Duct Repair from a Licensed Silicon Valley HVAC Contractor
Three sealing methods dominate the Bay Area duct repair market and they have wildly different applications. Mastic and fiberglass mesh, applied by hand to every visible joint, is the gold-standard durable seal for accessible ductwork — but it requires the duct to be physically reachable, and most duct leaks are in inaccessible boots and saddle-tap branches buried in framing. UL-181B-FX listed mastic (RCD #6, Hardcast DT-5300) is the only mastic legally compliant for California Title 24 work; aluminum foil tape (despite the marketing) is not approved for primary sealing on residential ductwork per the IECC and CMC adoption. We see a lot of failed aluminum-tape jobs — the adhesive lets go after 5-7 years in attic temperatures.
Aeroseal aerosol duct sealing is the technology that changed the math on duct repair in the Bay Area. The system uses a temporary blower at the air handler to pressurize the duct system to 100 Pa, then injects an atomized polymer aerosol (vinyl acetate-ethylene copolymer) that travels with the airflow and deposits on the edges of leaks — never on the interior walls. The system measures leakage in real time and stops when target leakage is hit. A typical Bay Area home that pre-tests at 280 CFM25 (about 30% leakage) will post-test at 35-65 CFM25 (under 7%) after a 90-minute Aeroseal application. That is HERS-pass territory on existing ductwork — without opening a single wall or attic.
When ductwork is past serviceable life, sealing is throwing good money after bad. We replace flex duct that shows inner liner shedding, kinks under 90% of nominal diameter, R-4.2 insulation (current code is R-8), or visible animal damage. Our standard replacement specification is R-8 insulated flex on branches (Atco 36 series, RFL 2-Ply liner), 26-gauge galvanized metal trunks wrapped with R-8 fiberglass blanket and FSK facing for trunks over 12 inches, all joints sealed with UL-181A-M listed mastic and supported per SMACNA HVAC Duct Construction Standards (every 4 feet on horizontal flex, every 5 feet on metal trunks). We do not run flex through framing penetrations or sharp 90s — the supply path matters as much as the seal.
Insulation upgrades are commonly bundled with sealing scope because attic insulation degrades just like duct insulation. R-4.2 to R-8 on a typical 2,200 sq ft home (about 240 linear feet of flex branches) costs $1,800-$3,200 and reduces conduction losses 35-45% on the duct system. Combined with Aeroseal and sealed plenum returns, we routinely deliver 18-25% reduction in HVAC runtime — measurable in the next month's PG&E bill. Aeroseal-only on tight 1990s-2000s ductwork is often unnecessary; pre-testing with the duct blaster tells us before quoting whether sealing is the right scope. Some 2010+ tract homes test under 8% as-built and need nothing.
What's Included in Every Duct Repair Job
- Pre-repair duct leakage test with calibrated Retrotec DucTester (CFM25 documented)
- Visual inspection with photos of every accessible plenum, trunk, and branch
- Aeroseal aerosol sealing OR hand mastic + UL-181 mesh, scope-appropriate
- R-4 to R-8 insulation upgrade per Title 24 Part 6 minimums
- Flex duct replacement where liner is shed, kinked, or rodent-damaged
- Boot and register sealing with mastic at the drywall/ceiling penetration
- Return-side air leakage repair (often the largest single source)
- Post-repair duct leakage test with CFM25 reduction documented
- HERS rater coordination if required by scope (full system replacement triggers it)
Common Duct Repair Issues We Resolve
Hot upstairs / cold downstairs in two-story home
Cause: Trunk leakage in attic dumping conditioned air before it reaches upstairs branches
Fix: Aeroseal sealing or trunk re-wrap, sometimes combined with zone control add-on — $1,800-$5,500
Bedroom 12+ feet from air handler always 4-6°F off setpoint
Cause: Long flex run with kinks, leaks, or inadequate insulation; airflow loss compounds with distance
Fix: Branch replacement with R-8 insulated flex, kink elimination, register damper rebalance — $450-$1,200 per branch
Energy bills 40-60% higher than neighbors with same equipment
Cause: Duct leakage in unconditioned attic or crawlspace; heated/cooled air dumped outside building envelope
Fix: Pre-test with duct blaster, full Aeroseal + insulation upgrade — $2,800-$5,500 typical
Visible mastic cracking or aluminum tape lifting at plenum
Cause: Original installer used non-listed materials, or 15+ year service life exceeded
Fix: Strip and re-seal with UL-181 mastic + mesh — $450-$1,200
Rodent damage in flex duct (chewed liner, droppings)
Cause: Mice or rats entered crawlspace through unsealed penetration; common in older Palo Alto, Menlo Park homes
Fix: Affected branch replacement, NADCA system clean, crawlspace seal coordination — $1,200-$3,500
New AC short-cycling on system installed 6 months ago
Cause: Existing return ductwork undersized for new equipment airflow; high static pressure trips limits
Fix: Return upgrade — typically add second return drop or upsize trunk — $850-$2,400
Condensation dripping from duct exterior in summer
Cause: R-4 or worse insulation on cold supply duct in humid attic; dewpoint reached on outer surface
Fix: R-8 insulation upgrade, vapor barrier verification — $850-$2,200
Our Duct Repair Process
Pre-Test Diagnostic
Calibrated duct blaster pressurizes system to 25 Pa, measures CFM25 leakage. Visual inspection photographs plenum, trunks, branches. Pre-test report documents starting condition.
Scope Decision
Based on leakage % and visual condition: hand mastic + mesh for accessible severe leaks, Aeroseal for diffuse hidden leaks, full replacement for past-life flex. Often a combination.
Repair Execution
Hand sealing with UL-181 mastic and fiberglass mesh on accessible joints, Aeroseal aerosol injection for diffuse leakage, R-8 insulation wrap, flex replacement where warranted.
Post-Test Verification
Duct blaster re-test at 25 Pa, post-CFM25 documented. Side-by-side report shows leakage reduction. Title 24 / HERS verification scheduled if scope triggers it.
Performance Walkthrough
Static pressure measured at supply and return, room-by-room temperature differential checked, register airflow rebalance if needed, before/after PDF emailed within 48 hours.
Duct Repair Pricing in the Bay Area
Typical duct repair pricing in our Silicon Valley service area runs $850 – $8 500 per system. Most jobs complete in 4-8 hours for sealing only; 1-3 days for replacement scope.
Every quote is flat-rate and provided in writing before work begins. Diagnostic fees are waived when repair is approved. We never use time-and-materials billing surprise pricing.
Local Context: Duct Repair in Silicon Valley
Bay Area duct repair is dominated by three building stock patterns. Eichler-tract construction (Palo Alto Greenmeadow, Fairmeadow; Sunnyvale Vista Park; Mountain View Monta Loma) typically has very limited duct space — flat roof construction with no attic — so most original Eichlers used radiant slab heating instead of forced-air ductwork. Where retrofitted, the ductwork is usually high-velocity small-duct (SpacePak, Unico) or ducted mini-split cassettes routed through tight ceiling chases. Sealing options are limited; we usually replace rather than seal in these homes. Mid-century ranch homes (1955-1975) throughout Old Palo Alto, Crescent Park, and the Menlo Park flats typically have R-4.2 flex in vented attics, with original installer mastic that has cracked and aluminum tape that has failed — these are prime Aeroseal candidates and routinely yield 70%+ leakage reduction. Newer tract construction (1990s+) in San Jose, Cupertino, and Fremont generally tests under 15% as-built and may not need sealing at all unless renovation has been done. Coastal homes in Pacifica and Half Moon Bay see condensate damage on duct insulation from marine humidity hitting cold supply duct, requiring vapor barrier verification along with R-value upgrade. Title 24 Part 6 (2022) HERS verification on duct leakage applies to full system replacement — not standalone duct sealing — so most sealing-only scope does not require third-party verification, though we measure pre/post on every job for our own quality control.
HVAC Brands We Service for Duct Repair
Duct Repair FAQ
How much air does my ductwork actually leak?
Most Bay Area homes built 1960-1995 leak 22-38% by duct blaster measurement. Newer construction (2005+) typically tests 8-15% as-built. Title 24 Part 6 (2022) requires under 5% on full system replacements. We pre-test every job with a calibrated Retrotec duct blaster so the scope is data-driven, not guesswork. The pre-test result drives whether sealing alone is enough or whether replacement is warranted.
What is Aeroseal and how does it work?
Aeroseal is a patented aerosol sealing process. We pressurize the duct system to 100 Pa with a temporary blower at the air handler, then inject an atomized vinyl acetate-ethylene copolymer that flows with the airflow and deposits on the edges of leaks — like sealing a tire from the inside. The control software measures leakage in real time and stops when the target is hit. A typical 90-minute application drops leakage 70-85%, and the polymer is UL Greenguard Gold certified for indoor air quality.
Should I do Aeroseal or full duct replacement?
Depends on duct condition. If the flex liner is intact, the trunks are properly sized, and the leakage is diffuse (lots of small joint leaks), Aeroseal is the better economic choice — $2,500-$4,500 for a typical home vs $5,500-$12,000 for full replacement. If the flex liner is shed, trunks are undersized, R-value is below R-8, or there is animal damage, replacement is the right call. We document the actual condition with photos and video before quoting.
Why does my old aluminum tape keep failing?
Aluminum foil tape (the silver tape sold at Home Depot as "duct tape") is not actually approved for primary sealing on residential ductwork in California — UL-181B-FX listing is required, and most foil tapes do not carry that listing. Adhesive degrades in attic temperatures (140°F+ in summer) and the tape lifts within 5-10 years. Mastic + fiberglass mesh is the durable solution for hand sealing; Aeroseal is the durable solution for diffuse hidden leakage.
Is duct sealing eligible for rebates?
Yes, often substantial. BayREN Home+ rebate covers duct sealing as part of comprehensive home upgrades (typically $1,000-$3,000 toward sealing scope). PG&E rebates vary by program. SVCE and PCE both have weatherization rebates that include duct sealing. Federal IRA 25C does not directly cover duct sealing as a standalone, but if it is part of a qualifying heat pump installation it folds into the broader $2,000 credit.
Will sealing my ducts make my house too tight?
No — duct sealing seals the duct system, not the building envelope. Your home still has the same air infiltration through windows, doors, and wall penetrations. Duct sealing keeps conditioned air inside the duct system rather than dumping it into the attic or crawlspace. If anything, it can improve indoor air quality because the system stops pulling unconditioned attic air through return-side leaks.
How long does Aeroseal last?
Aeroseal carries a 10-year manufacturer warranty and field studies show effective sealing performance at 15+ years. The polymer cures hard and bonds to duct interior, similar to how mastic cures. Unlike aluminum tape, it does not degrade with attic temperature swings. We provide our own 5-year workmanship warranty on top of the manufacturer warranty.
Do I need to be home during duct sealing?
For Aeroseal: yes, ideally — we need access to the air handler and seal off all supply registers with foam plugs while the equipment runs. Pets and people stay in a separate area; the polymer aerosol is non-toxic and Greenguard Gold certified but mildly irritating. For mastic-only hand sealing in an accessible attic, we work independently after the initial walkthrough. Either way, the visit is 4-8 hours for sealing scope.