If your AC is 'fine' but some rooms never get there, or the attic feels like the system is heating the insulation, start with ducts. Not the SEER2 sticker. Not a new thermostat.
A typical older San Gabriel Valley duct system leaks 25–40% of the air you paid to cool. That air goes into the attic or crawlspace. You then run the equipment longer to make up for it, which costs money and wears the compressor.
Cloth 'duct tape' is not a sealing method
The silver cloth tape people mean when they say duct tape fails on ducts. The adhesive dries, the fabric lets go, and you have a decorative wrapper around a leak. Codes and manufacturers want UL 181 listed tapes or mastic for a reason. If a quote says 'we taped the ducts' and they mean the hardware-store roll, that work will not last.
Foil tape (UL 181)
Proper foil tape is a legitimate seal on metal seams and some fittings if the surface is clean and the tape is applied to bare metal, not dusty insulation facing. It is the right tool for accessible, well-fitted sheet-metal joints.
It is the wrong tool as the only strategy on a 30-year-old flex system buried in an attic. You cannot tape what you cannot reach, and flex leaks at the connections more than in the middle of a run.
Mastic
Mastic is a thick sealant brushed or gloved onto joints. It stays flexible, fills gaps tape skips, and is what most careful residential techs should use on boots, plenums, and takeoffs they can actually get to.
Cost is mostly labor and access. In a clean attic with room to move, mastic on every accessible joint is one of the highest-ROI hours we spend. In a tight, 120°F attic over a 1960s ranch, 'every joint' may mean 'every joint we can reach without destroying the ceiling.'
Mastic does not fix torn flex, crushed runs, or a return that was never sized. It seals leaks. It does not redesign the system.
Aeroseal (and similar aerosol sealing)
Aeroseal pressurizes the duct system and blows a sealant fog that sticks at the leaks from the inside. You temporarily block the registers. The machine treats the leaks you cannot crawl to.
It is the right conversation when leakage is high, access is ugly, and the ducts are otherwise worth keeping. It is not magic: large disconnected runs, collapsed flex, and missing returns still need a human with a screwdriver.
Expect it to cost more than a mastic afternoon and less than a full duct replacement. You should get a before-and-after leakage number. If a company cannot measure leakage, they are guessing.
What each one costs in the real world
| Method | Typical job | Ballpark (SGV) |
|---|---|---|
| UL 181 foil + mastic, accessible joints | Boots, plenum, obvious takeoffs | $400–$1,500 |
| Thorough mastic + repairs | Half-day to full-day attic work | $1,200–$3,500 |
| Aerosol sealing | Whole-system leak reduction | $2,500–$6,000 |
| Replace ducts | New flex or sheet metal, sized right | $6,000–$15,000+ |
Those ranges move with access, number of returns, and whether we find asbestos tape, rodent damage, or a furnace closet that was framed around the plenum.
How I decide on a house
- Look at the returns first. A leaky attic return sucks insulation and hot air — that is comfort and IAQ, not just efficiency.
- Check connections at the air handler and boots. Those are mastic jobs.
- If rooms are starved and we cannot reach the runs, talk aerosol sealing or replacement.
- If flex is crushed, disconnected, or the layout is wrong, do not seal a bad design. Redesign it.
The test that ends arguments
A duct leakage test (and static pressure) tells you whether sealing is a $800 mastic visit or a replacement. I will not sell Aeroseal on a system that needs new ducts, and I will not sell new ducts if mastic will get leakage into a sane range.
If your upstairs never cools or the attic insulation is blowing around the return, call. We will tell you which of the three methods is worth doing — and which one would just look busy.