Sealing fixes leaks. Replacement fixes leaks plus everything else wrong with the duct system: undersized runs, crushed flex, thin insulation, a layout that never served the back bedroom. If the ducts are the right size, in decent shape, and just leaky, seal them. If the material is failing or the design was wrong from the start, sealing turns a bad duct system into a tight bad duct system.
What sealing actually is
Sealing closes the openings where air escapes. On accessible joints, that means mastic, a thick paste brushed over every seam, collar, and takeoff, sometimes with fiberglass mesh for larger gaps. Foil tape rated for ductwork (it will say UL 181 on the roll) handles some connections. Boots get sealed to the drywall from inside the house. The plenum seams and the air handler cabinet get the same treatment.
For ducts nobody can reach, there is aerosol sealing. The system is blocked off at the registers, pressurized, and a fog of sealant particles is pushed through it. The particles collect at the edges of each leak and build up until it closes. It works well on the small, scattered leaks that add up to most of the loss in a typical attic system, and it reaches runs buried under insulation.
What sealing does not do: add insulation, fix a crushed run, enlarge an undersized return, or change where the air goes.
When sealing is enough
- The ducts are sheet metal, or flex duct with an intact jacket and a readable R-6 or R-8 label.
- A leakage test shows the loss concentrated at connections, not spread through split or rotted material.
- The rooms are roughly balanced already. The complaint is dust, efficiency, or one weak register, not a whole wing of the house that never works.
- The layout makes sense. The air handler is reasonably central and the runs go where they are going by a sensible route.
- The equipment is staying. You are not about to replace the furnace or AC with something that moves more air.
That describes a lot of 1980s and 1990s duct systems in Pasadena, CA and the San Gabriel Valley that were built decently and simply never had their joints sealed.
When replacement is the honest answer
- Gray or black flex duct that flakes when you touch it. The plastic jacket on older flex duct goes brittle in a hot attic. Once it starts crumbling, the inner liner is usually not far behind, and there is nothing sound to seal to.
- R-4.2 or unlabeled insulation. Sealing a thin-wrapped duct leaves it losing heat through the blanket in a 130-degree attic. New flex at R-8 is the fix.
- Crushed, kinked, or sagging runs. A flex duct that was pinched under a walkway or run sixty feet to a room fifteen feet away restricts airflow no matter how tight its joints are.
- Undersized returns. The single small return grille in the hallway of a 1950s Reseda ranch is the most common airflow problem in the San Fernando Valley. The system cannot breathe in enough air to push out. Sealing does not help. Adding or enlarging the return does.
- Rodent damage, water damage, or mold inside the ducts. Chewed liner and contaminated insulation get replaced, not sealed.
- Old wrap or tape that may contain asbestos. Metal ducts in houses from the 1950s and 1960s sometimes carry it. Testing and removal are a separate licensed trade. Once that is done, the ducts get replaced.
- New equipment on the way. A modern high-efficiency AC or a heat pump moves more air than the furnace it replaces, and a heat pump in particular delivers heat at a lower temperature than gas, so it depends on volume. Ducts sized for a 1970s furnace choke it.
The three things a tech measures
Recommending sealing or replacement without measuring is guessing. Three readings settle it.
- Leakage. A duct blaster test pressurizes the system and measures how much air escapes, and a smoke pencil or pressure pan at each register shows where. Loss concentrated at joints favors sealing. Loss spread through the runs favors replacement.
- Static pressure. A manometer at the air handler reads how hard the blower is working to push air through the ducts. High static means the ducts are too small or too restricted for the equipment. No amount of sealing lowers it. Only bigger or straighter ducts do.
- Material and insulation. Jacket condition, R-value labels, liner condition at the collars. This one is a flashlight and a hand, but it decides whether there is anything worth sealing to.
Then airflow at each register gets compared to what each room needs. A room getting half its required air with tight ducts has a sizing problem, not a leak.
It is often not all or nothing
The most common outcome is a partial job. The metal trunk lines from the 1960s are fine and get sealed. The flex branch runs from the 1980s are rotted and get replaced. The return is too small and gets a second grille. The plenum gets rebuilt because it was never sealed to the cabinet. That is duct repair and duct installation on the same visit, and it costs less than starting from scratch when the trunk is sound.
What you can check yourself
- From the attic hatch, look at the color and condition of the flex duct. Gray or black jackets that look chalky are near the end. Silver or white jackets are usually newer.
- Read the R-value printed on the jacket.
- If you can reach one safely from the joists, press the jacket gently. Powdery or cracking means done.
- Count the return grilles. One small grille for a whole house is a red flag on its own.
- Compare airflow at the registers with a tissue.
- Think about the equipment. If the furnace or AC is near the end of its life, sealing now and replacing the ducts in a couple of summers means paying for the attic twice. Plan them together.
When to call a licensed tech
- Any duct work beyond sealing the boots you can reach from inside the house.
- Suspected asbestos wrap. Do not touch it.
- Equipment replacement. California requires a duct leakage test after most duct replacement work and most equipment replacements, and the ducts need to match the new system's airflow.
- Rooms that have never balanced.
Our ductwork page goes through what a full assessment covers.
Common questions
How long do ducts last?
Sheet metal trunk lines often outlast the house. Flex duct in a hot attic is a different story. Roughly 15 to 25 summers is a fair expectation, and original 1980s flex is usually past it.
Should I seal the ducts before replacing the AC?
Do them together. The leakage test is required with the equipment anyway, the new system's airflow gets verified on the finished ducts, and the attic only gets opened up once.
Is aerosol sealing better than mastic?
Different tools. Aerosol reaches what hands cannot. Mastic handles the plenum, the boots, and the big accessible joints. A thorough job usually uses both. Neither one fixes a crushed run or a small return.
If you are not sure whether your ducts need sealing or replacing, we do a $69 diagnostic, credited toward the work, and put the findings and both options in writing. Call the Boost Services team at 818-277-5056 between 6am and 10pm, any day, or reach us here.
Written with AI assistance by Boost Services, Heating and Cooling team, and reviewed by our licensed technicians before publishing. Boost Services Heating and Cooling, C-20 #1119892.