Short answerIn pre-1980 Southeast Michigan homes, ductwork commonly leaks 20 to 30 percent of the air the furnace moves, mostly at unsealed sheet-metal seams, panned joist-bay returns and crushed or disconnected branch runs in basements and crawlspaces. The result is rooms that never warm up, a dusty house, higher DTE bills and a furnace that runs longer than it should. Sealing with mastic and reconnecting runs fixes most of it.
How ducts were built in the decades that matter
The furnace in a 1955 ranch in Westland or a 1928 bungalow in Ypsilanti's Historic East Side was installed with a sheet-metal supply trunk down the center of the basement ceiling, round branch takeoffs to each room, and a return system that was often not ductwork at all. Returns were built by nailing sheet metal across the bottom of a joist bay, using the floor above and the joists on either side as the other three walls of the duct. That "panned joist" return runs to a hole in the floor under a grille in the hallway, and at the furnace end it drops into the return plenum.
Every joint in that system was assembled with sheet-metal screws and, if the installer was careful, a wrap of cloth duct tape. Nobody used mastic. The joist-bay returns leak at every subfloor gap, every joist notch, every wire and pipe penetration. The tape dried out and fell off within a decade. And over the following fifty years, plumbers, electricians, cable installers and basement finishers cut holes, pulled branches loose and crushed runs to make room for their work.
Houses from the 1960s and 70s added flex duct for the branch runs, and flex that has been hanging in a crawlspace or above a drop ceiling for 50 years has sagged, kinked, split at the inner liner and pulled off its collars.
Where we find the leaks
- Trunk seams and takeoffs. The longitudinal seam on a rectangular trunk and the collar where each round branch attaches are the biggest supply leaks. Put your hand near one with the blower running and you can feel it.
- The furnace plenum connection. The transition between the furnace and the supply plenum and between the return drop and the furnace is often held with a few screws and nothing else. We find gaps you can slide a finger into.
- Panned joist returns. The single largest leak source in most pre-1970 houses. The return is pulling air from the basement through every gap, which means a portion of what comes out the registers upstairs is basement air.
- Return grille boots. Where the floor grille meets the duct, the boot is usually not sealed to the subfloor, and in a house with a finished basement it may be drawing from a wall cavity that is open to the attic.
- Disconnected or crushed branches. A branch that fell off its takeoff is dumping conditioned air into the basement or crawlspace. A flex run that got stepped on during a plumbing job is delivering half the air it should.
- Supply boots in exterior walls or the attic. In two-story colonials from the 1960s in Canton and Saline, the second-floor supplies often run up through exterior wall cavities or across the attic, and the boots leak into unconditioned space.
What the leaks actually cost
A duct leakage test on a typical pre-1980 house in our area measures 20 to 30 percent of the blower's airflow lost outside the conditioned space, and we have tested houses that were above 40 percent. Think about what that means on a January night with the furnace running. Thirty percent of the heated air never reaches a room. The furnace runs longer to satisfy the thermostat, so your gas use climbs. The rooms at the end of the longest runs, which are usually bedrooms, get the least air and stay coldest. The return leaks pull in basement air, which in a Michigan basement in winter is cold and sometimes damp, so the furnace is heating air it should never have seen.
In summer the picture flips. Supply leaks in a hot attic lose cold air into a 130-degree space, and return leaks pull hot attic or humid basement air into the system. The AC runs longer, the house never quite dries out, and the upstairs stays warm. We go deeper on that in uneven temperatures between floors.
There is a safety dimension too. Return leaks near the furnace and water heater can depressurize the basement enough to backdraft an atmospheric flue, which puts combustion gases into the house. And return leaks anywhere pull in whatever is around them: dust, insulation fibers, crawlspace air. That is a big part of why older houses are dusty, as we cover in why is my house so dusty.
How we find them
The simplest approach is a walk of the basement with the blower running on high, feeling and listening at every seam and joint. It finds the big ones. For a more complete picture we use a duct blaster test, which pressurizes the duct system with the registers taped off and measures how much air it takes to hold that pressure; the number is the total leakage. Smoke pencils and a thermal camera help pinpoint the leaks the test says exist. A camera inside the ducts is useful for finding a disconnected branch inside a finished ceiling without opening it up.
How they get fixed
The product is mastic, a thick paste applied by brush or glove over every seam, joint and takeoff, with fiberglass mesh tape across larger gaps. It stays flexible, it does not dry out and fall off like cloth tape, and it is the only material the code recognizes for sealing ducts. Foil-backed UL 181 tape is acceptable on certain joints but is a distant second. The old gray cloth duct tape is not a duct sealing product at all.
The order we work in: seal the furnace plenum and return connections first, because that is where the pressure is highest and the leakage per foot is greatest. Then the trunk seams and takeoffs within reach in the basement. Then the panned joist returns, which means sealing the subfloor gaps and joist penetrations inside the bay and the pan seams outside it, or in bad cases abandoning the bay and running a proper sheet-metal return. Then reconnecting and supporting any loose branches, replacing crushed flex, and sealing boots to the floor or drywall from the register side.
A full ductwork repair of this kind on a ranch with an accessible basement is a one-day job. A two-story with returns in finished walls takes longer and involves choosing which leaks are worth opening a wall for. Where the system is beyond sealing, with rusted trunk, undersized returns and flex that has disintegrated, a ductwork replacement is the honest recommendation, and we size the new system to the furnace rather than copying the old layout.
When this matters most
Three moments make duct sealing worth doing. The first is when the furnace is being replaced. A new 96-percent furnace feeding a system that leaks a third of its air is not delivering 96-percent efficiency to the rooms, and the variable-speed blower on a new unit will fight an undersized, leaky return. We check and seal the connections on every furnace installation. The second is when a basement is being finished and the ducts are about to be covered forever; seal them while they are exposed. The third is when a room has never been comfortable; before buying a space heater or a window unit, find out whether the duct to that room is even connected.