Last reviewed: September 2026 · By the HVAC Brief Editorial Team · Our sourcing and editorial standards
Duct problems are leakage problems, and leakage is measured rather than guessed: a pressure test reports it in cubic feet per minute at 25 pascals, CFM25. ENERGY STAR’s threshold for a certified new home is total duct leakage of “the greater of ≤ 4 CFM25 per 100 sq. ft. of CFA or ≤ 40 CFM25” at rough-in.
Sealing fixes most systems. Replacement is for ducts that are crushed, disconnected, undersized or unreachable, which is a different diagnosis from a leaky one.
How leakage is actually measured
A duct blaster pressurises the duct system to 25 pascals and measures the airflow needed to hold that pressure. The higher the flow, the leakier the ducts. Testing follows ANSI/RESNET/ICC 380.
| ENERGY STAR test | Limit |
|---|---|
| Total leakage, rough-in | The greater of 4 CFM25 per 100 sq ft of conditioned floor area, or 40 CFM25 |
| Total leakage, final | The greater of 8 CFM25 per 100 sq ft, or 80 CFM25 |
| Leakage to outdoors | The greater of 4 CFM25 per 100 sq ft, or 40 CFM25 |
| Systems with three or more returns | 6 CFM25 per 100 sq ft at rough-in, 12 at final |
Those are new-construction certification thresholds, not a code compliance test for your existing house, but they give you a number to judge a result against. A tester who reports “your ducts are leaky” without a figure has not tested anything.
Leaks versus restriction: two different problems
Leakage wastes conditioned air. Restriction starves the system of airflow. They produce overlapping symptoms and opposite fixes, which is why measurement comes first.
| Symptom | Leakage | Restriction |
|---|---|---|
| Weak airflow at far registers | Possible | Likely |
| High energy use, comfortable house | Likely | Possible |
| Dusty air, smells from crawl space or garage | Likely, return side | No |
| Furnace shutting off on limit, coil freezing | No | Likely |
| Measured static pressure high | No | Yes |
Static pressure is the restriction measurement; see ductwork and airflow. A blower straining against restriction also shortens motor life, covered in blower motor symptoms.
Sealing: what works
- Mastic, not duct tape. Brush-applied mastic on joints and seams, or foil tape rated for the purpose. Cloth-backed duct tape fails.
- Start at the air handler and the boots. The connections at the equipment and where ducts meet floors and ceilings leak the most.
- Seal the return side. Return leaks in a crawl space, attic or garage pull unconditioned, sometimes contaminated, air into the house.
- Insulate after sealing, where ducts run outside conditioned space.
- Retest. A sealing job without a before and after number is not a result.
Aerosol sealing from inside the ducts exists for systems that cannot be reached, and is priced accordingly.
They are sold together and they do different things. Cleaning removes dust; it does not change leakage, restriction or capacity, and no federal source establishes a health or energy benefit from routine cleaning. Sealing changes measured performance. If a company quotes cleaning for an airflow complaint, ask what it measured.
When replacement is the answer
- Crushed or kinked flex duct, which cannot be repaired to its original area.
- Disconnected runs in attics or crawl spaces, common after other trades have worked there.
- Undersized trunks or runs, where static pressure stays high after sealing.
- Systems being converted, for example to a heat pump that moves more air; see heat pump installation.
- Contaminated or deteriorated material, including old duct board that is shedding.
Partial replacement is normal. Replacing the accessible trunk and boots while sealing the rest is often the cost-effective route, and it should be quoted with a target leakage figure.
What it costs
No federal series measures duct repair or replacement prices, so we publish none. Make quotes comparable by requiring a measured starting point, the scope in linear feet and locations, the sealing method, and a target CFM25 result with a retest.
Frequently asked questions
How do I know if my ducts leak?
By a pressure test. A duct blaster pressurises the system to 25 pascals and reports leakage as CFM25. For reference, ENERGY STAR certified new homes must reach the greater of 4 CFM25 per 100 square feet of conditioned floor area or 40 CFM25 at rough-in. Without a measured number, a leakage claim is an opinion.
Should ducts be repaired or replaced?
Seal when the ducts are intact but leaky, which covers most systems. Replace when runs are crushed, disconnected, undersized, deteriorating, or when the system is being converted to equipment that needs more airflow. High static pressure after sealing points to replacement.
Is duct cleaning the same as duct sealing?
No. Cleaning removes dust and does not change leakage or airflow. Sealing changes measured performance. They are frequently sold together, and only one of them addresses an airflow or energy complaint.
What should duct sealing cost?
No federal source measures it. Compare quotes on scope and evidence instead: a measured leakage figure before work, the method used, which sections are included, and a retest showing the result afterwards.
Methodology and limitations
Leakage thresholds are quoted from the ENERGY STAR Single-Family New Homes National Rater Field Checklist. The test method is ANSI/RESNET/ICC 380, a private standard we name but do not reproduce.
- ENERGY STAR limits apply to certified new homes, not to existing houses as a code requirement.
- Local energy codes set their own duct testing requirements, which vary by jurisdiction and adoption year.
- We publish no prices, because no federal series measures them.
Sources
- ENERGY STAR, Single-Family New Homes National Rater Field Checklist, items 6.4 and 6.5.
The HVAC Brief is an independent trade publication. We do not sell equipment, take manufacturer advertising, or accept payment for placement in our research. Corrections welcome.
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