Duct Sealing in Redwood City: Cooler Rooms for Older Homes, Hillside Houses and Townhomes

Key Takeaways

1. Redwood City's Climate and Your Ducts

Redwood City's "Climate Best by Government Test" slogan dates to a 1925 Chamber of Commerce contest. The weather is mild most of the year, but summers are warmer than many people expect. Average July highs run around 81°F, the city sees about 21 afternoons a year at 90°F or above, and it hit 110°F on September 6, 2022.

On hot afternoons, leaks into the attic or crawl space hit the rooms at the end of the longest runs first. ENERGY STAR estimates a typical home loses 20–30% of the air moving through its ducts to leaks, holes and poor connections.

2. Older Homes Near Downtown

Redwood City's population more than doubled in the 1930s and again in the 1940s, reaching about 46,000 by 1960. Many neighborhoods near downtown have homes from those decades and earlier. Homes that age usually started with a simple furnace, and air conditioning or a heat pump came later on ducts that were extended, rerouted or patched during remodels.

In raised-floor homes, the ducts often run through the crawl space, where joints separate, insulation falls away and leaky returns pull damp, dusty air into the system without passing the filter. Leaks inside walls and floor cavities can't be reached by hand.

3. Emerald Lake Hills and Hillside Homes

Emerald Lake Hills sits in the oak-covered hills between Redwood City, Woodside and San Carlos. It's unincorporated San Mateo County, and we serve it the same way we serve homes inside the city. The area began around 1920 as a resort community, and many of the cabins and vacation houses have since been replaced or rebuilt as full-time homes.

Hillside homes bring their own duct challenges:

• Ducts in stepped crawl spaces that are tight to reach
• Long runs to rooms on different levels
• Additions and rebuilds tied into older duct sections
• Rooms on the far side of the house that never catch up in summer

Aeroseal works from inside the ducts, so we can seal leaks along runs we can't crawl to. See ducts in crawl spaces.

4. Signs Your Ducts Are Leaking

• Upstairs or far rooms stay hot on summer afternoons
• Some registers blow hard while others barely move air
• A new AC or heat pump feels weaker than expected
• Dust or a musty smell when the system starts
• Ducts run through a crawl space, attic or garage
• A remodel or addition extended the original ducts

A duct leakage and airflow test measures the leakage and the air reaching each room.

5. Crawl Space and Attic Duct Repairs

Some problems need hands-on repair before sealing. Here is how we handle the common ones:

We'll point out crawl space problems we see along the way.

6. Townhomes and Two-System Homes

If you live in a townhome or condo with its own furnace, air handler or heat pump, your ducts can be tested and sealed like any house. We work only on your unit's system and will tell you if anything appears to connect to a shared building system. Check your HOA's rules on contractor access. For building-wide questions, see condo and HOA duct sealing.

Larger homes often have a separate system for each floor. We test and report each one separately, and multi-system discounts apply. In one two-system home from our Bay Area data, the upstairs system (H-03) went from 469 to 92 CFM25 and gained 81% airflow. CFM25 means cubic feet per minute of leakage at 25 Pascals of pressure.

7. How Aeroseal Seals Without Opening Walls

Most duct leaks hide inside walls, floors and ceilings. Aeroseal reaches them 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 larger ones get hand-repaired first.

Our Typical Aeroseal Duct Repair Process

1. Free consultation. At your home or by video.
2. Baseline test. A calibrated Retrotec fan measures leakage in CFM25, and a flow hood reads every register.
3. Hand repair. We reconnect separated joints, replace crushed sections and close gaps over 5/8 inch.
4. Aeroseal sealing. A computer tracks leakage falling in real time. Aeroseal recommends occupants are out during this stage.
5. Final test and report. You get before and after leakage and airflow in writing.

Most jobs are done in a few hours.

8. What Changes After Sealing: Bay Area Data

These are MaxSeal's Bay Area-wide averages from 38 sealed systems. They are not Redwood City results.

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

9. Heat Pumps, Smoke Season and Title 24

If you're replacing a furnace with a heat pump, the ducts will carry cooler supply air in winter, and leaks make it feel cool at the register. California's Title 24 generally requires duct leakage testing with third-party HERS verification when equipment is replaced, so confirm the target with your contractor or building department. MaxSeal works on ducts only, so we have no equipment to sell.

During wildfire smoke season, sealed return ducts pull less unfiltered crawl space and attic air into the system, so more of your indoor air passes through the filter. See Title 24 duct testing and air quality.