There is an abundance of information out there regarding duct sealing; however, when specifying ducts for a new construction project there is not a straightforward solution.
The primary reason for the focus on sealing duct work is energy loss. The EPA Energy Star program estimates that as much as 20% of the air moving through a duct system is lost due to leaks, holes, or improper installation.
Keep the metal ductwork
The lack of information on "which duct product is best" is justified because the short answer is: "it depends."
- To properly specify duct work for your home, a mechanical engineer or contractor needs to perform calculations specified in Manual D Residential Duct Systems.
- This manual is an ANSI-recognized set of instructions for calculating proper duct size for a home.
Having seen the results from a duct blaster on many different configurations of residential duct work, I would recommend keeping the metal duct work and cleaning it.
- Flex duct has an inherent flaw based on the very attributes that make it flexible. The ribs on the inside of a flexible duct reduce the air flowing through it measured in cubic feet per minute (cfm) causing back pressure to the source.
- There are mathematical calculations required to properly size flexible ducts for use in a residential home. Most often, flex duct is used to reduce cost or in places where metal duct work will not fit.
Re-insulating your ductwork
You mentioned that you were considering re-insulating your current metal duct work. This is a great decision!
If your budget allows, you may want to consider resealing your duct work with a mastic product. More than likely, the ducts are currently sealed with tape. Mastic is a superior sealant to tape and I've had severe difficulty in some cases passing the Energy Star duct air leakage test without mastic (i.e., using tape).
For more information:
See theEPA's Energy Star websitefor more information on air leakage and sealing your ducts.
Read Daniel Glickman's Q&A, "Should we replace our PVC-lined flexible heating ducts with another type of duct work?"to learn more about choosing duct work for a healthy home.