How best to insulate my 3" x 2.5' semi rigid aluminum outside air duct

[vermonty]

I installed my 3" outside air duct (which is 2.5 feet in length - see attached photo) for my Hearthstone GM40 and would like to insulate it. My original plan was to wrap koawool insulation around this pipe and then install 4" semi rigid aluminum pipe over the 3" pipe and insulation. In looking into the koawool, it has respiratory concerns related to it so I'm not comfortable with having it out in my living space, even if I try to seal the ends with high temp caulk.
As this pipe is visible in our living space, I was hoping it won't be overly obtrusive looking. I was going to paint the pipe matt black when done.
Many thanks.

 

[Highbeam]

Have you experienced a need to insulate it? I have heard that there is the potential for frost if outside air temps are low enough.

Mine is not insulated and has no frost issues. The pellet stove people are big on OAK connections and might have some ideas. Don't think I've ever seen anybody insulate a OAK duct.

 

[vermonty]

Even in this autumn weather (50's), the colder air rushing in is making the pipe cold to the touch. I suspect when our Vermont winter starts, it'll get real cold!