Duct Sealing & HERS Testing in California

If your older North Bay home has hot and cold rooms, dusty registers, and high energy bills, leaky ducts are a likely culprit — and in California, sealing them is often not optional. When you replace a furnace, air conditioner, or heat pump, the state’s Title 24 energy code generally requires a third-party HERS (Home Energy Rating System) rater to test your ducts for leakage and verify they meet the standard. Done right, duct sealing recovers air you’re already paying to heat and cool, and it’s one of the most cost-effective comfort upgrades we make.

What duct sealing and HERS testing are

Duct sealing is exactly what it sounds like: finding and closing the gaps, disconnections, and failed joints where conditioned air escapes the duct system before it reaches your rooms. We seal with mastic and proper fittings — not the cloth “duct tape” that ironically fails on ducts.

HERS testing is the verification step. A HERS rater is an independent, certified third party (not the installing contractor) who measures duct leakage and confirms the work meets code. Leakage is measured in CFM25 — cubic feet per minute of air lost when the duct system is pressurized to a standard 25 pascals. The rater compares that number against the maximum the code allows for your project and issues a pass/fail. If a term here is unfamiliar, our HVAC glossary defines duct leakage, CFM25, and Title 24.

Why ducts leak — especially in older homes

Ducts leak for predictable reasons that get worse with age:

• Failed sealant — old tape and undersized fittings dry out and let go at the joints.
• Disconnected runs — a duct knocked loose in the attic or crawlspace dumps conditioned air into unconditioned space.
• Panned returns and building-cavity ducts — older construction sometimes used wall and floor cavities as “ducts,” which leak badly.
• Crushed or kinked flex duct — restricts airflow and raises static pressure across the system.

The result is air you paid to heat or cool leaking into the attic, crawlspace, or walls — and unconditioned (often dusty) air getting pulled back in to replace it.

When HERS testing is required

In California, certain HVAC work triggers Title 24 verification. The most common triggers we see:

Project	Typically triggers HERS duct testing?
Replacing a furnace, AC, or heat pump	Yes, in most cases
Installing new ductwork	Yes
Replacing a significant portion of duct runs	Usually
A simple repair with no equipment change	Often not

These projects also require a permit, and the HERS result is part of closing it out. Because the exact triggers and thresholds change with code cycles and local jurisdiction, the specifics matter — read permits and HVAC code in California for the bigger picture. [CONFIRM: verify the current Title 24 duct-testing triggers and the leakage threshold that apply to your project and jurisdiction.]

The comfort and efficiency payoff

This is the part homeowners feel:

• Even temperatures. Sealed ducts deliver air to the rooms it was meant for, so the back bedroom stops being 8 degrees off from the living room.
• Lower bills. Industry studies commonly cite that older duct systems can lose on the order of 20–30% of conditioned air to leaks. [CONFIRM: verify current duct-loss figures for the North Bay housing stock.] Recovering even part of that is real money.
• Less dust. Sealing stops the system from pulling attic and crawlspace air — and its dust — into your living space.
• Right-sized equipment. Sealing ducts before sizing new equipment means you buy the system the house needs, not one oversized to fight leaks; see what size HVAC system you need.

If you’re weighing where to spend, duct sealing reliably ranks among efficiency upgrades that actually pay off.

Failure modes and honest caveats

• Some duct systems can’t hit the target without significant rework. When that happens, the path is more sealing or partial replacement — there are limited code allowances, but they aren’t a free pass.
• Sealing won’t fix a badly sized system. If equipment is oversized or undersized, sealing helps but doesn’t cure comfort problems; sizing has to be right too.
• Don’t seal a contaminated duct. If there’s mold or rodent contamination, that gets addressed first — sealing dirty ducts just traps the problem.
• The rater is independent on purpose. A contractor who offers to “skip the HERS test” is offering to skip code compliance, which can haunt you at resale or permit close-out.

What we see in North Bay homes

Across Sonoma, Marin, and Napa, much of our work is on mid-century homes with ducts in vented attics and crawlspaces — exactly the conditions where leakage runs high. It’s common for us to open an initial duct-leakage test on an older Rohnert Park or Santa Rosa home and find a system losing a large share of its air before sealing. Homeowners who’d lived with one stubbornly hot bedroom for years are often surprised that the fix was air leaking out long before it reached the register. Because so many local homes pull from leaky attics and crawlspaces, duct sealing also pairs naturally with air-quality work — sealing the path is step one toward cleaner indoor air.

What a duct sealing and HERS visit looks like

For homeowners who’ve never been through it, the process is straightforward:

1. Initial test — the duct system is pressurized and leakage is measured in CFM25, giving you a real number for how leaky it actually is.
2. Sealing — we access the ducts in the attic or crawlspace and seal joints, connections, and failures with mastic and proper fittings, reconnecting anything that’s come loose.
3. Verification test — once the work is done, the independent HERS rater re-tests and confirms the system meets the code threshold.
4. Permit close-out — the passing HERS result becomes part of closing your permit, which protects you at resale.

Because the rater is independent of the installer, the number you get is verified by someone whose job isn’t selling you the work — which is the entire point of the HERS system. If a contractor proposes to skip it, that’s worth pausing over.

Your next step

If you’re replacing HVAC equipment in California, plan for a permit and HERS duct testing from the start — and treat duct sealing as part of the project, not an upsell. You can review our home services to see how we handle sealing and testing together. And if a contractor quoted you a new system without mentioning HERS testing or a permit, request a free second opinion — that’s a red flag worth a second look.

Where we most often find leaks

In the older North Bay homes we service, leakage clusters in a handful of predictable spots:

• Disconnected or crushed runs in attics and crawlspaces.
• Failed cloth-tape joints that dried out years ago.
• Boots and registers where ducting meets the floor or ceiling.
• The plenum connections right at the furnace or air handler.
• Building cavities used as return “ducts” in older construction.
• Gaps where ducts pass through framing into unconditioned space.
• Loose or uninsulated flex duct sagging between supports.

Knowing where to look is half the job — these are the spots a thorough sealing visit targets first.

Frequently asked questions

Is duct sealing and HERS testing required by law in California?

In most cases, yes — when you replace a furnace, AC, or heat pump, Title 24 generally requires duct leakage testing verified by an independent HERS rater as part of the permit. The exact triggers depend on the project scope and your local jurisdiction’s code cycle. [CONFIRM: verify the current requirements for your project.] A reputable contractor will build the permit and HERS verification into the job rather than skipping it.

How much air are leaky ducts really losing?

Industry studies commonly cite that older duct systems can lose roughly 20–30% of the air they carry through leaks, though every home is different. [CONFIRM: verify current figures for the North Bay.] You feel that loss as uneven temperatures, a system that runs longer than it should, and dusty air pulled in from attics and crawlspaces. A duct-leakage test puts an actual number on your home rather than a guess.

Will duct sealing lower my energy bills?

It usually does, because you stop paying to condition air that escapes into unconditioned space. The size of the savings depends on how leaky your ducts were to begin with — a tight system won’t change much, while a badly leaking one can see a meaningful drop. We never promise a specific dollar figure sight-unseen; the honest answer comes from a leakage test before and after.

What happens if my ducts fail the HERS test?

A failed test means the system leaks more than code allows, so the work isn’t done yet. The fix is additional sealing or, in some cases, replacing the worst duct sections, then re-testing. There are limited code allowances for systems that genuinely can’t reach the standard, but they aren’t a shortcut. The point of the independent test is to make sure you actually get the sealed system you paid for.

Can I get a rebate for duct sealing?

Sometimes. Various California utility and statewide energy programs have offered incentives tied to duct sealing and verified efficiency improvements, but the programs, amounts, and eligibility rules change frequently. [CONFIRM: verify current duct-sealing rebates and incentives available in the North Bay.] We’d rather point you to a current, verified program than quote a figure that may have expired — ask us what’s active when you plan the project.

Does duct sealing help in summer as well as winter?

Yes — leaks waste conditioned air in both seasons. In summer, sealed ducts keep the cool air you paid for from dumping into a hot attic; in winter, they keep heated air from leaking into a cold crawlspace. Because the North Bay swings from hot, dry summers to cool, damp winters, a tight duct system pays off year-round rather than in just one season.

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