Why does my Toronto home ductwork sweat and drip water onto the basement floor in summer?

Your ductwork is sweating because cold supply ducts running through your warm, humid basement are dropping below the dew point of the surrounding air — condensation forms on the duct surface exactly the same way a cold glass of water sweats on a humid summer day.

This is one of the most common ductwork complaints in the GTA, and it's almost entirely a moisture and insulation problem, not a duct problem per se. When your air conditioner is running, supply ducts carry air chilled to roughly 12–15°C. Your basement in July or August might have air at 24–28°C with relative humidity above 60% — a humidex situation that pushes the dew point up to 15–18°C. The moment your cold duct surface drops below that dew point, moisture condenses out of the air and onto the duct. Enough condensation accumulates to drip onto the floor, stain ceiling tiles, and eventually promote mould growth on the duct exterior, surrounding insulation, and wood framing.

Toronto's summer humidity is the key aggravating factor here. The GTA regularly sees humidex values above 40 from late June through August, and Lake Ontario's moderating effect keeps lakefront neighbourhoods — the Beaches, Mimico, Port Credit, Lakeshore — more persistently humid than inland areas. An uninsulated galvanized steel trunk duct in a Scarborough or Etobicoke basement that barely sweated 20 years ago may now drip constantly because the home has been air-sealed more tightly over the years, trapping more indoor humidity, or because the air conditioner was upgraded to a larger, colder-running unit.

The fix is duct insulation, not dehumidification alone. Wrapping your supply ducts with R-8 duct wrap insulation — fibreglass blanket with a foil vapour barrier — raises the outer surface temperature of the duct above the dew point, stopping condensation at the source. R-8 is the minimum recommended value for GTA basements and is non-negotiable for attic duct runs. The vapour barrier facing must face outward and all seams must be sealed with UL 181-rated foil tape to prevent humid air from reaching the cold duct surface through gaps in the insulation. If you use duct mastic to seal the duct joints first, then wrap with R-8, you've addressed both air leakage and condensation in one pass.

Before wrapping, seal the duct joints. Leaking duct joints in the basement are doubly problematic in summer — they lose cold conditioned air into the basement (wasting energy) and create cold spots on the duct exterior that condense even more aggressively. Apply duct mastic to all joints and seams before insulating. Never use grey cloth duct tape for this — it dries out and falls off within a few years in the temperature cycling of a GTA basement. Only UL 181 foil tape or mastic belongs on ductwork.

Check your return air situation as well. Many post-war Toronto homes — particularly those built between 1945 and 1975 in North York, Scarborough, and Etobicoke — use floor joist cavities as return air plenums. These "panned joist" returns are extremely leaky and pull humid basement air directly into the system, increasing the moisture load on your air conditioner and worsening condensation on supply ducts. If you have this setup, replacing panned joist returns with proper sheet metal return ductwork is one of the highest-impact ductwork upgrades you can make.

What you can DIY versus what needs a pro: Wrapping accessible basement ductwork with R-8 duct wrap is a reasonable DIY project for a capable homeowner — the materials are available at HVAC supply houses, and the technique is straightforward. Sealing joints with mastic beforehand is also manageable. However, if your ductwork runs through finished ceiling spaces, if you suspect mould has already established on the duct exterior or surrounding framing, or if you have panned joist returns that need replacing, bring in a professional. Mould remediation before insulating is essential — trapping active mould growth under insulation makes the problem significantly worse.

Professional duct insulation for a typical GTA basement runs $1,500–$4,000 depending on the amount of exposed ductwork and whether sealing is included. If Aeroseal duct sealing is warranted for inaccessible sections, budget an additional $1,500–$3,500.

Need help finding a ductwork contractor to assess and insulate your basement duct system? Toronto Ductwork can match you with local professionals for free — browse contractors through the Toronto Construction Network directory at torontoconstructionnetwork.com/directory?trade=hvac.