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Building Science

The Science of Air Flow in Flex Duct

A Texas A&M study shows how much of a hit you take when you let flex duct sag

Sagging flex duct is bad for air flow. We all know it. We all talk about it. Here's the research data that prove it.
Image Credit: Energy Vanguard

Sagging flex duct is bad for air flow. We all know it. We all talk about it. It turns out there’s research data to prove it, too. Texas A&M did a study a few years ago to look at the pressure drop that occurs for different levels of compression. If you’re not familiar with this study, the results may astound you.

Papers about scientific research aren’t always the easiest reading, but stick with me and I’ll see if I can make this one as painless as an Irish Car Bomb. (I’m talking about the drink, not an actual bomb, silly!) The paper I’m writing about here is Static Pressure Losses in 6, 8, and 10-inch Non-Metallic Flexible Ducts, (pdf) by Kevin Weaver and Dr. Charles Culp. If you’d like to read it, click that link in the last sentence and dive in. If not, let me give you what I see as the big takeaways in their work.

What they did

Briefly, the researchers set up a test apparatus that let them control the air flow through a section of duct, both rigid and flex. Then they changed some of the properties of the flex duct to see what happened to the air flow and static pressure. The diagram below (Image #2 at the bottom of the page) shows the test apparatus.

The variables they looked at were:

Compression of flex duct – How long was the duct compared to its maximum stretch? They looked at 5 variations here: maximum stretch and compressions of 4%, 15%, 30%, and 45%. Note that compression in this use refers to the degree that the duct wasn’t stretched out to its full length. It has nothing to do with being squeezed around the middle.

Support – They supported the flex duct in two…

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  1. George Frick | | #1

    High Velocity Systems?

    What effects would you say this has on the choice of and installation of high velocity systems? They seem to be built around more and smaller flex ducting.


    George Frick

  2. User avater GBA Editor
    Allison A. Bailes III, PhD | | #2

    Response to George Frick
    Good question. This study didn't look at high velocity systems, which operate at higher pressures and use smaller ducts, but perhaps we can infer a bit from the graphs. Smaller ducts suffer more from compression. That's because the surface plays a much bigger role, and in smaller ducts there's a lot more surface for a given volume of air. So if you do a high-velocity system with flex duct, it seems that compression could hurt you a lot.

    Anyone know of data on this subject?

  3. Greg Labbe | | #3

    Diameter penalty
    Could we also conclude that the effect is more pronounced in smaller diameter ducts? Is it also fair to say that 4" diameter ducts would have a steeper penalty?

  4. User avater GBA Editor
    Allison A. Bailes III, PhD | | #4

    Response to Greg Labbe
    Yes! Just look at the graphs above and you can see that that's true.

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