Dust returns within days of cleaning
The return is breathing in from somewhere it should not, a roof void, a subfloor or the inside of a stud bay, and carrying whatever lives there into the rooms.
Duct sealing in La Honda, CA, with a leakage figure before and a leakage figure after
Conditioned air escaping into an attic or a crawl space is paid for and then thrown away. The only honest way to sell sealing is to measure it twice.
Duct leakage is the largest single efficiency loss in a great many buildings, and it is invisible. A supply trunk running through an unconditioned attic with open joints is heating the attic in winter and cooling it in summer, while the rooms it was meant to serve get whatever is left. Nobody sees it, nobody hears it, and the only symptom is a bill that seems higher than it ought to be.
The return side is worse in a different way. A leaky return running through a crawl space or an attic does not lose air, it gains it, pulling unconditioned and often dirty air into the system and distributing it through the building. That shows up as dust returning quickly after cleaning, as humidity that will not come down, and occasionally as an air quality complaint nobody can explain.
Measured twiceLeakage tested before the work and again after
MasticJoints sealed with mastic, not with duct tape
Both sidesSupply and return treated, not just the visible runs
RecordedBefore and after figures handed to you
How leakage is found and quantified
Four methods, used in combination, because no single one finds everything.
The primary tool is a duct pressurisation test. The system is sealed at the registers, a calibrated fan pressurises the ductwork to a reference pressure, and the airflow required to hold that pressure is the leakage rate. That number is comparable against standards, comparable against itself after the work, and it is the only defensible way to state how leaky a system is.
Second is a visual and tactile survey of everything accessible. Attic, crawl space and basement runs get inspected joint by joint. Takeoffs, boots, plenum seams and the connection at the air handler cabinet are the usual offenders, and the cabinet itself is frequently the single largest leak in the whole system despite being the one nobody thinks of as ductwork.
Third is pressure diagnostics room by room. Closing an interior door and measuring the pressure difference across it while the system runs shows whether that room is being pressurised or depressurised relative to the rest of the building. A strongly depressurised room is pulling makeup air from somewhere, and that somewhere is usually outside.
Fourth, where a leak is suspected but not located, a theatrical fog introduced into the pressurised duct system makes it visible. It is a crude technique and it works extremely well, particularly for finding a separated branch inside a ceiling cavity where nothing else will.
Where the leaks usually are
Four locations that account for the majority of measured leakage on residential systems.
- The air handler cabinet itself, including panel seams and the filter slot
- Plenum connections at both the supply and return side of the equipment
- Branch takeoffs from the trunk, especially where secured with tape
- Boots at the register, where the duct meets the floor or ceiling opening
Signs of a leaking duct system
Six indicators, several of which are usually present together.
Leakage rarely announces itself directly. What it does is produce a cluster of smaller complaints that individually seem unrelated and together point clearly at the distribution.
The dust one is the most diagnostic. A building that gets dusty unusually quickly after cleaning is very often breathing through its return ducts.
Rooms furthest from the equipment underperform
Supply leakage along the run means the far branches receive what is left rather than what they were designed for.
Attic or crawl space noticeably conditioned
An attic that is warm in winter or a crawl space that is cool in summer is receiving air that you paid to heat or cool.
Humidity that will not come down in summer
Humid outside air being pulled into a leaky return bypasses the coil's ability to keep up, so the building never dries out properly.
Bills higher than the equipment rating suggests
Efficiency ratings assume the air goes where it is meant to. Leakage is the most common reason a rated system underdelivers.
Whistling or hissing near the equipment cabinet
Air escaping under pressure at a panel seam or plenum joint, which is audible precisely because the leak is substantial.
How the sealing work is carried out
Six stages, with a measurement at each end so the result is verifiable.
The before and after test is the part that matters commercially. Without it, duct sealing is a service you have to take on faith, and there is no reason to.
Baseline pressurisation test
System sealed and pressurised to a reference, with the leakage rate recorded before anything is touched.
Locate and document
Accessible runs surveyed joint by joint, pressure differences measured room by room, and fog used where a leak is suspected but hidden.
Seal the cabinet and plenums
The equipment cabinet, panel seams, filter slot and both plenum connections addressed first, because they are usually the largest single sources.
Seal joints, takeoffs and boots
Mastic applied at every accessible joint, with mesh tape reinforcement where a gap is wide, and straps fitted at flex connections.
Insulate where required
Runs through unconditioned space checked for insulation coverage, because a sealed but uninsulated duct still loses heat through its wall.
Retest and record
The pressurisation test repeated and the new figure recorded alongside the original, so the improvement is a number rather than a claim.
Why mastic and not duct tape
The product named after the job is the one that fails at it, and this is well established rather than a matter of opinion.
Cloth backed duct tape fails on ductwork. The adhesive dries out in the temperature cycling that ductwork experiences, the backing shrinks, and the tape peels away from the joint, usually within a few years and sometimes within one. This has been demonstrated repeatedly in laboratory testing and it matches what is found in every attic where somebody sealed a system with it a decade ago.
Mastic is a thick paste applied with a brush or a glove, which cures into a flexible permanent seal that moves with the duct rather than peeling off it. Applied over mesh tape where a gap exceeds a quarter inch, it seals joints that tape cannot bridge at all. It is messier to apply and it takes longer, and it lasts for the life of the duct.
There is one legitimate tape, which is foil backed tape carrying an appropriate listing, and it has a place on certain rigid connections. Even there, mastic over the top is the durable answer. The practical test when inspecting somebody else's sealing work is simple: if the joints are wrapped in cloth tape, the system will need doing again, and probably already does.
Materials used on a sealing job
- Water based mastic at every accessible joint and seam
- Fiberglass mesh tape embedded in mastic where the gap is wide
- Metal straps at every flexible duct connection, under the mastic
- Listed foil tape only where a rigid connection specifically calls for it
- Insulation repaired or added where runs pass through unconditioned space
Duct sealing questions
What it costs to find out, what improvement to expect and whether the inside of the ducts matters.
It depends entirely on the starting point, which is why the test comes first. A system with substantial leakage into an attic can improve dramatically. A basement system in reasonable condition, where any leaked air stays inside the building envelope, offers much less.
That distinction matters and it gets explained honestly. Air leaking from a duct in a conditioned basement is largely still heating your house. Air leaking into a vented attic is gone.
No, and they address different things. Sealing stops air escaping from or entering the duct system. Cleaning removes accumulated debris from inside it.
Sealing has a measurable effect on comfort and running cost. Cleaning is worth doing in specific situations, such as after construction work or where there has been a rodent or moisture problem, and it does not improve airflow or efficiency in a system that is merely dusty.
The accessible ones, to an extent. Mastic is not difficult to apply and joints in a basement or an open crawl space are reachable. The limitation is that you cannot measure what you have achieved, and the largest leaks are usually at the equipment cabinet and the plenums rather than at the obvious joints.
Anything inside a wall or ceiling cavity is not a homeowner job, and neither is sealing the interior of a system from the inside.
It is a legitimate technique, particularly for systems where the leaks are inaccessible, and it works by injecting a sealant aerosol that deposits at the edges of leaks from the inside.
It is more expensive than manual sealing and it is not always the right answer. Where most of the leakage is at accessible joints and at the cabinet, manual mastic work achieves the same result for less. The test results determine which makes sense.
The duct system, no. Ducts should not leak at all in an ideal world and sealing them simply delivers air where it was intended to go.
Building tightness is a separate question and a real one. A well sealed building needs deliberate ventilation rather than relying on random leakage, and that gets discussed where the testing indicates it is relevant.
A typical residential system with accessible runs is usually a day, including both tests. Systems with difficult access, or where insulation has to be added as well, take longer.
The before test and the survey are often done as a separate shorter visit first, so you get the numbers and a fixed price before committing to the sealing work itself.
Get the leakage measured before deciding anything
A pressurisation test in La Honda, CA gives you a number, and the number decides whether sealing is worth it.
Call with where your ductwork runs, if you know: attic, crawl space, basement or inside the building envelope. That single fact largely determines how much a sealing job is likely to be worth in your building.
If the complaint is dust rather than comfort or cost, mention that specifically. It points at the return side, which is tested slightly differently and is often the more rewarding half of the job.