Externally insulated sheet metal ducts are best. The smooth interior surface of metal allows the air to flow freely with little resistance for maximum efficiency. The smooth surface also makes it harder for dirt and contaminates to stick, and also discourages molds and fungi to grow. Metal can also be cleaned and last forever if installed and maintained properly. If you have an older but properly designed and installed metal duct system there is a good chance that the external insulation is deteriorating and is no longer as efficient as it could be. When I say properly designed, I mean if you are comfortable in your entire home the majority of the time regardless of the outdoor temperature then your duct system is probably properly designed. When it comes to proper installation, older homes were built to different codes and standards, many of which ignored energy efficiency. A lot of older round metal ducts were just stuck together with duct tape at best. Also up until the last few years, building codes in Texas did not require the seams to be sealed and secured. IF your externally insulated duct system is properly designed and is secure enough to not fall apart, then we can do what we refer to as a “peal and seal”. We remove the insulation, make sure each joint has at least 3 screws and seal all seams and joints with a code approved duct mastic. We also add or replace the hanging brackets and strapping to meet or exceed current code and then reinsulate the ducts with at a minimum 3” fiberglass duct wrap. 2” is still available but considering the high cost of energy the few bucks you would save upfront would not benefit you in the end.
Other types of insulation are available, of those, cotton is the very best available in my opinion. It’s much denser and will last longer than fiberglass and can be installed in some very tight applications where fiberglass is not as feasible. It does cost substantially more, so you have to consider all of your options.
Some other bubble foil type insulations are starting to expand in the market place. I am in favor of these, however the installation practices are much different so if choosing these, make sure it will be installed properly.