Duct Sealing in Atherton: Even Comfort for Multi-System Homes and Guest Houses

Key Takeaways

1. Why Leaky Ducts Matter in Atherton Homes

Atherton incorporated in 1923 as a residential town, and its zoning still sets a one-acre minimum for new lots in much of the town. Homes on lots that size tend to be wide rather than tall. A single-story wing can run a long way from the equipment, and many homes split heating and cooling across two or three systems to keep up.

Every joint, boot and takeoff along those runs is a place for air to escape. ENERGY STAR estimates a typical home loses 20–30% of the air moving through its ducts to leaks and poor connections. In a home with three systems, that loss happens three times, and it lands hardest on the rooms farthest from each air handler.

2. Signs Your Ducts Are Leaking

• A primary suite or far wing never quite reaches the thermostat setting
• One system runs much longer than the others to keep up
• A guest house feels weak at the registers on hot afternoons
• A home office gets stuffy by mid-afternoon
• Dust builds up quickly even with a good filter
• Ducts run through an attic, crawl space or garage

A duct leakage and airflow test measures each system on its own, so you can see which one is losing the most air and address it first.

3. Homes With Two or Three Systems

Larger homes often divide the load by floor, by wing, or between a main house and a guest unit. Each system has its own ducts and its own leaks. These are MaxSeal multi-system jobs from around the Bay Area, listed by job ID. None is an Atherton result.

Results varied between systems in the same house, which is why we test each one separately. Multi-system discounts apply when we seal more than one. See the two-system case study.

4. Guest Houses and ADUs

A detached guest house usually runs on a small system of its own, often with short but poorly connected ducts in a tight attic or under the floor. Because the system is small, a few loose joints can cost it a large share of its air. Guests notice a room that never cools down, even when the main house feels fine.

In our Bay Area data, an ADU system (M-08) went from 320 to 34 CFM25 and gained 24% airflow at the registers. CFM25 means cubic feet per minute of leakage when the ducts are pressurized to 25 Pascals, the standard test pressure. We test and seal a guest house the same way we do the main house, and it gets its own line in your report.

5. Clean Work With No Walls Opened

In a finished home, most duct leaks sit behind drywall, under floors or above ceilings. Reaching them by hand would mean cutting into finished rooms. Aeroseal works from the inside. We block every register, isolate the furnace and coil, pressurize the ducts and blow in a fine sealant mist. The particles stay airborne until they reach a leak, then build up at its edges until it closes.

Aeroseal seals gaps up to about 5/8 inch, so we hand-repair larger ones first. Aeroseal describes the sealant as non-toxic and ultra-low VOC, and it is used in hospitals and schools. We protect floors and finishes and leave each room as we found it. Read how it works.

6. Step-by-Step: Sealing Each System

Our Typical Aeroseal Duct Repair Process

1. Free consultation. In person or by video, we map your systems, including any guest house, and the rooms that bother you.
2. Baseline testing. For each system, a calibrated Retrotec fan measures leakage at 25 Pascals, and a flow hood reads every register.
3. Hand repairs. We reconnect loose runs, replace crushed flex duct and close gaps larger than 5/8 inch.
4. Aeroseal sealing. One system at a time, with live computer monitoring. Aeroseal recommends occupants are out during this stage.
5. Final test and report. You get before and after numbers for every system in writing.

Most jobs are done in a few hours. In a home with several systems, we plan the order and timing around your schedule.

7. What Changes After Sealing: Bay Area Data

These are MaxSeal's Bay Area-wide averages from 38 sealed systems. They are not Atherton results, and your numbers will depend on your home.

In theory, a 38% airflow gain means about 28% less runtime for the same delivered air. Results vary. See how much energy leaky ducts waste.

8. Wildfire Smoke Season and Filtered Air

On smoky days, you want indoor air to pass through a good filter. A leaky return duct in the attic or crawl space pulls air from spaces that connect to the outdoors, and that air skips the filter on its way into your rooms.

Sealing the returns means more of the air your system moves comes through the filter first, with less attic dust and insulation fiber along for the ride. We don't run lab tests for indoor pollutants. If ducts are too dirty to seal, cleaning runs $500–$800, and we only suggest it when there is visible mold, pests or heavy debris, in line with EPA guidance. See air quality.

9. Older Homes, Newer Rebuilds and New Equipment

Atherton's housing ranges from older homes that have been added onto over decades to recent rebuilds. Each has its own duct story. In an older home, ducts were often extended with every addition, and the newest rooms hang off the end of the oldest trunk lines. In a newer home, ducts can still leak at boots and connections that were rushed during a large build.

Variable-speed heat pumps are sized to deliver a set amount of air. Leaky ducts lose part of it before it reaches a room, and heat pump air runs cooler than furnace air, so the loss feels like weak or cool airflow. California's Title 24 generally requires duct leakage testing when equipment is replaced; confirm the target with your contractor or building department. MaxSeal works on ducts only and doesn't sell equipment, so our advice follows your test numbers. See fix ducts before replacing HVAC and meeting code in a new construction home.