The three main failure modes
Residential ductwork in the Sacramento region fails in a few predictable ways. Understanding which category your system is in points to the right fix.
- Leakage — joints come apart over time; mastic or tape fails; connections to boots and plenums loosen. Causes 10-30% air loss on an untested system.
- Disintegration — older flex duct (pre-2000) uses materials that break down from UV, heat cycling, and age. The inner liner collapses; insulation sheds; sections tear.
- Rodent damage — attic rats and squirrels chew through flex duct's inner liner. A single bad section leaks massively.
- Undersizing — new HVAC installed on an old duct system designed for lower-output equipment. The ductwork becomes the bottleneck; static pressure runs high; the new system wears faster.
R-6 vs. R-8 insulated flex duct
The 'R' value measures thermal resistance — higher R means more insulation. R-8 duct holds the conditioned air at the design temperature better than R-6, which matters most in hot Sacramento Valley attics that reach 140°F+ in summer.
California Title 24 currently requires R-6 minimum in unconditioned attic spaces for most residential installs; R-8 is the common upgrade and the default we recommend for new replacement work in our region. The incremental cost is modest; the thermal payoff over the duct's 15-20 year life is substantial.
Duct sealing — what it fixes and what it doesn't
Duct sealing addresses leakage at joints, boots, and plenum connections using mastic (brush-on compound) or Aeroseal (interior-sprayed polymer). Properly sealed, a system that was leaking 25% can be brought under 6% — the Title 24 threshold for new residential installs.
Sealing does NOT fix:
- Collapsed or torn flex duct — the duct itself has to be replaced
- Undersized ductwork — adding sealant to a system that's already too small for your HVAC just hits a different ceiling
- Rodent-damaged duct runs — the damaged section has to be replaced; sealing the rest doesn't help
- Hot/cold rooms caused by bad register placement — a design problem, not a leak problem
HERS testing — what the inspector actually checks
HERS (Home Energy Rating System) testing is a third-party verification required by Title 24 for most residential HVAC changeouts in California. A certified HERS rater pressurizes the duct system and measures leakage; the install only passes final inspection if duct leakage is under the threshold.
For most new installations in the Sacramento region, the HERS leakage limit is 6% of total system airflow (slightly different thresholds apply by jurisdiction and project type). Properly installed and sealed ductwork passes easily; untested or decades-old systems typically don't.
When ductwork replacement is the right call
Not every duct problem is a replacement candidate. Signals that point to full replacement rather than targeted repair or sealing:
- Flex duct older than 20 years, especially if visibly discolored or brittle
- Multiple torn or disintegrated sections across the system
- Rodent damage in several runs (pest control + duct replacement; the rodents will come back if entry points aren't addressed)
- Undersized ducts — static pressure runs 1.0+ IWC on a system rated for 0.5 IWC
- You're replacing the HVAC system itself and the ducts can't support the new equipment's airflow
- Mold or persistent moisture inside the duct system (requires scope evaluation)
Common questions
How much does a full duct replacement cost?
Depends heavily on home size, attic access, number of registers, and whether we're reusing the existing plenum. We won't publish a fixed price because the variables are too large. For reference, a straightforward single-story residential duct replacement in good attic conditions is typically a one-to-two-day job; see the HVAC cost guide for the factors.
Can I just seal my ducts instead of replacing them?
Sometimes — if the ducts are structurally sound and the problem is leakage at joints. If flex duct has disintegrated, insulation has collapsed, or rodents have torn through, sealing can't fix the physical damage. We tell you which category you're in after an inspection, not before.
What's the difference between R-6 and R-8 duct insulation?
Higher 'R' is more insulation. R-8 holds the conditioned-air temperature better than R-6, which matters most in Sacramento's hot attics in summer. California Title 24 requires R-6 minimum for most residential attic duct installs; R-8 is the common upgrade we recommend for new replacement work in our region.
What's HERS testing and does it apply to my project?
HERS (Home Energy Rating System) is third-party duct-leakage verification required by California Title 24 for most residential HVAC changeouts. A certified rater pressurizes the ducts and confirms leakage is under the Title 24 threshold. Your install isn't legally complete without passing it; we schedule it as part of the project.
Will new ducts actually improve my comfort?
If your existing ducts are leaking 20-30% of the conditioned air (very common on 15+ year-old systems), yes — significantly. If your discomfort is actually from undersized equipment or poor register placement, new ducts alone won't fix it. A proper diagnosis identifies the real bottleneck before quoting the work.
How long does duct replacement take?
Most residential duct replacements take one to two days depending on attic access, number of registers, and whether we're reusing the plenum. Larger multi-story homes or difficult attic access can push to three days.