How to Tell if Ductwork Is Leaking and What You Can Do About It

If you want to know how to tell if ductwork is leaking, start by checking for hot or cold spots in different rooms, a sudden spike in your monthly energy bills, or a layer of dust that seems to reappear immediately after cleaning. You can often confirm these leaks by inspecting the physical duct joints in your attic for disconnected sections or dark "filtration" stains on the insulation.

At Attic Bros, Inc. we see it all the time: a homeowner invests in a top-of-the-line HVAC unit, but the house still feels stuffy or expensive to run. Usually, the culprit isn't the machine. It's the "silent tax" of leaky ducts. In fact, most homes lose about 20% to 30% of their conditioned air before it ever reaches a vent. If you're tired of paying to heat your attic instead of your living room, here is the professional's perspective on air duct replacement and identifying the source of the problem.

Duct leaks reduce indoor air quality and force your heating and cooling system to work harder than it should.

Think of your ducts as the lungs of your home. When there's a hole in the system, it doesn't just let air out; it also pulls air in. If you have a leak in your return ducts, your system is likely sucking in dusty, unconditioned air from your crawlspace or attic.

Beyond just dust, these gaps can pull in allergens or odors, especially if your attic has become a popular spot during mouse season when pests leave behind debris that you definitely don't want to be breathing. Keeping the system sealed is the only way to maintain true climate control.

Modern energy codes require duct systems to meet strict air-tightness standards to prevent energy loss and protect indoor air quality.

Two major standards apply:

• International Energy Conservation Code (IECC) – establishes national minimum efficiency and duct leakage requirements.
• California Title 24 sets even stricter duct performance rules for homes in California.

Because San Jose follows California energy codes, duct leakage testing is usually required when replacing heating or cooling equipment.

National Standard (IECC)
Duct systems must be tested to ensure air leakage to the outside is kept to a low, measurable level.

California Standard (Title 24)
California measures duct leakage using a CFM25 test, which is the amount of air escaping at 25 Pascals of pressure, calculated per 100 square feet of conditioned floor area.

Typical limits include:

• New duct systems: 6 CFM25 per 100 sq ft or less
• Existing duct systems: 15 CFM25 per 100 sq ft or less

These limits ensure HVAC systems deliver the efficiency homeowners pay for.

Why This Matters
If you are pulling permits, upgrading equipment, or selling your home, failing a duct leakage test can delay final inspection and approval. Fixing leaks ahead of time helps avoid costly last-minute repairs.

Detecting leaks involves a progression from sensory observation to technical diagnostics, allowing homeowners to triage repairs before calling a specialist.

The simplest way to start is to turn your fan to the "on" position and walk to your attic or crawlspace. Run your hand along the joints of the metal or flex ducts. If you feel a "breeze" hitting your skin, you've found a leak. Pay close attention to the types of air ducts you have; flex ducts are prone to tearing, while metal ducts often leak at the elbows.

Fiberglass insulation is a great filter. If you see a section of insulation that looks dark gray or black compared to the bright pink or yellow around it, that's a "tell." It means air is blowing through that spot, and the fiberglass is trapping the dirt on its way out.

If you want to feel like a pro, light an incense stick and hold it near the duct seams. If the smoke gets sucked into a crack or blown away violently, you've pinpointed a high-pressure leak.

Safety Warning: This is the most important thing to remember: if you have a gas furnace, a leak in the return duct can be dangerous. It can create enough "suction" in the utility closet to pull carbon monoxide from your water heater vent back into the house. This is called backdrafting, and it's why we always recommend having a pro check your system's pressure.

• Internal Tasks: You can easily tape up a small visible hole in an accessible attic using UL 181 foil tape (the shiny stuff, not the cloth stuff).
• Specialist Tasks: If your ducts are buried under feet of insulation, located inside walls, or if you're dealing with a full disconnect, you'll need a pro.

• Certification Required: No for basic DIY; Yes for Title 24 compliance.
• Resource Investment: $50 for DIY tape; $500+ for professional sealing or repair.
• Compliance Risk: High if you are doing permitted work in California.

At the end of the day, your home is likely your biggest investment, and your HVAC system is one of its most expensive components. Learning how to tell if ductwork is leaking is a great first step, but the "patchwork" approach only goes so far.

Whether you've noticed a spike in your PG&E bill or you're tired of one bedroom feeling like an icebox, our team is here to help. We provide expert diagnostics and seamless solutions that meet all California regulatory standards, giving you peace of mind and a much lower energy bill.

Attic Bros, Inc. is a home performance contractor in San Jose, CA, specializing in attic cleaning, insulation services, rodent proofing, and air duct replacement for homeowners. Ready to stop wasting energy? Reach out to Attic Bros today for a comprehensive duct inspection.

In California, if you replace your AC or furnace, you are almost always required by Title 24 to have your ducts tested and sealed by a professional to meet state efficiency goals.

Funny enough, no. Standard cloth duct tape (the silver stuff from the hardware store) actually dries out and falls off within a year or two. You need "Mastic" or "UL 181" rated foil tape.

The Department of Energy says the average homeowner saves about 20% on their bills just by sealing up their ducts. In a place like San Jose, where energy prices are high, that adds up fast.

Yes. If your ducts are leaking in the attic, they are pulling in attic dust and blowing it straight into your living room. If you can't keep up with the dusting, your ducts are likely the cause.

A thorough inspection usually takes about 60 to 90 minutes. We check the plenums, the connections, and the overall "tightness" of the system to see where you're losing money.