Duct Sealing in Woodside and Portola Valley: Long Runs, Crawl Spaces and Cleaner Air in Smoke Season

Key Takeaways

1. Why Leaky Ducts Matter in Woodside and Portola Valley

Both towns sit on the eastern slopes of the Santa Cruz Mountains, and both have kept a rural character since incorporating, Woodside in 1956 and Portola Valley in 1964. Portola Valley set large minimum lot sizes for new homes, and many homes in both towns sit on an acre or more.

On a sloped, spread-out lot, a home often steps down the hill or stretches along it. Ducts follow, running long distances through crawl spaces and attics. ENERGY STAR estimates a typical home loses 20–30% of the air moving through its ducts to leaks and poor connections. On a long run, the rooms at the end of the line feel that loss first, and a guest house or studio on its own system can lose a large share of a small supply.

2. Signs Your Ducts Are Leaking

• A bedroom at the far end of the house stays cold in winter
• A studio, office or guest house feels weak at the registers
• The system runs a long time to reach the thermostat setting
• A smoky or musty smell indoors when the system starts
• Dust builds up quickly even with a good filter
• Ducts run through a crawl space, attic or garage

A duct leakage and airflow test measures the leakage and the air reaching each room, so you can see where your system loses the most.

3. Long Duct Runs on Hillside Lots

Every joint, boot and takeoff is a chance for air to escape, and long runs have more of them. Flex duct pulled across a long span can also sag or kink, which chokes airflow even where it doesn't leak.

In our Bay Area data, systems that started with weak airflow gained the most. Systems delivering under 400 CFM before sealing saw a median gain of 38%, while systems already delivering 700 CFM or more saw a median gain of 22%. CFM means cubic feet per minute of air. One Bay Area system (H-04) went from 238 to 670 CFM at the registers after sealing, a 182% gain. That is not a Woodside or Portola Valley result, and most homes gain less. See the 182% airflow case study.

If a far room is still short after sealing, the register readings show why, and air balancing can take it further.

4. Crawl Space Ducts Under Hillside Homes

Many hillside homes have raised floors with ducts in a crawl space that is tall at one end and tight at the other. Down there, ducts get crushed, separated and stripped of insulation, and rodents can damage flex duct. A leaky return pulls that crawl space air into your rooms without passing the filter.

We'll point out crawl space problems we see. See ducts in crawl spaces.

5. Wildfire Smoke Season and Filtered Air

Smoke from regional wildfires can settle over the Peninsula for days at a time. On those days, you want the air inside your home to pass through a good filter. A leaky return duct in a crawl space or attic 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, along with less crawl space dust and insulation fiber. Sealing doesn't replace a good filter, and we don't run lab tests for indoor pollutants. See air quality.

6. Clean Work With No Walls Opened

Most duct leaks hide inside walls, floors and ceilings. Aeroseal reaches them from the inside, so wood paneling, plaster and finished ceilings stay intact. 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, including separated joints we find in the crawl space. A computer tracks leakage falling in real time. Aeroseal describes the sealant as non-toxic and ultra-low VOC, and it is used in hospitals and schools. Read how it works.

7. Step-by-Step: What Happens on Your Job

Our Typical Aeroseal Duct Repair Process

1. Free consultation. At your home or by video, we map your systems, including any guest house or studio, and the rooms that bother you.
2. Baseline test. A calibrated Retrotec fan measures leakage at 25 Pascals (CFM25 means cubic feet per minute of leakage at that pressure), and a flow hood reads every register.
3. Hand repair. We reconnect separated joints, replace crushed or chewed sections and close gaps over 5/8 inch.
4. Aeroseal sealing. We seal with live monitoring. Aeroseal recommends occupants are out during this stage.
5. Final test and report. You get before and after leakage and room-by-room airflow in writing.

Most jobs are done in a few hours. Homes with several systems are scheduled around your calendar.

8. What Changes After Sealing: Bay Area Data

These are MaxSeal's Bay Area-wide averages from 38 sealed systems. They are not Woodside or Portola Valley 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.

9. Guest Houses, Studios and Multi-System Homes

On a large lot, the main house may run two systems, and a guest house or studio often has its own. Each one gets tested, sealed and reported separately, and multi-system discounts apply when we seal more than one. In our Bay Area data, an ADU system (M-08) went from 320 to 34 CFM25 and gained 24% airflow.

If you're planning to switch a furnace to a heat pump, sealing first helps. Heat pump air runs cooler than furnace air, so heat lost through leaks makes it feel cool at the register. MaxSeal works on ducts only, so we have no equipment to sell. See the three-system case study and heat pump cold blow.