I was not able to figure out which forum was the right one to ask this in. I'm a DIY homeowner (ex contractor) and the general forum asks homeowners to post in the AOP forums, and the AOP forums say no DIY.

I have a home in the SF bay area with an uninsulated attic. We have mild weather of course. The attic hosts a gas furnace and most of it's ductwork is what I call alumaflex - the core is thin aluminum, wrapped in a bit of fiberglass and plastic sheathing/skin. Most of the sheathing has disintegrated and is hanging down. Some of the fiberglass wrapping is hanging down too. This stuff is about 20 years old.

I would like to replace much of this ducting. The question is should I use alumaflex, or go with flex ducting (that is, wire coil, with insulated wapper)? I was going to use alumaflex because I'd guess it sends the air down with less resistance. But flex ducting is a lot easier to work with and cheaper. Could I just install a larger diameter duct - ie where now it has 8" alumaflex, install 10" flex? The idea being that the larger size would compensate for the increased resistance of the flex duct.

In some areas the ducting will lay on the rafters, in others it will need to be suspended.