You pay to heat and cool every cubic foot of air your furnace or heat pump moves. But a surprising share of that air never reaches a single room. According to ENERGY STAR, about 20 to 30 percent of the air moving through a typical duct system is lost through leaks, holes and poorly connected joints. That air ends up in your attic, crawlspace or garage — spaces you have no interest in conditioning.
Duct sealing is one of the few home energy jobs where the fix is cheap, the payback is fast, and a reasonably handy homeowner can do most of it in a weekend. Here is how it works and where to start.
Where the Air Actually Goes
Ductwork is assembled from sections, and every seam, elbow and register boot is a potential leak. Over years of thermal cycling, mastic cracks, tape peels and connections work loose. The result is a system that is quietly moving your conditioned air into unconditioned space.
The efficiency penalty is real. ENERGY STAR notes that leaky ducts can cut heating and cooling system efficiency by as much as 20 percent, and the U.S. Department of Energy warns that ducts leaking heated air into unheated spaces can add hundreds of dollars a year to heating and cooling bills.
Leaks also work in both directions. When return ducts leak, they pull air in from the attic or crawlspace — dust, insulation fibers, humidity and whatever else lives up there — and distribute it through the house. That is an indoor air quality problem, not just an energy one.
Signs Your Ducts Are Leaking
You rarely see a duct leak. You feel the symptoms instead:
- Rooms that never reach the thermostat setpoint, especially at the end of a long duct run
- Weak airflow at registers even with a clean filter and a healthy blower
- Attics, crawlspaces or garages that feel oddly warm in winter or cool in summer
- Dust that reappears within days of cleaning
- Heating and cooling bills that climbed without any change in habits or rates
If several of those apply, ducts are a strong suspect. Working through a room-by-room DIY home energy audit first will tell you whether the problem is the ducts, the building shell, or both.
What Duct Sealing Cost Depends On
Duct sealing cost swings widely, and anyone quoting you a single national number is guessing. Four things drive the price:
- Access. Ducts in an open basement or a walkable attic are cheap to seal. Ducts buried in insulation, wedged into a tight crawlspace, or hidden inside finished walls cost far more in labor.
- System size. More runs and more registers means more joints to seal.
- Method. Hand-sealing with mastic is the low-cost approach. Aerosol-based interior sealing, where a technician pressurizes the system and injects sealant that collects at the leaks, costs considerably more but reaches joints nobody can physically touch.
- Testing. A contractor who performs a duct blaster test before and after gives you documented results, which adds to the bill but proves the work.
The DIY route is dramatically cheaper. A tub of duct mastic, a brush, a roll of UL 181 foil tape and a few hours of your time will handle every joint you can reach. That accessible ductwork — attic, basement, crawlspace, garage — is typically where the biggest leaks live anyway.
Before hiring anyone, get at least two quotes and ask whether utility rebates or weatherization programs in your area cover part of the work. Many utilities do, and terms vary by state and provider.
Mastic, Foil Tape and What Not to Use
The Department of Energy is direct on this point: duct mastic is the preferred material for sealing seams and joints. It is more durable than any available tape and generally easier for a do-it-yourself installation.
Mastic is a thick, gray, paint-like compound. You brush or glove it onto the seam, work it into the gap, and let it cure. It stays flexible and it does not fall off.
What you should not use is ordinary cloth-backed, rubber-adhesive duct tape. Despite the name, DOE specifically advises against it because it tends to fail quickly. If you use tape at all, use foil tape rated UL 181. For gaps wider than about a quarter inch, bridge the opening with fiberglass mesh tape first, then coat it with mastic.
Priority order for a DIY session:
- The connections where duct branches meet the main trunk
- Register and grille boots where they meet the floor, wall or ceiling
- Seams on the air handler cabinet and plenum
- Any disconnected or crushed flex duct — reconnect and strap it properly before sealing
Insulate After You Seal
Sealing stops air from escaping. Insulation stops heat from bleeding through the duct wall. Ducts running through unconditioned space need both, and the order matters — seal first, then wrap.
For reference, the 2021 IECC calls for insulation of at least R-8 on ducts three inches in diameter and larger, and at least R-6 on ducts smaller than three inches. Check your local code, which may differ.
Even well sealed and insulated ducts lose some heat, which is why many new energy-efficient homes route ductwork entirely within the conditioned envelope — dropped ceilings, interior soffits, corners of rooms. That is not usually retrofittable, but it is worth knowing if you are planning a major renovation.
Where Duct Sealing Fits in the Bigger Picture
Duct sealing pairs naturally with two other jobs. The first is air sealing the shell of your house with caulk and weatherstripping — same principle, different surface. The second is bringing attic insulation up to spec, since attic ducts and attic insulation are usually addressed in the same visit.
The sequencing argument is strong if you are considering new equipment. Sealing ducts before you size a replacement system means the contractor sizes for the load you actually have, not the load your leaks create. Anyone weighing a heat pump against a gas furnace should fix the distribution system first — a high-efficiency unit pushing air through leaky ducts throws away much of what you paid for.
The Bottom Line
Duct sealing is unglamorous work with a better return than most efficiency upgrades. Between a fifth and a third of the air your system moves is escaping before it reaches a room, and a tub of mastic plus an afternoon in the crawlspace recovers a meaningful share of that. Start with the joints you can reach, use mastic rather than duct tape, insulate afterward, and call a contractor for the runs you cannot get to.
One caveat: pricing, rebates and utility incentive programs change frequently and differ substantially by state, province and utility. Confirm current numbers with your local utility or energy office before budgeting the job.
