Quick answer
Condensation forms on ducts when the metal is colder than the dew point of the air around it. In summer, supply ducts carry air at roughly 12–14 °C, and basement air at 21 °C sweats on them once humidity passes about 60%. Dry the basement to 50%, seal and insulate the ducts with foil-faced wrap, and fix low airflow from a dirty filter or an icing coil.
There is condensation on your ducts because the duct surface is colder than the dew point of the air touching it, the same reason a cold drink sweats on a patio table. An air conditioner sends air down the supply trunk at roughly 12 to 14 °C. Basement air at 21 °C and 50% relative humidity has a dew point near 10 °C, so the duct stays dry; at 65% the dew point is about 14 °C and the metal starts to bead. Either the air around the duct is too wet or the duct is running too cold, and often it is both.
Which ducts are sweating, and what that tells you
- Bare supply trunks and branches in the basement, wet along their whole length: the basement is above 60% relative humidity.
- Heaviest at the plenum and the first metre or two of trunk, sometimes with frost on the copper line: the duct is too cold because airflow is low.
- A wet jacket or a drip at one spot on a wrapped duct in an attic, crawlspace or garage ceiling: the vapour barrier is torn or a seam was never taped, so humid air reaches the cold metal and water collects inside the wrap.
- Water around a ceiling register upstairs: an uninsulated boot in the attic, or attic duct wrap that has been pulled back.
- Water on the floor at the furnace with dry ducts: that is the coil drain, covered in why an AC leaks water.
- In winter, a wet or frosty insulated duct running to the HRV: that duct carries outdoor air, and a torn jacket lets indoor moisture condense in the insulation.
Low airflow makes supply ducts colder than they should be
The less air that crosses the evaporator coil, the colder that air leaves it. A clogged filter, a matted coil, closed registers or a blower set to the wrong speed can drop the supply temperature by several degrees, enough to push a borderline duct below the dew point. If ice forms on the coil, the plenum sits near freezing and everything close to the furnace drips.
Look at the larger of the two copper lines where it enters the coil cabinet. Frost or ice there means the coil is freezing: switch cooling off, set the fan to On to thaw it, and read why an AC freezes up. A low refrigerant charge causes the same icing, and refrigerant work in Ontario has to be done by a technician carrying an ODP card.
Check this yourself before booking a visit
- Hang a hygrometer near the ducts. A reading over 60% means humidity is the first thing to fix.
- Replace the furnace filter if you cannot see light through it.
- Open every supply register. Closing basement registers to push air upstairs chills the trunk and makes the sweating worse.
- Shut the basement windows and put the HRV on its low or intermittent summer setting.
- Run your hand along wrapped ducts and look for tears, open seams and hanger straps that have cut through the foil.
- Note whether the ducts only get wet on muggy days.
How to stop ducts from sweating: dehumidify, seal, insulate, restore airflow
- Dehumidify to 45–50%. A portable unit handles one room; a ducted whole-home dehumidifier, $1,500–$3,000 installed, is the fix when the entire basement is damp. The setting is explained in what to set a basement dehumidifier to.
- Seal the joints first, with mastic or foil tape on every seam and take-off. Cloth duct tape dries out and falls off, and cold air leaking from a joint chills the wrap from the inside.
- Insulate with foil-faced fibreglass duct wrap. The foil is the vapour barrier, so each seam and each hanger penetration gets foil tape, and the wrap is laid on without being squeezed flat. One gap becomes one wet spot.
- Insulate attic boots and replace flex duct whose outer jacket has torn.
- Restore airflow: a new filter, a cleaned coil and the right blower speed. An AC tune-up with a refrigerant check and coil cleaning is $120–$250.
Sealing and wrapping has no flat published price, because it depends on the metres of duct, whether the joists are open or the ceiling is finished, and whether any of it is in an attic. You get a written quote after we have seen the runs.
Ceiling stains, rust and mould: what happens if duct condensation is ignored
A sweating duct is not an emergency this week, and it is not something to leave for a whole summer. The drips stain drywall and ceiling tiles, soak the insulation in the joist bays and rust the seams of galvanized duct. Mould grows on the dust that coats the duct and on the framing around it.
Where you see it first depends on the house. In 1950s and 60s bungalows the trunk runs exposed across an unfinished basement, so you find beads on the metal and drip marks on the boxes stored below. In 1970s to 90s subdivision houses with finished basements, the ducts are above a drop ceiling or boxed into a bulkhead, and the first sign is a brown ring on a tile. Houses near Lake Ontario in Burlington, Oakville and Stoney Creek see it during the muggiest weeks of July and August.
- Call now if a ceiling is sagging or dripping under a duct.
- Call now if there is ice on the coil or the copper line and it returns after thawing.
- Call now if water is reaching the furnace cabinet, its wiring or the floor around it.
- Call soon if you can see mould on the duct, the joists or the back of a ceiling tile.
ZK Mechanical measures the supply air temperature and the basement humidity, checks airflow across the coil, and quotes sealing and insulating the existing runs or replacing sections through our ductwork installation service. Phone (647) 801-1252 or describe where the water is showing up. The air quality and ducts section has the related humidity and airflow answers.