Duct Sealing — matched to a local tech

What is duct sealing, and how does it work?

Duct sealing closes the gaps, cracks, and loose joints that let heated or cooled air leak out of your ductwork before it reaches your rooms. A technician finds the leaks, then closes them from the outside with mastic paste or UL 181 rated tape at accessible joints, or from the inside with an aerosol-injected sealant that finds and plugs leaks under pressure. Good sealing work starts with a look at the whole system: where the runs go, which joints are accessible, and how the ducts were assembled in the first place. It usually ends with a pressure test showing how much the leakage dropped. DuctDove does not perform sealing ourselves; we refer you to a local, independent technician who does this work every week.

How much conditioned air do leaky ducts actually lose?

Enough to notice. ENERGY STAR states that in a typical house about 20 to 30 percent of the air that moves through the duct system is lost to leaks, holes, and poorly connected ducts. That is ENERGY STAR's figure, not ours, and your house may lose more or less, which is exactly why measurement beats guessing. A duct system that leaks into an attic or crawlspace is paying to condition space nobody lives in, and the rooms at the end of long runs feel it first. The honest way to learn your number is a duct leakage test before any sealing is proposed. A tech who quotes sealing without measuring, or who promises savings as a guaranteed percentage, is guessing on your behalf.

What is the difference between mastic and foil tape sealing?

Mastic is a thick, paintable sealant that gets brushed or gloved onto duct joints and seams, often reinforced with fiberglass mesh over larger gaps. It stays flexible, handles temperature swings, and is the workhorse for accessible sheet metal joints. Foil tape rated UL 181 is the right call for certain seams and for securing flex duct liners to collars, where mastic alone will not hold mechanical tension. The familiar cloth-backed duct tape from the junk drawer is neither: its adhesive dries out and fails, sometimes within a couple of seasons. A careful tech typically uses both mastic and rated tape, choosing by joint type rather than by habit. What matters is that every accessible connection ends up sealed, not which product got top billing.

How does aerosol-injected duct sealing work?

Aerosol sealing, with Aeroseal as the best-known brand, works from the inside of the ducts. The technician blocks your registers, pressurizes the system, and injects a fog of sealant particles into the airstream. Air escaping through a leak carries particles to the edges of the gap, where they build up and bridge it, sealing holes up to roughly the width of a finger. Because the sealant travels wherever the air goes, it reaches joints buried in walls, chases, and floors that no hand with a mastic brush can. The equipment measures leakage continuously, so you see the before and after numbers in real time. It costs more than hand sealing and does not fix crushed or disconnected ducts, so an honest tech inspects first and seals second.

How do technicians test duct leakage before and after sealing?

The standard tool is a duct leakage tester, often called a duct blaster: a calibrated fan that pressurizes the duct system with the registers sealed off, measuring how much air it takes to hold a set pressure. That airflow number is your leakage. Testing before sealing tells you whether sealing is even the right fix; testing after proves the work accomplished something. Some techs also use theatrical smoke, an infrared camera, or simply a hand held near joints while the blower runs to locate individual leaks. If a company proposes sealing without any measurement, ask how they will demonstrate the result. The good ones volunteer the before and after numbers, because those numbers are the product.

When does duct sealing matter more than duct cleaning?

Often. Cleaning removes dust from inside ducts; sealing stops conditioned air from escaping them. Those are different problems, and leakage is usually the one costing you comfort. The EPA notes duct cleaning has never been shown to prevent health problems. Leakage, by contrast, is measurable, and fixing it produces results you can verify with a pressure test. If your complaints are uneven temperatures, a system that runs constantly, or high utility bills, sealing addresses the cause; cleaning does not. There are real reasons to clean ducts, such as visible debris, rodent activity, or renovation dust, and NADCA publishes standards for doing it properly. But if a company leads with cleaning when your ducts leak into a hundred-degree attic, they are selling what is easy, not what you need.

Why are duct runs in unconditioned attics a special problem?

Because every leak there is a total loss. A supply duct leaking inside your conditioned space at least dumps air into the house; a duct leaking in a vented attic dumps cooled air into a space that can top well over a hundred degrees in summer, or heated air under a frosty roof deck in winter. Return leaks in attics are arguably worse: they pull hot, dusty attic air into the system and distribute it through the house. Attic runs also take the biggest temperature beating, so their joints and tapes age fastest. ENERGY STAR specifically calls out ducts in attics, crawlspaces, and garages as the priority for sealing. If your ducts live in an unconditioned attic, sealing there gives the biggest return of any duct work.

What are the signs my ducts are leaking?

Rooms that never match the thermostat, usually the ones farthest from the air handler. Utility bills that climb even though your habits have not changed. A system that runs long cycles and still cannot keep up on the hottest and coldest days. Dust streaks radiating from register edges or at duct joints you can see, which mark air being pushed through gaps. Flexible ducts in the attic that are visibly detached, torn, or held together with brittle, flaking tape. A whistling or rushing sound near the air handler. None of these alone proves leakage, since undersized ducts and failing equipment can mimic several, which is why the next step is an inspection and a pressure test, not a contract. A tech should show you the evidence before proposing anything.

Which duct joints leak most often?

The usual suspects are the connections nobody can see without crawling. Where flex duct meets a metal collar or boot, the inner liner is often secured with nothing but a zip tie and dried-out cloth tape. Where the supply plenum meets the air handler, factory openings and field-cut holes are commonly left rough. Boot-to-drywall gaps at each register let air slip into wall and floor cavities. Sheet metal trunk lines leak at every slip-and-drive joint and every seam that was never sealed at installation, which in many houses means all of them. Panned returns, meaning building cavities used as ducts, leak almost by definition. A methodical tech works joint by joint outward from the air handler, because leakage close to the fan sees the highest pressure and loses the most air.

Can I seal ducts myself with hardware store tape?

You can seal some of them, and a careful homeowner with mastic and UL 181 foil tape can make a real dent on accessible joints in a basement or attic. Skip the ordinary cloth duct tape; despite the name, it fails on ducts as the adhesive bakes out. The limits of do-it-yourself sealing are reach and verification: you cannot hand-seal joints buried in walls and floors, and without a pressure test you will not know how much you accomplished. There is also a safety line: sealing should never block combustion air for a furnace or water heater, and return-side changes can affect equipment airflow. Handle the easy wins yourself if you like, then have a technician test, seal the rest, and confirm the equipment still gets the airflow it needs.

How long does professional duct sealing take, and how long does it last?

Hand sealing accessible joints in an average single-family house is typically a one-day visit, sometimes less; add time for a duct leakage test before and after. Aerosol sealing usually runs several hours, including setup, register blocking, injection, and verification. Done properly, the results are durable: mastic stays flexible for decades, and aerosol sealant manufacturers publish long service-life data for the material inside ducts. What shortens the life of any sealing job is movement, meaning ducts that sag, get stepped on in the attic, or vibrate loose at unsupported joints, which is why decent techs correct support problems as they seal. Ask the technician what is included, what is excluded, and how the after-test result will be documented. A written leakage number outlives everyone's memory of the visit.

How does DuctDove connect me with a duct sealing technician?

DuctDove is a referral service, and we say so plainly: we do not seal, clean, or repair ducts ourselves. You call our toll-free number, describe the house and the symptoms, and we connect you with a local, independent duct technician who handles sealing work in your area. The tech you meet works for their own company, not for us; they inspect on site, quote in writing, and you decide with no obligation. We may be compensated for the referral, which costs you nothing and does not change the quote. What we bring is a filter: we look for techs who test before they sell and who put leakage numbers, methods, and scope in writing. If a visit goes badly, we want to hear about it.