Ducts run through an attic, crawl space or garage
This is the strongest case. Every leak outside the conditioned envelope is capacity delivered to somewhere nobody lives.
A number, not a promise
Duct Sealing in Akron, CO
- Leakage measured before and after
- Mastic and mesh, not tape
- Boot to drywall sealing included
- Flat rate agreed in advance
Duct sealing is the most over promised and under measured service in this trade. Somebody spends an afternoon with a roll of foil tape, presents an invoice and a claim about efficiency, and the homeowner has no way whatsoever to know whether anything improved.
The work is worth doing and the savings can be substantial, particularly where ducts run outside the conditioned envelope. But it only means something if the leakage was quantified at the start and quantified again at the end.
What measured sealing involves
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Leakage is quantified by pressurizing the duct system with a calibrated fan while the registers are temporarily blocked, then recording how much air the fan must deliver to hold the system at a reference pressure. The more the ducts leak, the more air it takes.
That single figure is the whole basis of the job. It establishes whether sealing is worth doing at all, it identifies how much there is to gain, and repeated at the end it proves what was achieved.
Where duct systems actually leak
Leakage is rarely a hole. It is the accumulation of dozens of small gaps that each seem trivial and collectively amount to a substantial opening.
Longitudinal seams on rectangular trunk, and the joints between sections, are the first source. Branch takeoffs are the second, because a takeoff collar fitted into a trunk without sealant leaks around its entire circumference. Plenum connections at the air handler are the third, and they are frequently the worst single location in the system because pressure is highest there.
The connection between the duct boot and the drywall or subfloor is the one most often missed entirely. Air escapes into the wall or ceiling cavity around the boot rather than through the register, and because the register plate hides the gap nobody ever sees it. In houses with ducts in unconditioned space, boot sealing alone can account for a large share of the total.
Return side leakage deserves separate mention because it is the more damaging direction. A leaking supply loses conditioned air. A leaking return actively pulls air in from wherever the leak is, which in a crawl space, attic or garage means drawing in dust, moisture, insulation fibers and whatever else is in that space and distributing it through the house.
Materials, and why tape is not the answer
Mastic is a thick sealant applied by brush or glove that cures into a flexible permanent seal. On gaps wider than a small fraction of an inch it is applied over fiberglass mesh tape, which bridges the opening and gives the mastic something to hold.
Adhesive tapes fail. Not immediately, and not universally, but the environment they live in is hostile: an attic swinging through a wide temperature range across a year, with dust settling on surfaces that were never properly cleaned before application. Adhesive lets go, the tape curls at the edges and the joint opens again, typically well before anyone thinks to look.
Aerosol sealing from inside the ducts is a legitimate alternative where the system is inaccessible, sealing from within by injecting a sealant that accumulates at the leak edges. It is more expensive and it needs the right conditions, but on ducts buried in finished construction it can achieve what no amount of manual work could reach.
Whichever method, the sequence is the same: measure, seal every accessible location systematically rather than opportunistically, then measure again. The second measurement is what separates this from guesswork.
What sealing will and will not achieve
Where ducts run through an attic, crawl space, garage or any other unconditioned area, sealing delivers real and immediate gains. Conditioned air that was being delivered outside the building now reaches rooms, and return leakage that was drawing in unconditioned air stops doing so. Comfort improves at the same time as running cost falls.
Where the entire duct system is inside conditioned space, the honest position is that the gains are smaller. Air leaking from a basement trunk into a basement is not lost from the building. It is delivered to the wrong place, which still affects room by room balance and still matters, but the energy case is weaker and it will be stated that way rather than oversold.
Sealing also does not fix undersizing. A duct system that is too small remains too small once it is airtight, and in fact sealing a leaky undersized system raises static pressure because the air that had been escaping now has to go through the intended path. Where the measurement shows both problems, the sizing is addressed first or alongside, and that sequencing is explained rather than left to produce an unwelcome surprise.
Sequencing sealing against other work
Sealing is cheapest and most effective when it happens at the right point relative to everything else. Where equipment is being replaced, sealing belongs before commissioning, so the new system is balanced against a distribution that will not change afterward. Balancing a system and then sealing it means the balance is no longer valid.
Where insulation work is planned in an attic or crawl space, sealing goes first while the ducts are still accessible. Burying leaking ducts under new insulation is a common and expensive sequencing mistake, because reaching them afterward means disturbing the work that was just paid for.
And where the assessment finds both leakage and undersizing, the order depends on severity. A badly undersized system that is also leaky will see static pressure rise once sealed, so the sizing correction is scheduled alongside rather than afterward. That interaction is explained at the assessment rather than discovered when the blower starts working harder.
Leak locations and their relative importance
| Location | Why it leaks | Typical significance |
|---|---|---|
| Plenum connections at the equipment | Highest pressure point in the system | Usually the single largest source |
| Branch takeoff collars | Fitted into the trunk without sealant | Large in aggregate across many branches |
| Boot to drywall connection | Hidden behind the register plate | Substantial where ducts sit outside the envelope |
| Longitudinal and transverse seams | Sheet metal joints never sealed | Moderate but widespread |
| Return side joints and panning | Building cavities used as return path | Serious, because it draws air in |
| Filter rack and access panels | Gaps around removable components | Small individually, easy to correct |
How the job is carried out
01
Measure leakage first
Registers temporarily blocked, system pressurized with a calibrated fan, and the baseline figure recorded and shown to you.
02
Locate and prioritize
Accessible runs inspected, the largest sources identified, and the return side given priority because it draws air in.
03
Seal with mastic and mesh
Every accessible joint, seam, takeoff and plenum connection, with mesh bridging any gap too wide for mastic alone.
04
Seal boots to the building
The boot to drywall or subfloor connection at each register, which is the step most commonly skipped altogether.
05
Measure again and report
The same test repeated, the improvement quantified, and any remaining inaccessible leakage identified honestly.
When sealing is worth doing
Dust accumulates faster than it should
Return side leakage pulling in air from cavities and unconditioned spaces, then distributing it through the house.
Rooms that are unevenly conditioned
Leakage upstream of a branch reduces what reaches the end of it, which produces exactly the same complaint as an undersized duct.
Building cavities used as return ducts
Common in older construction, effectively unsealed by design, and one of the largest sources of leakage in any system built that way.
A musty or crawl space smell when the system runs
The system is pulling air from somewhere it should not be. That is a health consideration as much as an efficiency one.
Running costs that maintenance has not explained
Where the equipment tests healthy and the bill is still high, distribution losses are the usual remaining explanation.
You are about to replace equipment
Sealing before commissioning means the new system is balanced against a distribution that will not change afterward.
A garage that smells of exhaust when the system runs
Return side leakage in an attached garage draws in whatever is in that space and distributes it through the house. This one is a safety finding, not an efficiency one.
Insulation fibers appearing at registers
Material from an attic or crawl space is being pulled into the distribution through an opening on the return side.
Duct sealing questions
What Measured Sealing Should Involve
Sealing is easy to sell and hard to verify, which is exactly why the measurement matters more than the materials.
More than most homeowners would believe. Older systems commonly lose a substantial fraction of the air passing through them, and systems using building cavities as return paths can be worse again. There are standards expressed as a percentage of system airflow, and newer construction is held to far tighter figures than older housing stock achieves. The useful number for you is not the standard, it is your own before and after pair, because that is what quantifies the work you paid for.
They solve different problems. Hand applied mastic is more durable, cheaper and entirely adequate wherever the ducts can be reached. Aerosol sealing from inside the system is the answer where the ducts are buried in finished walls and ceilings and cannot be accessed at all. The right choice follows from the building rather than from a preference, and on many properties the answer is hand sealing on the accessible sections with the inaccessible remainder reported honestly.
No. Duct sealing seals the ducts, not the building envelope, and a duct system is supposed to be a closed path from the equipment to the room. Sealing it returns it to how it was intended to work. Whole house ventilation is a separate consideration that applies to envelope tightness, and if a property is genuinely tight enough to need mechanical ventilation that would be raised as its own subject rather than as a reason to leave ducts leaking.
You can improve matters at accessible joints, and doing so is better than nothing. Two cautions. Use mastic rather than adhesive tape, because tape in an attic fails within a few years and you will have bought yourself a repeat of the same job. And without a before and after measurement you will not know what you achieved, which matters because the largest leaks are usually at the plenum and in the return, both of which are harder to reach than the joints people naturally start with.
It can, and on a system that is already restrictive it is a genuine consideration. Air that had been escaping through leaks now has to travel the intended path, so if that path was already undersized the pressure rises. This is why static pressure is measured alongside leakage rather than separately. Where both problems exist, the sizing work is sequenced first or carried out at the same time, and that is explained before anything is agreed.
Most homes are a single day, including both measurements. Larger properties, systems with difficult access, or jobs that include boot sealing at a large number of registers can run into a second day. The flat rate is quoted against a defined scope after the initial assessment, so the number does not move because the work took longer than expected.
Frequently more, and it is the benefit households notice first. Supply side leakage loses conditioned air, which shows up on the bill. Return side leakage actively draws air in from wherever the gap is, and in an attic, crawl space or attached garage that means pulling dust, insulation fibers and moisture into the system and distributing them through every room. Sealing the return side usually produces a visible reduction in how quickly surfaces need dusting, which is why it is prioritized over the supply side.
Also in the catalog
Work Often Paired With Duct Sealing
Leakage findings usually arrive alongside sizing and airflow findings.
Quantified, not claimed
Get the Leakage Measured Before Anybody Opens a Tub of Mastic
The before figure tells you whether the work is worth doing. The after figure tells you whether it was done.
Where the honest answer is that your ducts are inside conditioned space and the gains will be modest, that is what you will be told.
- Calibrated leakage test before work begins
- Return side prioritized, because it draws air in
- Mastic and mesh throughout, not adhesive tape
- Boot to building sealing at every accessible register
- The same test repeated at completion
- Remaining inaccessible leakage reported rather than hidden