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Duct Leakage: The Signs, How It Is Tested, and What Counts as Acceptable

A furnace can be working perfectly and still lose part of its heat before it reaches a room. The ducts that carry warm air through a house are joined at dozens of seams, and each seam, connection and boot where a duct meets a register is a place air can escape. When that happens in an attic, a crawlspace or a garage, the heat goes into space nobody is trying to warm.

This guide covers three things. First, the signs that point to leaky ducts, from ENERGY STAR’s own list. Second, how duct leakage is actually measured, which is a specific test with a specific result. Third, what the energy code says counts as acceptable, and why that answer depends on which edition your town has adopted, and on whether the code applied to your ducts at all.

How much air ducts can lose

ENERGY STAR puts a figure on it. “In houses with forced-air heating and cooling systems, ducts are used to distribute conditioned air throughout the house,” its duct-sealing page explains. “In a typical house, however, about 20 to 30 percent of the air that moves through the duct system is lost due to leaks, holes, and poorly connected ducts.” The result, it says, is “higher utility bills and difficulty keeping the house comfortable, no matter how the thermostat is set.”

That figure is ENERGY STAR’s estimate for a typical house, not a measurement of yours. The only way to know how much your ducts lose is to test them, which is covered below.

The signs of leaky ducts

ENERGY STAR lists the signs it associates with poorly performing ducts:

  • “you have high summer and winter utility bills”;
  • “you have rooms that are difficult to heat and cool”;
  • “you have stuffy rooms that never seem to feel comfortable”;
  • “your ducts are located in an attic, crawlspace, or the garage”;
  • “you find tangled or kinked flexible ducts in your system.”

Two of those are about where the ducts are, not how they behave. Ducts that run through an attic, a crawlspace or a garage run through space outside the heated part of the house, so air that leaks from them is lost to the outdoors in all but name. ENERGY STAR also points to the register connections: the places where ducts “meet the floors, walls, and ceiling” are “common locations to find leaks and disconnected ductwork.”

None of the signs is proof on its own. High bills have other causes, and a room that will not heat can be an insulation problem rather than a duct problem. They are reasons to test, not a diagnosis.

What a homeowner can look at

A homeowner’s part stops at the registers and grilles. With the system running, a supply register that delivers noticeably weaker air than the others, or a register boot with a visible gap where it meets the floor, wall or ceiling, is worth pointing out to a technician. Keep supply registers and return grilles open and clear. Anything behind the register, in the ductwork itself, is for the person doing the test.

How duct leakage is tested

Duct leakage is not judged by eye. It is measured, and the energy code sets out how.

The International Energy Conservation Code is the model energy code that towns adopt, with or without local changes. Its 2021 edition requires that ducts “be pressure tested in accordance with ANSI/RESNET/ICC 380 or ASTM E1554 to determine air leakage”, with total leakage “measured with a pressure differential of 0.1 inch w.g. (25 Pa) across the system”. For the test, “registers shall be taped or otherwise sealed”, and “a written report of the results of the test shall be signed by the party conducting the test and provided to the code official.”

The 2024 edition keeps the same approach. It requires that “each duct system shall be tested for air leakage in accordance with ANSI/RESNET/ICC 380 or ASTM E1554”, with total leakage “measured with a pressure differential of 0.1 inch water gauge (25 Pa) across the duct system”, including “the measured leakage from the supply and return ductwork”, and again with “a written report of the test results”.

In plain terms: the registers are sealed, the duct system is brought to a set pressure, and the amount of air needed to hold that pressure is measured. That air is leaking out somewhere. The result is a number, and it is written down. A test report is the thing to ask for if you are told your ducts leak, and it is the thing to ask for again after they are sealed.

What counts as acceptable, and which edition applies

This is where the answer depends on your town. The energy code is adopted by each town, edition by edition, and towns in the same county can be on different editions at the same time. Before relying on any figure below, confirm with your own town’s building department which edition it has adopted and whether it has amended the duct provisions.

Across the towns we serve, the adopted editions differ. Mead, Frederick and Dacono list the 2024 International Energy Conservation Code on their building-codes pages, and Erie adopted the 2024 edition with local amendments in April 2026. Firestone’s building-permits page lists the 2018 edition. Each of our city pages records what that town’s own building pages say.

The 2021 edition

The 2021 IECC sets the limits in one section. For a test after construction, “total leakage shall be less than or equal to 4.0 cubic feet per minute (113.3 L/min) per 100 square feet (9.29 m2) of conditioned floor area.” Where the test is done at rough-in with the air handler installed, the same 4.0 figure applies; with the air handler not yet installed, the limit is 3.0. Where “all ducts and air handlers are located entirely within the building thermal envelope”, the limit is 8.0.

The 2024 edition

The 2024 IECC replaces the single figure with a table, Table R403.3.8, and the section that introduces it says the total measured leakage “shall not be greater than the values in Table R403.3.8, based on the conditioned floor area, number of ducted returns, and location of the duct system.” For duct systems serving more than 1,000 square feet, the table’s limits run from 3 to 12 cubic feet per minute per 100 square feet, depending on whether the equipment is installed at the time of the test, whether the ducts are entirely inside conditioned space, and whether the system has fewer than three ducted returns or three or more. For smaller systems, it sets totals in cubic feet per minute rather than a rate.

So the same house can face a different number depending on the edition, the stage of the test and where its ducts run. That is why this guide does not give one figure as “the” standard.

What the limits apply to, and what they do not

These limits are requirements for new construction, and for work where the adopted code requires a duct test. They are how an inspector judges a new or permitted duct system. They are not a grade on the ducts in an existing house that was never required to meet them, and a house built before its town adopted these provisions was never tested against them.

For an existing house, the figures are a useful benchmark. If a test shows your ducts leak far more than a new system would be allowed to, that tells you there is a lot to gain from sealing. It does not mean your house is failing a standard, and nobody should present it that way.

Why it matters, and where the heat goes

ENERGY STAR’s benefits page sums up the case for a sound duct system: “A duct system that is well-designed and properly sealed can make your home more comfortable, energy efficient, and safer.” It then takes those in turn.

Comfort. “Sealing and insulating ducts can help with common comfort problems, such as rooms that are too hot in the summer or too cold in the winter.” Its duct-sealing page names the cost of leaks as “higher utility bills and difficulty keeping the house comfortable”.

Efficiency. “Leaky ducts can reduce heating and cooling system efficiency by as much as 20 percent,” ENERGY STAR says. That is an upper figure, not a measurement of any one house. It adds that if you are planning new equipment, “a well-designed and sealed duct system may allow you to downsize to a smaller, less costly heating and cooling system”. Where replacement is on the table, a duct test belongs before the sizing decision, not after it.

Indoor air. “Fumes from household and garden chemicals, insulation particles, and dust can enter your duct system, aggravating asthma and allergy problems,” the benefits page says. ENERGY STAR’s view is that “sealing ducts can help improve indoor air quality by reducing the risk of pollutants entering ducts and circulating through your home.”

Safety. This is the one that matters most in a house with gas appliances. ENERGY STAR explains that furnaces, water heaters and clothes dryers “release combustion gases (like carbon monoxide) through their ventilation systems”, and that “leaky ductwork in your heating and cooling system may cause ‘backdrafting,’ where these gases are drawn back into the living space, rather than expelled to the outdoors. Sealing leaks can minimize this risk.” The homeowner’s part here is the same as always: test the carbon monoxide alarm. Anything involving the appliances or their venting is for a technician.

The ducts are also part of the furnace’s installation, not separate from it. One manufacturer’s installation manual requires a furnace to be installed “with a duct system which has external static pressure within the allowable range, as specified on the furnace rating plate”. That reading is taken at the furnace, and it is one reason a duct test and a furnace tune-up belong together.

How leaky ducts are fixed

ENERGY STAR is specific about the method. It describes “sealing air leaks using mastic sealant or metal tape” and “insulating all the ducts that you can access (such as those in attics, crawlspaces, unfinished basements, and garages)”. It is blunt about one material: “Never use duct tape, as it is not long-lasting.” It also names the register connections as a place to seal: “make sure that the connections at vents and registers are well-sealed where they meet the floors, walls, and ceiling.”

It notes too that repair can be hard: “Because ducts are often concealed in walls, ceiling, attics, and basements, repairing them can be difficult.” This guide keeps the homeowner’s part at the registers and grilles, and leaves sealing to the person who can test before and after, so the result is a number rather than a hope.

The sequence that makes sense is test, seal, and test again. The first test shows how much leakage there is. The second shows how much was fixed. Both belong in writing.

For the readings a technician takes on the furnace side, see heating maintenance and tune-ups. When a room stays cold for reasons that turn out to be the furnace itself, see furnace short cycling, furnace blowing cold air and how a furnace’s safety switches work. For how this fits into heating a home here, see the guide to winter heating in Boulder, and for repairs, furnace repair.

Frequently asked questions

How do you know if your ductwork is leaking? ENERGY STAR lists the signs: high summer and winter bills, rooms that are hard to heat and cool, stuffy rooms, ducts in an attic, crawlspace or garage, and tangled or kinked flexible ducts. Those are reasons to test. The test itself, a pressure test with the registers sealed, is what tells you how much the ducts leak.

How do I test for duct leakage? The energy code describes the test: the duct system is pressure tested to a named standard, ANSI/RESNET/ICC 380 or ASTM E1554, at “0.1 inch w.g. (25 Pa)”, with the registers sealed, and the result is recorded in a written report. It is done with test equipment by the person conducting it, not by a homeowner.

How much duct leakage is acceptable? It depends on the code edition your town has adopted, and your building department can confirm which one applies. The 2021 IECC sets 4.0 cubic feet per minute per 100 square feet for a post-construction test. The 2024 IECC uses a table with limits from 3 to 12 for larger systems, depending on returns, test stage and duct location. These limits apply to new construction and code-required work, not to an existing house that was never tested against them.

How can I fix leaky ductwork? ENERGY STAR describes sealing with “mastic sealant or metal tape”, insulating accessible ducts, and sealing register connections, and says to “never use duct tape”. Test before and after sealing, so the improvement is measured.

Does my existing house have to meet the duct leakage limits? Not as a rule. The limits apply to new construction and to work where the adopted code requires a test. An existing house that was never required to meet them is not failing a standard. The figures are a useful benchmark for how much a test result could improve.

How do I know if my ductwork needs to be replaced? Leakage alone is a sealing question, not a replacement question. A test shows how much the ducts leak, and inspecting what can be seen shows whether sections are crushed, kinked, disconnected or undersized. Whether to seal, repair or replace follows from those findings.

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