Dryer Transition Ducts: Is Foil Better Than Semi-Rigid Aluminum?

I've always told my Minnesota home inspection clients that the best type of dryer duct material is a completely rigid metal duct, with as short of a run as possible.  I've recommended using semi-rigid metal when necessary, as a way of transitioning from the dryer to the duct.  Semi-rigid aluminum is all that anyone recommends!

• There was an article published in the ASHI Reporter in April 2005 saying that semi-rigid metal was the only good substitute for smooth metal (click here to see it).
• If you search online for the best type of dryer duct, the only advice you'll get is to use semi-rigid aluminum as a transition.  This information is repeated on every web site or blog dealing with dryer duct safety.

After doing some research on the semi-rigid transition ducts, I've learned that they might not be allowed in Minnesota.

To understand why, you need to know the difference between a clothes dryer duct and a transition duct. They're not the same thing, and they're not interchangeable.  According to the Minnesota Mechanical Code, a dryer duct is required to be constructed of metal, and can only have a smooth interior finish (section 504.6).   This means that flexible materials are not allowed for dryer ducts; not foil, not semi-rigid aluminum.

A transition duct is flexible, which allows the dryer to be connected to the duct, then pushed back up against a wall so it's not sitting in the middle of the room.  A transition duct is allowed by the Minnesota Mechanical Code to connect the dryer to the dryer duct... but that's all.  These transition ducts are limited to single lengths of eight feet or less, and must be listed and labeled for the application.

Keyword: Listed

I went shopping at a few retailers for dryer transition ducts, and I was quite surprised to learn that the semi-rigidmaterial I've always recommended is not a UL listed product!  If there are any manufacturers that do make a UL listed semi-rigid duct, they've done a good job of hiding their products - even online.

Semi-rigid aluminum duct

Home Depot sells 'dryer stuff' from Deflect-o, while Menards and Lowes sells 'dryer stuff' from Dundas-Jafine.  After browsing through Deflect-o's web site and Dundas-Jafine's web site, I learned that the only UL listed dryer transition ducts are the ones that look like they're made from foil, pictured below.  These transition ducts are actually made from aluminum.

UL Listed Dryer Transition Duct (foil)

Interesting.   I won't be reporting the semi-rigid stuff as unsafe - I have it in my own house, and I feel like it's perfectly safe... but if it's not listed, it's a code violation and I won't recommend it any more.

I wonder why the semi-rigid material isn't listed?