Leaky Ducts & Duct Sealing: Is It Worth It?

Almost every duct system leaks. The U.S. Department of Energy estimates that 20–30% of the air moving through a typical system is lost to loose joints, gaps, and poorly connected runs — and if those ducts pass through an unheated basement, attic, or crawlspace, that's air you paid to heat, escaping where it does you no good. Sealing the leaks is one of the best-value things you can do to a forced-air system. But "seal the ducts" isn't one job with one price, and it isn't the right fix for every airflow problem. Here's how to tell what your ducts need.

What leaky ducts actually cost you

A duct leak isn't dramatic — there's no puddle, no alarm. It just quietly bleeds off a share of every dollar you spend on heat. When that leak is in a finished, conditioned space, the escaped air at least stays in the house. When it's in an unconditioned space — the basement, an attic, a crawlspace, a garage — it's a total loss. You're heating the joists before you heat the bedrooms.

The signs are familiar and easy to write off:

• Higher heating bills than the house's size and insulation should call for.
• Rooms that never quite warm up, especially the ones farthest from the furnace or air handler.
• A dusty house, because return-side leaks pull dust and attic air into the system.
• Whistling or a rush of air audible near the ducts when the system runs.

None of it screams "duct leak," which is why leaks go unaddressed for years while the meter keeps running.

Where ducts actually leak

Leaks concentrate at the connections, not in the middle of a straight run:

• Joints and seams where sections of duct meet, especially if they were only tacked with screws and old tape.
• Register and grille boots where the duct meets the floor or wall.
• The plenum and takeoffs at the furnace or air handler — big surfaces, lots of seams.
• Disconnected or fallen runs that came apart entirely and now dump into a joist bay or the basement.

The leaks that matter most are the ones in unconditioned space. Sealing a joint in a finished, heated basement saves a little; sealing the same joint on a run through a cold attic saves a lot.

The three ways to seal ducts

There's no single "duct sealing." There are three approaches, and good work often combines them:

1. By hand with mastic. For joints and seams you can reach, water-based mastic is the gold standard — it brushes on as a paste and cures into a durable, flexible, airtight seal that outlasts any tape. This is the workhorse of a proper sealing job.
2. UL-181 foil tape for smaller cracks and tight spots where a brush won't fit. It's the right tape for the job — and worth saying plainly: never use cloth "duct tape." Despite the name, it dries out, lets go, and peels off within a few years.
3. Aeroseal for the leaks you can't reach. It's an aerosolized sealant blown through the pressurized duct system that finds and plugs leaks from the inside — including ones buried in walls, ceilings, and chases where no brush or hand can go.

Which combination is right depends entirely on where your leaks are and how accessible they are.

Is Aeroseal worth it?

This is the question most people are really asking, so here's the honest version. Aeroseal genuinely works: it can seal up to around 90–95% of measured leakage, with real-world results more often in the 80s, and it reaches hidden leaks that hand-sealing simply can't. A typical residential job runs about $1,500–$3,000, and in a leaky home the energy it saves can pay that back over several years.

It's worth it when a meaningful share of your ductwork is inaccessible — buried in walls and ceilings, in a finished space, or in a sprawling attic run — because that's exactly the leakage you can't get at by hand.

It's not worth it when your leaks are accessible basement joints. Those can be sealed by hand with mastic for a fraction of the cost, and paying for an aerosolized system to do what a brush and an afternoon can do is spending money for its own sake.

And it's not a cure-all either way. Aeroseal seals gaps; it can't resize an undersized duct, add a return that was never there, or reconnect a run that's fallen apart. Anyone selling it as the answer to every airflow complaint is overselling it. We treat it as one tool among several, and only recommend it where it actually pays off.

What sealing can't fix

Sealing solves one specific problem: air escaping through gaps. It does nothing for the problems that get blamed on leaks but aren't:

• Undersized ducts that can't carry enough air no matter how airtight they are.
• Too few cold-air returns, which starve the system regardless of leakage.
• Crushed or kinked flex that chokes airflow at a pinch point.
• A layout that was never balanced and favours the nearest rooms.

These need physical work — resizing, adding returns, replacing bad sections — not sealant. If your real issue is one of these, sealing alone will disappoint you. It's why we diagnose the whole system before reaching for any one fix; the full rundown of what goes wrong is in our guide to common ductwork problems in older Halifax homes. Sealing is also a standard part of a furnace conversion, where the ducts are getting reworked anyway.

Where we fit in

Duct sealing is air-side work — our trade. We don't lead with one method and sell it to everyone; we look at where your ducts run, find where they leak, and seal the joints that are costing you, using mastic and proper tape where we can reach and recommending Aeroseal only where hidden leaks justify it. If your bills are high, a room won't warm up, or you just want the system you have to stop wasting what it makes, that's a straightforward thing to assess across Halifax HRM — and often a cheaper fix than people expect.