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Ductwork · SealingDuct Sealing
Duct sealing closes the gaps at the joints of your ductwork so the air the furnace and air conditioner produce reaches the rooms instead of the basement ceiling, the garage or the attic. It is worth doing first when ducts run through unheated space or when the returns are panned joist bays, and the proof is a before-and-after reading of static pressure and register airflow. Ducts that are disconnected or crushed need repair before sealing, because sealant does not bridge a missing joint.
Ducts leak at the same places in almost every house. The joints where sections of trunk meet, the take-offs where each branch leaves the trunk, the gap between a register boot and the hole cut in the subfloor, and the plenum connections at the furnace all leave openings, and in most older houses none of them were ever sealed. The return side is usually worse than the supply. In houses from the 1950s through the 1990s the return is often a joist bay with a sheet of metal nailed across the bottom, and that panned return pulls basement air in along every edge.
The materials are specific. Mastic is a thick water-based sealant brushed over a joint, and it stays flexible for decades; foil tape made for ductwork carries a UL 181 listing printed on the backing. Cloth duct tape is neither, and it dries out and lets go within a few heating seasons on a warm duct. Gaps wider than about 6 mm get fibreglass mesh bedded in the mastic, and the furnace's filter door and service panels are taped, not glued, so they still open.
Joints you can see are sealed from the outside. Joints you cannot see are reached from the inside: a boot is sealed to the subfloor through the register opening, and the first joint behind it can usually be reached by hand. For runs buried behind finished ceilings, the method that exists is an aerosol sealant blown through the pressurized ducts, which is a specialized process. We tell you plainly whether your house needs that or whether the accessible joints are where the loss is.
The result is measured. Before sealing we record static pressure at the blower and airflow at the registers that were weak; afterward we take the same readings in the same places. Airflow at the far registers should rise, and static pressure may rise slightly too, because air that used to escape now has to travel the whole duct. If static was already near the blower's limit, sealing gets paired with a return or trunk fix through ductwork installation, or you can ask us to measure first.
Duct sealing pays off when
- Ducts run through an attic, a crawlspace, a garage ceiling or an unheated cold room
- The room over the garage or at the end of the longest run never matches the thermostat
- The basement is the warmest room in winter and the coldest in summer while the bedrooms lag
- Returns are panned joist bays and the furnace filter loads with dust faster than it should
- Dust streaks fan out from duct joints or around the register boots
How it works
Static pressure at the blower, airflow at the weak registers, and a walk along every accessible run with the blower on, feeling for leaks and marking them. You get the readings in writing with the quote.
Plenum connections at the furnace, then trunk joints and take-offs, panned returns, and boots from the room side. Metal is wiped clean so the mastic bonds, and wide gaps get mesh.
Same instruments, same locations. You get the before-and-after numbers in writing, along with anything we could not reach and what it would take to get to it.
Common questions
Is duct sealing worth it if my ducts are all inside the house?
Less so for the gas bill, since air leaking into a heated basement still heats the house. It is still worth doing for comfort: leaks near the furnace rob the far bedrooms, and a leaky return pulls in basement dust and humidity. Where ducts cross a garage, attic or crawlspace, the leaked air is paid for and lost, and sealing comes first.
Can leaky return ducts be a safety problem?
They can. A return leak in the furnace room pulls that room into negative pressure, and a natural-draft water heater or an older furnace venting up a chimney in the same room can then spill flue gases instead of drafting them out. The return joints nearest the furnace are the first ones we seal, and a draft check at the water heater afterward confirms the result.
How much does duct sealing cost?
No flat price is honest, because the work is counted in joints and access. An open basement with exposed trunks is a short job; panned returns, a finished ceiling or an attic run add hours. We measure first and quote in writing, and the published bands for the rest of our work are on the pricing page.
Will sealing fix a room that is always cold?
Sometimes. If the run to that room leaks at the take-off or the boot, sealing recovers the lost air and the reading at the register shows it. If the run is too long, too small or has no return path, sealing changes little, and air balancing or a duct change is the fix. The first measurement tells us which.
How long does duct sealing last?
Mastic stays bonded for the life of the duct as long as it went onto clean, dry metal, and listed foil tape holds for decades when it is pressed down over a clean seam. What undoes a sealing job is later work: a renovation that moves a run, or a new furnace set on an old plenum. Disconnected runs need duct repair before any sealant goes on.