In Daly City, most forced-air ducts run through spaces nobody lives in: the ceiling of a ground-floor garage, a crawlspace under a hillside house, or an attic. Air that leaks out of a joint there heats the garage instead of the bedroom. Air that leaks in brings garage dust and damp crawlspace air into the system. Duct sealing measures those leaks with a pressure test and closes them at the joints, where most of them are.
Where ducts leak
Sheet-metal and flex ducts rarely leak in the middle of a straight run. They leak where two pieces meet:
- At the furnace. The supply plenum and the return connection at the cabinet are often loose or held only by old tape. In a house built over its garage, that joint sits a few feet from the car.
- At the takeoffs. Every branch that leaves the trunk is a joint. Many older systems were put together with screws and tape and little or no sealant.
- At the boots. The gap between a register boot and the hole in the floor or ceiling lets air escape into the joist bay instead of the room.
- In panned returns. Many older homes use a joist bay closed off with a sheet-metal pan as the return duct. Its edges are seldom airtight, so the return draws air from the garage, the crawlspace or the wall cavities.
- Along seams of older rectangular trunk, where the seam lock has opened over the decades.
Why a return leak in the garage matters
A supply leak wastes heat. A return leak does something else: it pulls in air from wherever the leak is. When the return runs through a garage, that can mean car exhaust, paint and solvent fumes and garage dust, carried through the furnace and out of every register. When it crosses a damp crawlspace, it pulls in moist, earthy air, one common reason a house smells musty. We seal return-side joints in a garage first. If a return grille opens into the garage itself, we point it out: a garage should have no return opening at all.
How we test and seal
- Leakage test first. The registers are covered, a calibrated fan is connected to the system and the ducts are brought to a standard test pressure. The fan reading shows how much air the ducts lose. That is your baseline.
- Find the leaks. With the ducts under pressure, leaks show up by feel and with a smoke pencil at each joint.
- Fasten, then seal. Loose joints are screwed tight first, because sealant is not a fastener. Then each joint gets brushed-on mastic, over fiberglass mesh tape where the gap is wide, or UL 181 foil tape where mastic is not practical. Boots are sealed to the floor or ceiling.
- Close the insulation. Where duct wrap was opened to reach a joint, it is closed and taped again so the duct does not sweat or lose heat to a cold space.
- Test again. A second test shows what leakage is left, and you get both numbers.
For runs out of reach, such as ducts inside walls or between floors, aerosol sealing is an option: a fine sealant mist blown through the pressurized ducts builds up at the leaks from the inside. It closes small gaps, not open joints or torn ducts, so those are fixed by hand first.
Sealing, repair or something else?
Sealing works on intact ducts. A crushed run or one that has dropped off its fitting needs ductwork repair. A bedroom without a return or a run that is too long and too small is a layout problem: ductwork modification. Ducts full of remodel dust need duct cleaning first; sealing afterwards keeps new dust from being drawn in.
California’s energy code can also call for duct testing and sealing when a furnace or air conditioner is replaced or ducts are added, so sealing is often part of a replacement job.
What affects the price
Duct sealing typically runs $500 to $2,000. The low end is a compact system with the furnace and trunk in an open garage. The price rises when:
- a finished garage ceiling hides the trunk, so joints can only be reached through access openings;
- the crawlspace is low or the attic is tight, and every joint takes longer to reach;
- the system has many branches, or a panned return that has to be sealed along its whole length;
- aerosol sealing is added for runs that are out of reach.
Most jobs take 3 to 8 hours. You get a written estimate before any work begins, and we go through it with you.
Local notes
Across Daly City’s mid-century neighborhoods, Westlake and Serramonte among them, many homes have the furnace in a garage under the living space, so the supply and return pass through a cool, often damp garage on their way up. In Brisbane’s hillside homes and on the slopes of South San Francisco, ducts often cross a crawlspace under the lower level, where the air stays damp most of the year. Joint tape that has spent decades there is usually dry and curled.
If the heat runs long on foggy evenings, energy bills keep climbing and the garage stays oddly warm, leaking joints are a reasonable first suspect. The fog-belt humidity guide covers the moisture side.