Never seen flex duct that is. They have liner that is and coatings that can be used. But in the real wold mold will not grow on metal so I would not waste the money.

 

Mold will grow on any surface that collects dust and is near 100%RH for a day or two. Regards TB

 

Take a look at QuietFlex AMG:
http://contractingbusiness.com/video/quietflex-takes-first-1109/

And SilverGuard from United McGill for the non-flex duct:
http://www.mcgillairflow.com/newsDocs/newsPDFS/silverGuard_WP.pdf

 

Don't fall for the marketing gimmicks about mold resistent flex duct. All flex duct is made from polyester and the one of the UL 181 tests for flex duct is mold growth. No inner core of flex duct can grow mold growth in order to pass the test. Thus, all flex duct doesn't grow mold.

NOW, if you have moisture sitting in the flex duct, there is nothing that will inhibit the mold growth.

I would recommend, ATCO, www.atcoflex.com, they are by far 3 times the size of any other flex company and have been around longer than any other flex company.

 

Why not just use ridgid duct and insulate it if it's a concern?