Ducts In The Cold
A duct running through an unheated Toronto space loses heat at one end of the year and condenses at the other, and both are avoidable.
Short answer
In winter a duct in a cold space loses heat and delivers cool air to the room at the end of the run. In summer, cooled air in a warm space condenses on the duct exterior, and warm moist air meeting a cooled duct interior condenses inside. Sealing then insulating addresses both, in that order.
Where it happens in Toronto
An attached garage ceiling, where duct serves a room above and the garage is unheated.
A crawlspace under an addition or a rear extension, which is extremely common on Toronto housing.
An attic, where duct was run to serve a converted upper floor.
An unheated or partly heated basement, where the duct is in the space but the space is cold.
A cold room or a root cellar, which some older Toronto houses still have.
And a joist space against an exterior wall, which is effectively outdoors in January.
Seal before you insulate
Insulating a leaky duct wraps the leak. The air still escapes, it just escapes through insulation, and on a cooling system the moisture in that leaking air condenses inside the insulation. Seal the joints with mastic first, then insulate. In that order, every time.
What goes wrong
Winter heat loss, so the far room never warms up and the homeowner closes the registers in other rooms to compensate, which makes the system worse.
Summer condensation on the outside of an uninsulated cooled duct, which drips — in a garage ceiling that shows as a stain on the ceiling below.
Condensation inside the duct where warm moist air meets a cold duct, which is a mould source.
A flexible duct with a wet liner, which is not cleanable and gets replaced.
Insulation fallen off and lying in the crawlspace, which is the usual finding.
And rodents in insulated flexible duct in a crawlspace, which is a Toronto classic.
Doing it properly
Seal every joint with mastic, including the branch takeoffs and the boot connections.
Support the duct properly so it is not sagging, which both restricts flow and collects water.
Insulate to an appropriate level for the space, with the vapour barrier on the correct side.
Replace flexible duct that is crushed, torn or has a wet liner rather than patching it.
Consider rerouting out of the unheated space entirely where the layout allows, which is the best answer.
And in a crawlspace, deal with the space itself — a vapour barrier on the soil and the perimeter — because an insulated duct in a wet crawlspace is still in a wet crawlspace.
Common questions
Should I seal or insulate first?
Seal first. Insulating a leaky duct just wraps the leak.
Why is the room above the garage always cold?
Usually uninsulated leaky duct in the garage ceiling.
Can crushed flexible duct be fixed?
Replace it. Patching does not restore the airflow.