What should I do if I find mould inside my Toronto home ductwork?

If you discover mould inside your ductwork, the most important thing to understand is that cleaning the mould without addressing the moisture source that caused it will only result in the mould returning. Mould needs moisture to grow, and the presence of mould inside ducts tells you that there is a condensation or water intrusion problem in your duct system that must be solved first.

In GTA homes, the most common cause of mould in ductwork is condensation on uninsulated or under-insulated ducts in unconditioned spaces. During Toronto's humid summers, when supply ducts are carrying air-conditioned air at 12-15 degrees Celsius through a hot, humid attic or basement, the temperature difference causes moisture to condense on the cold duct surfaces — the same way a cold glass sweats on a humid day. This condensation soaks into duct insulation, collects in low points of flex duct, and creates the persistent moisture that mould needs to colonize. Ducts in unconditioned spaces need a minimum of R-8 insulation with an intact vapour barrier facing outward to prevent this condensation cycle. Lakefront neighbourhoods like the Beaches, Mimico, and Port Credit are particularly vulnerable due to Lake Ontario's humidity effect.

Before doing anything else, stop using the system if mould is extensive. Running the blower circulates mould spores throughout every room in the home. Then assess the extent of the problem. Surface mould on the interior of galvanized steel ducts can be cleaned — the metal itself does not support mould growth, so once cleaned and the moisture source is eliminated, mould will not return on steel surfaces. However, mould on flex duct inner liners, fiberglass duct board, or duct insulation generally cannot be effectively cleaned. These porous materials absorb moisture and mould penetrates below the surface where cleaning cannot reach. Contaminated flex duct and duct board sections must be removed and replaced.

Professional mould remediation in ductwork typically involves several steps. A qualified contractor will inspect the entire duct system to map the extent of contamination, identify the moisture source, clean salvageable metal duct sections using HEPA-filtered equipment and antimicrobial treatment, remove and replace contaminated flex duct or duct board, insulate or re-insulate ducts in unconditioned spaces with R-8 duct wrap and properly sealed vapour barrier, and seal all duct joints with mastic to prevent air leakage that contributes to condensation. The cost for professional duct mould remediation in the GTA ranges from $1,500-$5,000 depending on the extent of contamination and how much ductwork needs replacement versus cleaning.

Do not attempt DIY mould cleaning inside ductwork. Disturbing mould without proper containment releases massive quantities of spores into the air. Professional duct cleaning companies with NADCA certification use truck-mounted HEPA-filtered equipment that maintains negative pressure in the duct system during cleaning, preventing spores from entering living spaces. Legitimate duct cleaning in the GTA costs $300-$700 for a standard home — be extremely wary of companies offering $99 whole-house deals, as these are bait-and-switch operations.

After remediation, the moisture source must be permanently addressed. This may mean adding or upgrading duct insulation, repairing vapour barriers, fixing a roof leak that drips onto attic ducts, or improving ventilation in a damp basement. Without solving the moisture problem, you will be paying for mould remediation again within a year or two. If your home has persistent humidity issues contributing to duct condensation, an ERV or HRV system ($2,500-$6,000 installed) can help manage indoor moisture levels while maintaining fresh air exchange.