Duct Leakage Testing Explained: CFM25 Standards for HVAC (2026)

Duct leakage testing is one of the four key measurements that determine whether a home passes ENERGY STAR, NGBS Green, EarthCraft, or DOE ZERH. This page explains what duct leakage is, why it matters, what the pass/fail thresholds are, and who is qualified to perform the test.

What Duct Leakage Is

Duct leakage is air that escapes from the forced-air heating/cooling system before it reaches the supply registers or after it passes through the return registers. Leakage can occur at:

• Joints between duct sections (unsealed elbows, takeoffs)
• Connections to plenums, boots, and registers
• Boots where ducts meet the drywall ceiling
• Penetrations where ducts pass through walls or floors
• Seams in the duct plenum itself
• Ducts that run through unconditioned spaces (attic, basement, garage) and have holes

For a typical 2,000 sq ft home with leaky ducts, 10–25% of the conditioned air can be lost to leakage. The HVAC system has to run longer to maintain setpoint, wasting energy and never reaching design performance.

Why Duct Leakage Matters

Leaky ducts cost homeowners real money. A study by LBNL showed that an average duct system in a typical American home leaks 6.3% of its airflow to outside. But leaky ducts cost more than just energy loss:

• Reduce comfort — some rooms get over-conditioned, others under.
• Strand humidity — AC in humid climates can’t dehumidify if ducts leak.
• Draw contaminants into the living space — ducts in crawlspaces or garages pull dust, mold, radon, garage fumes.
• Shorten equipment life — HVAC runs longer, wears faster.

For builders, sealed ducts are required by every major green program (ENERGY STAR, NGBS Green, EarthCraft, DOE ZERH). A duct leakage test is mandatory before any of these certificates can be issued.

How Duct Leakage Is Measured

Duct leakage is measured using the same door-fan equipment as a blower door test, plus a smaller fan that pressurizes or depressurizes the duct system. The test procedure is documented in ANSI/RESNET/ACCA Std 310 (Standard for Grading the Installation of HVAC Systems) and ASTM E1554-07.

Two Measurements: Total vs To-Outside

Duct leakage is reported as two distinct measurements:

1. Total Duct Leakage — all the air leaking out of the duct system, regardless of where it goes. Includes leakage inside the conditioned space (e.g., ducts in attic leak air but air stays in attic).
2. Duct Leakage to Outside — only the air leaking to areas outside the conditioned envelope (attic, crawlspace, garage, outside). This is the metric that affects home energy efficiency.

Total leakage and to-outside leakage are measured by combining a blower-door test with a duct-blower test. The math: turn on the duct-blower at a fixed pressure, turn on the blower-door at the opposite pressure, and compute the CFM at each step.

CFM25, The Industry Standard Pressure

All leakage measurements are reported at a standard pressure of 25 Pascals (about 1/100 psi). This makes measurements comparable across homes regardless of HVAC fan strength.

The reported metric is usually CFM25 per 100 sq ft of conditioned floor area, sometimes written as CFM25/100sqft.

Pass/Fail Thresholds

For a 2,000 sq ft home seeking ENERGY STAR, the home must have:

• Blower door test ≤ 5 ACH50
• Duct leakage to outside ≤ 4 CFM25/100sqft = ≤ 80 CFM25 to outside

The Duct Sealing Process

After the test, if ducts fail, a duct sealing process follows:

1. Locate leaks — using a hand-held anemometer, duct pressurization, or smoke pencil.
2. Seal joints — with mastic (preferred), aluminum foil tape, or approved aerosol-based sealants.
3. Re-test — confirm sealing worked.
4. Document — provide a post-repair report for the green certification file.

Proper sealing is permanent. Most duct sealing work holds up 20+ years.

Who Performs Duct Leakage Testing

An NCI-certified technician or RESNET HERS rater performs duct leakage testing. The technician uses:

• A calibrated door-fan (blower door) — typically a Retrotec or Blower Door brand fan
• A calibrated duct-blower — smaller fan that connects to the return or supply
• Pressure gauges and a manometer

HECS USA technicians are NCI-certified and carry calibrated equipment on every truck.

Frequently Asked Questions

Can I test my own ducts?

Some ENERGY STAR and utility programs allow homeowner self-tests for rough-in (pre-drywall) leakage. Final tests must be performed by a certified third party. HECS USA performs final certified tests that satisfy every major green program.

How long does a duct test take?

A typical single-family duct leakage test takes 45–75 minutes. Setup (sealing registers, connecting fans) is the bulk of the time.

Do I need to do anything before the test?

The HVAC system should be off. All registers and grilles should be unobstructed. The technician handles setup — you don’t need to do anything.

What if ducts are in the conditioned space?

Ducts in conditioned space (in interior walls, between floors) are exempt from to-outside leakage testing for some programs, but total leakage is still measured.

Get Your Ducts Tested

HECS USA provides certified duct leakage testing, blower door testing, and HVAC performance verification across Kentucky, Indiana, Ohio, Tennessee, Illinois, and Missouri. Our technicians seal the test as well as the leaks. Call (859) 983-7382 or get a quote online.