Duct Sealing in Ontario: Is It Worth It, and How Is It Done?

Duct sealing means closing the gaps at seams, joints and connections in your forced-air ductwork so heated and cooled air goes where it is supposed to. It is often worth doing in Ontario when ducts run through unheated spaces or when some rooms are noticeably colder than others, but the energy savings are usually smaller than in climates where ducts sit in hot attics.

This guide explains how much leaky ducts matter in a typical Ontario home, the methods and costs, and how to decide whether to do it yourself, hire a contractor, or put the money elsewhere.

How much air do leaky ducts waste?

ENERGY STAR and other sources commonly estimate that a typical duct system loses around 20–30% of the air it moves through leaks, holes and poorly connected sections. Real homes vary widely; some newer systems are fairly tight, while older ones with panned joist returns (where the space between floor joists is used as a return duct) can be very leaky.

Where that air goes matters:

Duct location	What happens to leaked air	Energy impact
Inside a heated, finished basement	Leaks warm the basement	Small direct loss, but upstairs rooms get less air
Unfinished but heated basement	Warms the basement and its walls	Moderate; more heat lost through uninsulated foundation walls
Attic, garage, vented crawl space	Leaks go outdoors or into unheated space	Large loss; top priority
Exterior wall cavities	Leaks into walls, sometimes outdoors	Moderate to large; can cause cold spots

In most Ontario houses with basements, the main benefit of sealing is better air delivery to the rooms that need it, especially second floors and rooms at the end of long runs. Our guide to fixing cold rooms covers this in detail.

Why return-side leaks deserve attention

Return ducts carry air back to the furnace or air handler. Leaks on the return side pull air from wherever they are, including basements, crawl spaces and, in some cases, near a gas water heater or furnace flue.

Caution: Large return leaks in a mechanical room can depressurize the space and contribute to backdrafting of an atmospherically vented gas water heater or older furnace, pulling exhaust gases back into the home. Have return leaks near combustion appliances sealed, and make sure working carbon monoxide alarms are installed. A gas technician or energy advisor can do a combustion safety test.

How are ducts sealed?

Method	How it works	Best for	Typical cost (2026 estimate)
Mastic (brush-on sealant)	Thick paste applied over joints and seams, sometimes with mesh tape	Accessible joints in basements, attics	DIY materials $30–$100; contractor often $300–$1,200 for accessible ducts
UL 181 foil tape	Metal-backed tape rated for ducts	Seams and smaller gaps, quick fixes	DIY materials $20–$60
Aerosol sealing (such as Aeroseal)	Ducts are pressurized and a sealant mist seals leaks from inside	Ducts hidden behind finished walls and ceilings	Roughly $1,500–$3,500 for a typical home
Duct insulation	Wrapping ducts in unconditioned spaces after sealing	Attics, garages, crawl spaces	Varies with length and access

Mastic is durable and inexpensive, and it is what most energy advisors prefer for accessible ductwork. Foil tape is fine for flat seams if the surface is clean and dry. Aerosol sealing is useful when most ducts are hidden in finished spaces, and it usually comes with before-and-after leakage measurements.

Do not use ordinary cloth-backed "duct tape." Its adhesive dries out with temperature cycling and it usually falls off within a few years.

Can you seal ducts yourself?

Yes, for accessible ductwork in an unfinished basement. A typical DIY approach:

Turn off the furnace or air handler at the thermostat and switch.
Start at the furnace: seal where the supply plenum and return connect to the cabinet, without covering access panels, filter slots or labels.
Seal joints and seams along the main trunk lines, especially where branch ducts take off from the trunk.
Seal the boots: where ducts meet floor and ceiling registers, seal the gap between the boot and the subfloor or drywall from the room side or below.
Address panned returns: seal the edges of sheet metal panning across joists, or ask a contractor about replacing it with proper ducting.
Insulate any ducts that pass through unheated spaces, after sealing.
Replace the filter and restart the system; debris may be disturbed.

Avoid sealing over dampers you need to adjust, and do not block combustion air openings. If your home may have older duct insulation or tape containing asbestos (more common in homes built before the 1980s), do not disturb it; have it tested first.

Is duct sealing worth it?

It usually is when:

Ducts pass through an attic, garage, crawl space or exterior walls.
Some rooms are much colder or hotter than others, and the problem is airflow.
An energy assessment or duct leakage test shows high leakage.
You are installing a new furnace or heat pump; sealing first helps the new system perform as designed, and heat pumps are more sensitive to airflow.
You are finishing a basement, which is your last easy chance to reach the ducts.

It is usually a lower priority when ducts are entirely inside a heated, finished basement, the home is comfortable, and bills are typical. In that case, attic insulation and air sealing the building itself usually save more. See our air sealing guide and home energy assessment guide.

How is duct leakage measured?

Energy advisors and some HVAC contractors use a duct pressurization test (often called a duct blaster test), which seals the registers and measures how much air leaks at a set pressure. A blower door test on the whole house can also give clues. Before-and-after testing confirms that the work made a difference, and is worth asking for on larger jobs.

Tip: After sealing, the system may push noticeably more air to distant rooms. Have the dampers rebalanced, and check that the filter is not now the main restriction. See our furnace filter guide.

Frequently asked questions

Will duct sealing lower my heating bill?

In homes where ducts run through unheated spaces, savings can be meaningful. Where ducts are inside the heated basement, energy savings are usually modest; the bigger gain is comfort and more even temperatures.

Is duct cleaning the same as duct sealing?

No. Cleaning removes dust and debris; sealing closes leaks. Cleaning rarely saves energy. In Ontario, unsolicited door-to-door contracts for duct cleaning are banned, so be cautious about at-the-door offers.

Should I close registers in unused rooms?

Closing a few registers is generally fine, but closing many increases pressure in the ducts, which pushes more air out through leaks and can strain the blower. Sealing ducts and balancing dampers is a better approach.

Does sealing ducts affect my HRV or furnace warranty?

Sealing duct joints does not affect equipment warranties. Do not seal over equipment panels, labels or service openings, and do not modify the furnace cabinet itself.