How Long Does Ductwork Last? Why the Age Number Misleads

So instead of trusting a number, judge your ducts by condition. Below is where the published figures come from, what each one actually measures, and a short inspection you can do with a flashlight.

Where the lifespan numbers come from

The most cited survey is the National Association of Home Builders' 2007 Study of Life Expectancy of Home Components, produced with Bank of America Home Equity. NAHB built it from a 2006 telephone survey of manufacturers, trade associations and researchers. Its HVAC table lists ducting at 10 years, dampers at 20+ and diffusers, grilles and registers at 25.

That 10-year figure surprises people, and it should be read with the report's own caveats. NAHB says life expectancy depends on installation quality, maintenance, weather and climate, and intensity of use, and that the table is "a general guideline only," not a guarantee for any product.

A second data point comes from Lawrence Berkeley National Laboratory. In a 1998 write-up of duct sealant aging tests, researchers Max Sherman and Iain Walker noted that houses are supposedly designed to last 30 years, while flex duct systems are often rated for 15.

Our reading: these numbers describe when parts of a duct system commonly start needing work, not a date when sheet metal stops carrying air. The rest of this guide covers the parts that wear out.

What fails first

The sealant at the joints

Every duct joint is a potential leak, and the material holding it shut has a shorter life than the duct. Berkeley Lab ran 19 sealant samples through a rig that cycled hot and cold air through standard duct joints. Eleven duct tape products failed within days, some falling right off, and only one duct tape survived three months. Clear tape, foil-backed tape, mastic and aerosol sealant kept good seals for the whole test. The researchers suspected heat was degrading the tape's adhesive, and noted that attic temperatures can easily reach 150°F.

They also pointed out that UL ratings for duct tape cover strength and fire resistance, not longevity. A "contractor grade" label tells you nothing about how long it will hold a joint.

Flex duct supports

Flexible duct depends on how it is hung. Building America's flex duct support guidance calls for supports at the manufacturer's spacing but never more than 4 feet apart, no more than half an inch of sag per foot between them, and straps at least 1.5 inches wide. Spaced too far apart, the duct sags and restricts airflow. Too narrow or too tight, and the strap crushes the inner liner and compresses the insulation.

In our view, that sensitivity to installation is the main reason flex gets a shorter rating than rigid metal.

Insulation and moisture

Insulated ducts in attics, crawl spaces and garages can take on water from condensation or leaks. EPA's duct cleaning guidance is blunt here: if insulated ducts get wet or moldy, the insulation cannot be effectively cleaned and should be removed and replaced. EPA adds that moisture can enter through leaks or poor installation, and that mold returns if the cause is not fixed.

What leaky, aging ducts cost you

ENERGY STAR estimates that in a typical house about 20 to 30 percent of the air moving through the ducts is lost to leaks, holes and poor connections. Its signs list is a decent self-check: high summer and winter bills, rooms that are hard to heat or cool, stuffy rooms, ducts in an attic, crawl space or garage, and tangled or kinked flex duct.

There is a safety angle too. ENERGY STAR's duct sealing benefits page says leaky ducts can reduce system efficiency by as much as 20 percent and can cause backdrafting, where combustion gases from a furnace, water heater or dryer get pulled back into the house. If you have gas appliances and leaky return ducts near them, that is worth a professional look regardless of age.

A 20-minute duct check

You can see most of what matters on a cool day with a flashlight and the system running. In an attic, stay on the joists.

1. Start at the registers. ENERGY STAR names the connections where ducts meet floors, walls and ceilings as common places to find leaks and disconnected ductwork. Pull a register cover and look for gaps around the boot.
2. Feel the joints. Run a hand along accessible seams and branch takeoffs. Moving air on your skin means a leak.
3. Read the tape. Dried, curling cloth tape can mean the joint is already leaking, given how quickly duct tape failed in Berkeley Lab's tests. Foil tape or gray mastic smeared over the seam is what you want to see.
4. Sight down the flex runs. Look for sag between straps, kinks at bends, crushed sections and runs lying on insulation or framing instead of hanging.
5. Look for water. Stained insulation, rust on metal, drips below the duct or a musty smell are moisture problems. If the system smells musty when it runs, connect the two.
6. Listen. Popping or banging from metal ducts is a separate issue, and our guide to ducts that pop and bang covers that separately.

Dust blowing from registers is a different question with its own answer. See whether your ducts actually need cleaning before paying for it.

Repair, seal or replace?

Seal when the ducts are structurally fine but leaky. ENERGY STAR says to use mastic sealant or metal tape on accessible joints, insulate accessible ducts, and never use duct tape. Many homeowners hire this out, and ENERGY STAR notes most heating and cooling contractors also repair ductwork.

Repair or re-hang when flex is sagging but intact, or a single section has pulled off its collar.

Replace a section when insulation is wet or moldy, flex is crushed or torn, or metal has rusted through.

Consider a larger redo when you are replacing equipment and the existing ducts are undersized, badly routed or leaking throughout. ENERGY STAR notes a well-designed, sealed duct system may let you choose a smaller, less costly unit. If you are sizing new equipment, our explainer on what size central air unit you need shows why duct capacity belongs in that conversation, and the same replacement visit is a good time to check how much life your evaporator coil has left.

The short version

Published figures put ducting at 10 years (NAHB's 2007 table) and flex systems at a 15-year rating (Berkeley Lab), but those are guidelines about when work starts, not expiration dates. In our view, duct runs can stay in service longer than those figures when the joints are sealed, the flex is supported and everything stays dry. Inspect the joints, supports and insulation, fix what fails, and replace only the sections that cannot be saved.