Duct Sealing & HVAC Airflow Glossary: 25 Terms Every Homeowner Should Know
When you read a duct test report or a contractor’s quote, a few terms do most of the work. The three that matter most are CFM (cubic feet of air per minute), CFM25 (duct leakage measured at 25 Pascals of pressure) and register airflow (how much air actually reaches each room). Everything else on a duct report builds on these. Use this glossary to look up the rest, from Aeroseal and air balancing to HERS raters and Title 24.
Key Takeaways
| Common Question | Short Answer |
|---|---|
| What’s the difference between CFM and CFM25? | CFM is any airflow measurement. CFM25 specifically means leakage measured while the ducts are held at 25 Pascals, so results can be compared between homes. How to read a duct leakage test |
| What’s the difference between total leakage and leakage to outside? | Total leakage counts every leak. Leakage to outside counts only leaks into unconditioned spaces like attics and crawl spaces, which waste the most energy. How much energy leaky ducts waste |
| What is a flow hood used for? | To measure how much air comes out of each register. It shows which rooms are starved for air. Duct leakage test |
| What is Aeroseal? | A patented process that seals duct leaks from the inside. The duct system is pressurized and a sealant mist builds up across leaks up to about 5/8 inch. How Aeroseal works |
| What is a HERS rater? | A certified third party who verifies duct leakage and other energy measures for California Title 24 compliance. Title 24 explained |
| What is air balancing? | Adjusting dampers and registers so each room gets its designed share of airflow. It’s best done after ducts are sealed. Air balancing vs. duct sealing |
| What is static pressure? | Resistance to airflow in the duct system. Too high reduces airflow and strains the blower. Manual J and full diagnostic |
1. Glossary of Duct Sealing and Airflow Terms
Use these plain-English definitions when you read a duct leakage report, an airflow reading or a sealing quote.
Aeroseal
A patented process that seals duct leaks from the inside. The duct system is pressurized and a sealant mist builds up across leaks up to about 5/8 inch.
Air balancing
Adjusting dampers and registers so each room gets its designed share of airflow. Best done after ducts are sealed.
Air handler
The indoor unit with the blower that moves air through the ducts. In many homes it’s part of the furnace.
Boot
The fitting that connects a duct to a register opening in a floor, wall or ceiling. A common leak point.
CFM (cubic feet per minute)
The standard unit for airflow and air leakage in HVAC.
CFM25
Duct leakage measured in CFM while the duct system is held at 25 Pascals of pressure. The standard number on duct leakage reports.
Conditioned air
Air your system has heated or cooled.
Conditioned space
The heated and cooled living area of the home. Attics, crawl spaces and garages are usually unconditioned.
Damper
An adjustable plate inside a duct that controls how much air flows down that branch.
Duct leakage test
A test in which a calibrated fan pressurizes the duct system and measures how much air escapes, reported in CFM25.
Duct mastic
A thick, paste-like sealant brushed onto duct joints and seams. ENERGY STAR recommends it over duct tape.
Flex duct
Flexible round duct with a plastic liner, wire coil and insulated jacket. Prone to kinks, sags and disconnections.
Flow hood (capture hood)
A tool placed over a register to measure the airflow coming out of it in CFM.
HERS rater
A certified third party who verifies duct leakage and other energy measures for California Title 24 compliance.
Leakage to outside
Duct leakage into unconditioned space only. Measured with a duct tester and a blower door running together.
Pascal (Pa)
A unit of pressure. Duct tests are standardized at 25 Pa, roughly what ducts experience in normal operation.
Plenum
The large box attached to the furnace or air handler that feeds the supply ducts (supply plenum) or collects return air (return plenum).
Register (vent)
The grille where air enters a room (supply) or leaves it (return).
Return duct
A duct that carries room air back to the system. Return leaks pull in attic, wall or crawl-space air.
Static pressure
Resistance to airflow in the duct system. Too high reduces airflow and strains the blower.
Supply duct
A duct that carries conditioned air from the system to the rooms.
Title 24
California’s building energy code. It generally requires duct leakage testing when HVAC equipment is replaced or ducts are altered.
Total leakage
All air leaking from the duct system, both inside and outside the conditioned space. The number most sealing reports show.
UL-181 tape
Foil tape rated for duct sealing. The right choice when tape is used on ducts.
Runtime reduction
The drop in how long the system runs to deliver the same air after airflow improves. Approximately 1 − (airflow before ÷ airflow after).
Conclusion
CFM, CFM25 and register airflow are the three numbers that matter most on a duct report. Once you know them, the rest of the terms are easier to follow.
Keep this glossary handy when you compare test results or quotes, so you know exactly what each number and term describes.
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Related: How Aeroseal works · Why ducts leak · FAQs