Why is air leaking from the joints in my Toronto home ductwork?

Air leaks at duct joints in Toronto homes are caused by failed sealant, thermal expansion and contraction from the GTA's extreme temperature swings, age-related deterioration, and in many cases, joints that were never properly sealed in the first place. A typical older GTA home loses 20 to 40 percent of its heated and cooled air through duct leaks, which translates directly into higher energy bills, uneven room temperatures, and reduced indoor air quality.

The most common culprit is cloth duct tape — the grey tape that ironically shares a name with ductwork but is completely unsuitable for HVAC applications. Cloth duct tape was widely used on GTA homes built from the 1950s through the 1990s, and it fails predictably within two to five years. The adhesive dries out, the fabric deteriorates, and the tape peels away from the joint, leaving gaps that leak conditioned air into basements, crawlspaces, wall cavities, and attics. If you see grey tape hanging off your duct joints, it has failed and the joints are leaking.

Toronto's climate accelerates joint failure significantly. With winter temperatures dropping to minus 20 degrees Celsius and summer temperatures exceeding 35, your metal ductwork expands and contracts through a temperature range that stresses every joint connection. Over 50 freeze-thaw cycles per winter work the joints back and forth like a slow-motion wrestling match between metal and sealant. Connections that were tight when installed gradually loosen, and sealant that was marginal to begin with cracks and separates. Snap-lock round duct joints and rectangular duct S-clips and drives are particularly vulnerable to this thermal cycling.

Vibration from the furnace blower also contributes to joint failure over time. Every time the system cycles on, the blower motor creates vibration that travels through the ductwork. Over tens of thousands of cycles across years and decades, this vibration loosens mechanical connections and cracks brittle sealant.

To fix leaking joints, the proper approach is to seal every accessible joint with duct mastic — a water-based, fibre-reinforced sealant that you apply with a brush or gloved hand in a thick layer over the joint. Mastic remains flexible permanently, adheres to both galvanized steel and flex duct collars, and does not degrade with temperature cycling. For a belt-and-suspenders approach, apply mastic first and then cover with UL 181-rated foil tape. A gallon of duct mastic costs $15 to $30 and covers 30 to 50 joints, making this one of the most cost-effective home improvements available.

In an unfinished Toronto basement with exposed ductwork, sealing joints yourself is a practical DIY project. Start at the furnace plenum and work outward along each trunk and branch run, applying mastic to every joint, seam, and connection including takeoff collars, register boots, and duct-to-plenum connections. For ducts concealed in walls, ceilings, and finished spaces, Aeroseal is the professional solution — a patented process that seals ducts from the inside by blowing aerosolized polymer particles through the system that accumulate and seal at leak points. Aeroseal costs $1,500 to $3,500 for a typical GTA home but can reduce duct leakage by 90 percent, often paying for itself within a few years through energy savings. If you need help finding a ductwork contractor to address leaking joints, Toronto Ductwork can match you with local professionals for free estimates.