Leaky Ducts
A leaky duct system wastes heat, pulls dusty air from the basement into the house, and makes rooms uneven — and it is a cheaper fix than most people think.
Short answer
Supply leaks dump heated air into unconditioned space; return leaks pull basement, crawlspace and garage air into the supply stream. Both cost money and both affect air quality. The high-value targets are the filter slot, the cabinet, the plenum joints, and any cavity return — and sealing them is mastic and labour rather than new ductwork.
Why it matters
Heated air leaking into a basement or a crawlspace is heat you paid for and cannot use.
Return leaks pull air from wherever the duct runs, which in a Toronto house is frequently a dusty basement, a crawlspace or an attached garage.
Garage air in particular is a genuine air quality problem — exhaust, solvents and whatever is stored there.
Leakage unbalances the system, which is why one room is cold and another is stuffy.
A depressurized basement can backdraft a gas appliance, which is the serious version of this.
And it reduces the effectiveness of filtration, because air bypassing the filter is unfiltered air.
The filter slot
The gap where the filter slides in is frequently wide open, and the blower pulls basement air straight through it, past the filter. It is the single highest-value seal in the whole system, it takes minutes with a proper cover or gasket, and it is open on a large proportion of Toronto installations.
The high-value targets
The filter slot and the cabinet access panels.
The joints at the supply and return plenums, which are the largest leaks in the system.
Branch takeoffs from the trunk.
Cavity returns — joist space and stud space — which leak enormously and which are the usual source of basement air in the supply.
The connection between the return duct and the furnace.
And any duct in an unheated space, which should be both sealed and insulated.
How it is done
Mastic, not tape. Duct tape fails within a few years and foil tape is a second choice; mastic is the material.
Mechanical fastening first where joints have separated, then sealing.
A proper filter slot cover or gasket rather than tape over the gap.
Insulation on anything in unheated space, after sealing rather than instead of it.
Converting cavity returns to sheet metal where access allows, which is the full answer.
And testing afterwards if the work is substantial, because a measured result is worth having and is how you know the money did something.
Common questions
What is the highest-value duct seal?
The filter slot. The blower pulls basement air straight past the filter through it.
Is duct tape acceptable?
No. It fails within a few years. Mastic is the material.
Why is one room always cold?
Frequently leakage unbalancing the system rather than the register setting.