How do I seal my Toronto home ductwork to reduce energy loss?

Sealing your ductwork is one of the most cost-effective energy upgrades you can make in a Toronto home — leaky ducts waste 20 to 40 percent of your heating and cooling energy, and proper sealing can noticeably improve comfort, reduce energy bills, and improve indoor air quality. The approach depends on whether your ducts are accessible (visible in an unfinished basement or crawlspace) or concealed (inside walls, above finished ceilings, or in enclosed chases).

For accessible ductwork in unfinished basements — which is the situation in most GTA homes with forced-air heating — sealing is a practical DIY project for a capable homeowner. Start by running your furnace fan and carefully feeling around every duct joint, seam, connection, and takeoff point for air leaks. You will feel them as cool or warm air blowing against your hand, depending on whether the system is heating or cooling. The most common leak points are where branch ducts connect to the trunk line, at register boot connections, where the trunk duct connects to the furnace plenum, and at any joints that have been taped with grey cloth duct tape (which is NOT rated for HVAC use and inevitably fails in GTA temperature extremes).

The proper sealing method uses duct mastic — a thick, water-based sealant that you brush or spread with a gloved hand over every joint and seam. Mastic stays flexible permanently, does not degrade with temperature cycling, and creates an airtight seal that lasts the life of the duct system. Apply a generous layer over the entire joint, extending at least one inch beyond the connection on each side. For gaps wider than about 3 millimetres, press fibreglass mesh tape into the mastic before applying a second coat — this bridges the gap and prevents the mastic from cracking. A gallon of mastic costs $15 to $30 and covers 30 to 50 joints, which is enough for most homes. For a belt-and-suspenders approach, apply UL 181-rated aluminum foil tape over the dried mastic. Never use grey cloth duct tape — despite its name, it dries out and falls off within two to five years.

For concealed ductwork that you cannot reach — ducts inside walls, above finished ceilings, or in enclosed soffits — professional Aeroseal treatment is the best option. Aeroseal is a patented process where polymer particles are blown through your sealed duct system from the inside. The particles accumulate at leak points and seal gaps up to 5/8 inch from within, without needing to access the ducts physically. This technology is particularly valuable in older GTA homes where decades of temperature cycling have loosened joints in concealed duct runs. Aeroseal costs $1,500 to $3,500 for a typical Toronto home and typically reduces duct leakage by 80 to 90 percent.

After sealing, insulate any ducts that run through unconditioned spaces — attics, crawlspaces, and unheated garages. Use R-8 duct wrap insulation with a foil vapour barrier, and seal all insulation seams to prevent condensation from reaching the duct surface. In GTA summers, uninsulated ducts carrying cold air through a humid basement sweat with condensation that promotes mould growth and rusts the ducts. Professional duct sealing and insulation for an entire home runs $1,500 to $4,000 in the GTA. If you want a professional assessment of your duct system's leakage, browse HVAC and ductwork contractors through the Toronto Construction Network directory.