How sealing is done properly
Measure, seal where it matters, then measure again
A duct system is a pressurised network, and pressurised networks leak at every joint, seam, boot and connection unless something was done to stop them. In a typical installation that was never sealed at build, the losses are large enough to matter financially rather than merely being a technical imperfection. Conditioned air escaping into an attic is air you paid to heat or cool being delivered somewhere it does nothing.
Quantifying it is the part that separates real work from a sales pitch. The system is pressurised with a calibrated fan while the registers are temporarily sealed, and the airflow needed to hold a given pressure is the leakage rate. That produces a number in cubic feet per minute, comparable against the system size, and it establishes what there is to gain before anyone quotes anything. Without that figure, sealing is being sold on faith.
Where the leaks are matters more than the total. Supply side leaks in unconditioned space lose conditioned air outright. Return side leaks in the same space do something worse: they pull attic or crawlspace air into the system and distribute it through every room, bringing dust, insulation fibres, humidity and whatever else is up there. A return leak also unbalances the building, pushing it slightly positive or negative and driving air through the envelope in ways that were never intended. Return side leakage therefore gets addressed first even where the supply side leaks more.
Materials decide whether the work lasts. Mastic, applied by brush or glove to a proper thickness with fabric reinforcement over larger gaps, stays flexible and bonded for decades. Mastic tape performs well on clean accessible seams. Cloth backed duct tape, despite the name, dries out, loses adhesion and releases within a few years, which is why finding it on a duct joint is a reliable sign that somebody has been here before and did the quick version. Connections at plenums, boots and branch takeoffs get the most attention because that is where the largest gaps are.
| Leak location | What it costs you | Access difficulty |
|---|---|---|
| Supply plenum seams | Conditioned air lost immediately at the highest pressure | Usually good, at the equipment |
| Branch takeoffs from trunk | Air lost before it enters the branch at all | Good where the trunk is accessible |
| Register boots at the floor or ceiling | Loss into the joist bay and wall cavity | Moderate, often from below or above |
| Return plenum and filter rack | Unfiltered air pulled straight into the blower | Good, and high value for the effort |
| Return runs in unconditioned space | Attic or crawlspace air distributed to rooms | Varies, and the priority regardless |
| Flex duct collar connections | Large leaks hidden under the insulation jacket | Moderate, the jacket has to be opened and reclosed |
| Duct sections inside finished walls | Loss with no practical remedy | Poor, and honestly reported as unreachable |
One more thing is worth measuring while the equipment is set up, which is the pressure difference between the main body of the house and any room with its door closed. Duct leakage and inadequate return capacity produce related symptoms, and the two remedies are different. A room that goes strongly positive with its door shut has a return path problem that sealing will not touch, and knowing that before the work begins stops the sealing job from being blamed for a comfort complaint it was never able to fix.
The second test closes the job. The same procedure is repeated, the new leakage rate recorded, and both figures handed to you along with a note of what was sealed and what was inaccessible. Some leakage is always unreachable behind finished surfaces, and saying so honestly is better than implying a sealed system when parts of it could not be touched.
Work sequence
Six stages of a measured sealing job
The first and last stages are the same test, which is what makes the middle four accountable.
Baseline leakage test
System pressurised with a calibrated fan and the leakage rate recorded before anything is touched.
Locate and prioritise
Leaks found by inspection and by feel under pressure, then ranked with return side leaks first.
Access preparation
Insulation jackets opened where needed, surfaces cleaned, because mastic will not bond to dust.
Seal with mastic
Brushed to proper thickness, with fabric reinforcement over any gap wide enough to need it.
Reinstate insulation
Jackets reclosed and resealed, since an opened jacket left loose creates a new problem.
Verification test
The same test repeated, both figures recorded, and anything inaccessible reported honestly.
In scope
What a duct sealing job covers
The testing either side of the work is included rather than being an add on.
Before and after testingThe same procedure both times, so the two numbers are genuinely comparable.
Mastic on all accessible jointsPlenums, takeoffs, boots, collars and seams, applied to thickness rather than smeared.
Return side priorityAddressed first, because those leaks pull contaminants in as well as losing air.
Insulation reinstatedJackets reclosed and sealed, so the work does not create a condensation problem.
Written resultsBoth leakage figures and a list of what was sealed and what could not be reached.
Static pressure recheckBecause sealing changes system pressure and the blower setup may need revisiting.
Customer testimonies
Feedback on duct sealing work
I was given a leakage number before and after. The previous company had just told me my ducts leaked and quoted a price.
The return in the attic was pulling insulation fibre into the house. That explained nine years of dust I had blamed on the neighbourhood.
They told me two sections inside a wall could not be reached and wrote it on the paperwork rather than pretending otherwise.
Sealing questions
Testing, materials, results and what sealing cannot fix
How much air is a typical duct system losing?
It varies enormously with how the system was built and where it runs, which is exactly why the measurement exists. Systems sealed properly at installation lose very little. Systems assembled with no sealing at all, particularly with flexible duct and many connections, can lose a substantial fraction of everything the equipment produces.
What matters for your decision is your number, not an average. The baseline test is short and inexpensive relative to the sealing work, and a system that turns out to be reasonably tight is a result worth having, because it redirects the money toward whatever is actually causing the complaint.
Why not just use duct tape?
Because cloth backed duct tape fails on ducts, which is one of the more ironic facts in this trade. The adhesive dries under the temperature cycling a duct experiences, the backing shrinks, and within a few years the tape curls away leaving the joint open and a residue that makes proper sealing harder.
Mastic is a thick paste applied by brush or glove that cures into a flexible permanent seal and bonds to both metal and flex duct collars. Foil backed mastic tape is acceptable on clean accessible seams. Finding cloth tape on a joint reliably means somebody sealed it quickly once and it has not been sealed since.
Will sealing reduce my energy bills?
Where leakage was significant and the ducts run through unconditioned space, yes, and the effect is direct rather than marginal. Air lost into an attic delivers nothing, so recovering it is a straightforward gain.
Where the ducts run entirely within conditioned space, the picture is more nuanced. Leaked air still ends up inside the building envelope, so the energy is not fully lost, though the distribution and the comfort problems remain real. We will tell you which situation you are in from the survey, because it changes what sealing is worth.
Does sealing help in summer, winter, or both?
Both, though the mechanism differs and one is usually larger than the other. In summer a supply leak in a hot attic loses cooled air into a space that is already far hotter than the house, and a return leak in the same place pulls that hot air straight into the system, so the equipment is fighting a load you created yourself.
In winter the supply loss is comparable but the return effect is milder, because the temperature difference between an attic and the house is generally smaller than it is at the peak of summer. Which season dominates depends on your climate and on whether the ducts sit in an attic or a crawlspace, and the leakage test does not care either way: the same sealing work addresses both.
Can sealing fix a room that has never been comfortable?
Sometimes, and sometimes not, which is why the survey looks at more than leakage. If that room's branch has a large leak upstream, sealing it will help immediately. If the branch is undersized, crushed, or the room has no return path, sealing will change very little.
This is worth being clear about before money is spent. Duct sealing and duct design are different problems with different remedies, and selling the first as a cure for the second is one of the more common disappointments in this trade.
Related services
What duct sealing usually connects to
Sealing sits within the wider distribution question and the air quality work that depends on it.
Dryer Vent Cleaning
Indoor Air Quality Testing
Air Cleaning & Humidifiers
HVAC Contractor
Start with a leakage number
The baseline test is short, and in Yountville, CA it tells you whether sealing is the right purchase or whether the money belongs somewhere else.
Written scope and a fixed figure before anything is ordered or dismantled.