Air leaves your air handler at the temperature the equipment produced, and some of it never reaches a room. The losses happen in three ways: air escapes through gaps in the duct system, heat passes through the duct wall into a hot attic, and a starved or unbalanced return keeps the system from moving the air it was designed to move. Each one has a different fix, and doing them in the wrong order wastes money.
For houses in Pasadena, CA and across the San Gabriel and San Fernando Valleys, nearly all of this happens in the attic, where the ducts spend the summer sitting in air far hotter than the house below them.
Where the energy actually goes
Ducts are assembled from pieces. Every collar, takeoff, boot, plenum seam and air handler cabinet joint is a place air can get out on the supply side and unwanted air can get in on the return side. Sealing matters most when the ducts run through an unconditioned area such as an attic or a vented crawlspace, because supply leaks push heated or cooled air straight out of unsealed joints and return leaks draw unconditioned air in.
The second loss is conduction. Even a perfectly sealed duct gives up heat through its wall, which is what the insulation around it is for. Even a well-sealed and insulated duct still loses some, which is why newer efficient homes put the duct system inside the conditioned space entirely. A Department of Energy guide for contractors recommends exactly that where it can be done.
The third is airflow. A system that cannot get air back to the air handler runs longer for the same result, and that shows up on the bill rather than as an obvious symptom.
Sealing comes before insulating
Insulation slows heat moving through a duct wall. It does nothing about a hole. Wrapping a leaky duct in R-8 buys you very little, so the sequence is seal first, then insulate, then check airflow.
The material guidance is specific, and worth repeating because the hardware aisle points the other way:
- Duct mastic is the preferred material for sealing seams and joints, more durable than any available tape. Its limit is that it will not bridge gaps over a quarter of an inch, which have to be bridged first with web-type drywall tape or a good quality heat-approved tape.
- Avoid cloth-backed, rubber adhesive duct tape. A Department of Energy guide for contractors says it should not be used, because DOE studies have shown it can dry out and fail within months. Use mastic, butyl tape, foil tape or other heat-approved tapes, and look for the Underwriters Laboratories logo on the roll.
- On cooling ducts, make sure a well-sealed vapor barrier exists on the outside of the insulation, or you trade a leak for condensation.
Sealing and insulating ducts in an unconditioned space is work for a qualified professional, so the right materials get used in the places that are hard to reach.
The duct insulation number California uses
Two figures matter here, and both come from the 2025 Building Energy Efficiency Standards.
The mandatory floor is R-6.0: Section 150.0(m)1B states that ducts shall have a minimum installed level of R-6.0, with narrow exceptions for duct inside the thermal envelope. The higher number applies when you are doing real work. Section 150.2(b)1D covers altered duct systems, and where more than 25 feet of new or replacement duct is installed, new duct in unconditioned space has to meet the insulation requirements of Table 150.2-D. That table sets R-6 for climate zones 3 and 5 through 7, and R-8 for zones 1, 2, 4 and 8 through 16. Our area falls in the R-8 column.
The same section sets the leakage targets. A complete replacement duct system, meaning at least 75 percent new duct material, has to measure "equal to or less than 5 percent of the system air handler airflow." An extension of an existing system has to hit 10 percent total, or 7 percent leakage to outside, or have all accessible leaks sealed and verified by visual inspection and a smoke test.
There is one more trigger people do not expect. Under Section 150.2(b)1E, replacing the air handler, the outdoor condensing unit of a split-system air conditioner or heat pump, or a coil requires the connected duct system to be sealed and confirmed through field verification and diagnostic testing. Exceptions exist for ducts documented as previously sealed and tested, systems with less than 40 linear feet of duct, and ducts containing asbestos.
The return side is half the system
Most attention goes to the supply. The return decides whether any of it works. If air cannot get back out of a room, the supply air stops going in, and the system runs against itself.
There is a check for this that needs no tools. Close all exterior doors and windows, close all interior room doors, turn on the central air handler, then crack each interior door and watch whether it swings closed or further open on its own. A door that moves marks a room with restricted return airflow. The fixes are ordinary: an added return grille, an undercut door, or a jumper duct connecting the room to the hallway.
A Department of Energy guide for contractors notes that door undercuts may not provide adequate airflow on their own, so a door that moves hard is worth mentioning to a tech.
What you can check yourself
- Run the door test above, room by room, and write down which doors move.
- Make sure furniture, rugs and curtains are not sitting over supply registers or return grilles.
- Vacuum the registers and grilles. Dust buildup on the grille is a real airflow restriction.
- From a safe standing position at the attic hatch, look at the ducts you can see: sections pulled apart at a collar, flex crushed under stored boxes, insulation torn open.
- Compare airflow room to room with your hand at each register. The weak one tells you where to look.
Stay off the attic framing, and leave the sealing itself to someone with the right materials and somewhere to kneel.
When to call a licensed tech
When a room never keeps up, when the system runs much longer than it used to, or when you are replacing equipment and the code test is coming anyway. A duct repair visit finds the leaks, seals what is accessible, and measures the result rather than guessing at it. If the ducts turn out to be crushed, undersized or original to the house, replacing the ductwork is usually the better spend than sealing something that was wrong to begin with. Either way, an annual HVAC maintenance visit is where airflow problems get caught before they turn into a July call.
Common questions
Does duct cleaning reduce energy loss?
No. Cleaning removes dust. It does not close leaks, add insulation or fix a starved return. It is a reasonable thing to do for other reasons, and it is not an energy measure.
Can I seal the ducts myself with mastic?
The accessible joints at the air handler are within reach for a careful homeowner. The rest of an attic duct system is not comfortable or safe to work in, and the leaks that cost the most are usually at boots and takeoffs you cannot get to without crawling over framing.
Is R-8 worth it if the code only requires R-6 on my job?
In an attic that runs well over 100 degrees on summer afternoons, more insulation on the duct is cheap at the time of install and expensive to add later. Ask for the higher number when the ducts are already open.
If one room never keeps up or your system runs all afternoon to hold a setting it used to hold easily, book a $69 diagnostic. We tell you the fee before booking and credit it toward the work. Call Boost Services at 818-277-5056 or send us a message.
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.