©2015 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
Ah, flex duct. That bane of home performance contractors and green builders everywhere. If you’ve seen only one forced-air duct system that uses flex, you’ve most likely seen a bad installation.
Good installations are certainly possible but are quite rare. Since I began blogging in March 2010, I’ve written quite a few articles about problems with flex ducts. In fact, whenever I can’t figure out what to write about, I look through my photos and often end up writing about another way that flex duct gets abused. It’s always fun to write these articles and as easy as shooting fish in a barrel (which I’ve never actually done but have always wondered why the fish were in the barrel to begin with and why someone wouldn’t just use a net to get them out).
I've posted plenty of photos of bad flex duct installations in the Energy Vanguard Blog  and have seen much more. Remember Stumpy, the flex duct amputee? This photo (above) has become so popular that it's showing up in other people's presentations (with no attribution, of course. Grrrrrrr!). I found ol’ Stumpy in a crawl space here in Decatur, Georgia (hovering above a dead squirrel).
Duct Leakage Testing 
Flex duct is almost never installed correctly. I've been in thousands of houses, both new construction and older homes, and I've seen a lot of flex duct. Mostly the installation is pretty bad. In new Energy Star homesA U.S. Environmental Protection Agency (EPA) program to promote the construction of new homes that are at least 15% more energy-efficient than homes that minimally comply with the 2004 International Residential Code. Energy Star Home requirements vary by climate. or EarthCraft Houses , it's usually better, but I don't think I've ever seen one that made me step back and say, “Now that's how you install flex duct.”
The most common problems I see with flex are:
So, yeah, flex duct has a lot of problems. Should it be banned completely? Should green building programs not allow it?
Energy Star  (which isn't a green building program but ducts are well within its purview) allows flex.
LEED for Homes  allows flex.
The NAHB Green Building Standard  allows flex.
Hmmmmm. I think Minnesota GreenStar doesn't allow it, but I don't know of any others. (Readers, please let me know of any programs you're aware of that ban flex duct.)
Well, if we're not going to use flex, what's left? There's duct board — rigid fiberglass with a foil facing — but, in my opinion, that's worse than flex. If any duct product should be banned, it's duct board.
That leaves rigid sheet metal, often called hard pipe. In my opinion, hard pipe is the best duct to install. Here's why:
But hard pipe is not without its own problems. Some of the leakiest ducts I've tested have been made of sheet metal, not flex. The hard pipe photo you see below is one that I tested and sealed several years ago, and it turns out that it was being held together mainly by the duct insulation! When I pulled the insulation off the branch you see there, it collapsed. Of course, it had been in that attic about 30 years.
Although I've been accused of wanting to take away your freedom of choice (because of my articles on can lights  and McMansions ), I do not think that flex duct needs to be banned. I'd just like to see it designed and installed properly. Its best use is for relatively short runs in a trunk-and-branch system, not entire air distribution systems.
Mike MacFarland, a home performance contractor in California, had some of the best advice I’ve seen for using flex. He wrote in a comment to one of my recent flex duct articles , “The trick to getting these installation right is to use rigid ductwork for all ‘deviations from straight,’ then pull the ductwork taut between the two ends which now point towards each other.” So if we’re going to ban something about flex duct, it should be banning it for changing the direction of air flow. I could almost get behind that. I do believe that long, gradual turns aren’t so bad, as long as the inner liner is pulled tight and the duct is properly supported.
The problem here is not the product; it's the process . It's also part of the larger issue of the HVAC industry underperforming , to put it nicely. Flex duct, I believe, can have a place in high performance homes. We just need tighter quality control.
Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog . You can follow him on Twitter at @EnergyVanguard .