiling space and condenses. The fix is air sealing and insulation, followed by removing the damaged tiles.
7. Verify. Recheck moisture readings after the next cold spell or wet season to confirm the area stays dry.

What the report should include

• The complaint, the areas inspected and the conditions on the day, including weather and indoor humidity.
• A plan or sketch showing stains, growth and moisture readings.
• The likely moisture source for each affected area, with the evidence for it.
• Materials that need cleaning, removal or replacement, and any hazardous materials to deal with first.
• Repairs needed to stop the moisture, in priority order.
• How and when to confirm the repairs worked.

Cleaning up mould

Small areas of mould on hard surfaces can often be cleaned by building staff or homeowners using detergent and water, with gloves, eye protection and a suitable respirator. Porous materials such as drywall, ceiling tile, insulation and carpet that have mould growing in them usually need to be removed and replaced.

Larger areas, mould inside wall cavities or HVAC systems, or mould in occupied buildings with vulnerable people such as hospitals and care homes, call for a professional remediation contractor. Remediation should use containment so spores and dust are not spread through the building. Health Canada and provincial occupational health and safety agencies publish guidance on mould; follow the guidance in your province, and remember that disturbing mould at work is a workplace hazard that needs to be assessed and controlled.

Is testing for mould useful?

Air sampling for mould spores can be useful in some investigations, such as checking whether cleanup was effective or searching for hidden growth. In most cases, though, visible mould or a musty smell is enough to act on. The priority is finding and fixing the moisture source.

How ehZee can help

As part of our commercial building inspections, ehZee looks for signs of moisture, leaks, condensation and mechanical issues that lead to mould. We can help you trace the likely source and plan repairs, and coordinate with remediation contractors where cleanup is needed.

Common questions