I’m replacing an undersized bathroom exhaust fan with a larger one that has a higher CFM.

When switching the fan out, I want to add insulation to the flexible aluminum duct that goes from the fan to the roof. The duct is roughly 3 11/32” in diameter and 10’ long.

All the articles and videos I’ve seen online on insulating ducts tends to show a plastic-like flexible duct with a fiberglass insulation wrap, which makes me wonder whether there is some issue I haven’t thought about related to adding the same kind of insulation to a flexible aluminum duct.

I was hoping someone could confirm that the same methods apply for flexible aluminum ducts, and also any recommendations for insulation to use.

  • 2
    I would think that what you're seeing in videos is a flexible duct that comes from the mfgr with the insulation already installed. What kind of "flexible aluminum duct" do you have? Can you get a nice, clear, focused picture or two and edit them into your post?
    – FreeMan
    Commented Jun 29, 2020 at 14:52

1 Answer 1


Insulated flexible plastic ducting is a common item available at any big box hardware store. Unfortunately not so much for preinsulated flexible aluminum ducting.

Sometimes code prohibits the use of non-metallic flexible ducting (for example often with range hood exhausts)

First I would check whether non-metallic ducting is allowed for your bathroom exhaust. If it is, then just purchase the insulated new ducting. This will be the easiest

If you are required to use metallic ducting, and the run isn't too long, then I have had good luck buying the insulated non-metallic ducting one size larger. You can then sleeve it over the metallic ducting. Sometimes it fits straight away, but sometimes you need to pull the inner ducting out leaving just the fiberglass lined sleeve.

Secure along the run with occasional zipties, and tape and tie at both ends.

  • I know you mentioned non-metallic, would something like this work? homedepot.com/p/… . After typing this I realized you meant to get the pre-insulated plastic one and using that. Just to have options though, would the one in the link work for my purpose?
    – Insu Q
    Commented Jun 29, 2020 at 16:38

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