I like the insulated board duct for the main trunk sections and galvanized pipe for the runners to help with noise control. I think the cfm rating on the flex duct is lower than galvanized. Insulating the galvanized would help with the noise I would think. I waited and found some rock bottom specials on insulated flex duct and removed the duct portion and insulated my galvanized runners much cheaper than buying insulation solely for galvanized.

I think the loss in air flow with flex is pretty high in comparison.

"I think the cfm rating on the flex duct is lower than galvanized. Insulating the galvanized would help with the noise I would think"

to move the same air that a hard pipe duct with flex the flex is sized one size up.
look at labor cost to install insulation on hard pipe
vs flex.

not poking @ you countryboy...just how my mind works.
how can I get same performance for lesser cost.

of course all should be mastic sealed...long seams,
joints & 90's in hard pipe PRIOR to insulating externally.
with flex it is the take off @ plenum and where flex connects to supply box. other areas to seal are plenum connections, returns and always hole in floor/ceiling/wall
where supply box attaches supply grill.
mastic mastic mastic...always seals, lasts for years
and pays back..in a very very short time.

best of luck OP
and hey countryboymo..things going ok in your part of the world?

After info here and further investigation on line seems like sticking with my galvinized pip is the best. Have done some tweaking on how the pipe is hung as well as clearances and have resolved some of the popping and cracking issues. Thanks for all inputs.

arlie - I saw your other thread as well. I am having airflow noise issues with my system. Why did you decide against insulated flex? Thanks.