The True Cost of Flex

Who is really saving money?

Polyester Flex

Polyester Flex has become a widely used residential duct material. Flex is cheap and easy to install and because it is “soft and flexible”, any size any length can be made to fit anywhere. It’s not surprising that flex is popular with the installers, but size for size, flex is significantly less efficient than steel duct and flex is too easily and too often installed improperly which further restricts air flow. As home builders rush to the “Energy Certification Programs”, they are discovering flex may not be such a good deal after all.


ENERGY STAR® is a joint program of the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Energy (DOE). The Energy Star Technical Standards “Minimum Standards and Recommended Practice for Energy Star Homes” publication, page 17, recommends “Avoid flex duct whenever possible” and further states “Flexible duct runs longer than 5’ can severely restrict airflow”.

Steel Duct

Steel Duct may cost more to install initially, but pays dividends in energy savings, equipment performance and indoor air quality. Steel Duct requires a higher level of skill to install, but when sealed and insulated properly Steel Duct is the most energy efficient material for air distribution systems.

Touchstone Energy®

Touchstone Energy® is an alliance of power cooperatives in 45 states. Touchstone Energy® Home Program standards provide homeowners with a more efficient Home, which translates into something we can all enjoy – savings. The 2006 Touchstone Energy® Home Program standards for the state of Indiana page 18, Mechanical Systems states; 3. “Metal Duct is preferred and maximizes HVAC system performance”. 4. “Flex Duct is prohibited for all applications”.

Independent Laboratory Research Proves There Is a Big Difference!

It’s time to take a closer look at Flex!

Energy Systems Laboratory (ESL) at Texas A&M University, jointly funded by Air Distribution Institute (ADI), American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), TXU Electric Delivery and Lennox Industries is conducting airflow efficiency research and published results in “Static Pressure Losses in 6”, 8”, and 10” Non-Metallic Flexible Duct” (page 13, Conclusions) state; “At compression values over 4%, non-metallic flexible duct exhibits 2 to 10 times increased pressure drops over sheet metal losses.”

Lawrence Berkeley National Laboratory published “A Study of Pressure Losses in Residential Air Distribution Systems” in 2004. “The flexible duct study covered compressibility and bending effects on the total pressure drop, and the results showed that the available published references tend to underestimate the effects of compression in flexible ducts that can increase pressure drops by up to a factor of nine.”