Installing ducts is not a good DIY project.
1) Sheet metal has the lowest resistance to air flow, but it tends to
be noisy and is more difficult to seal than other types of duct. It is
more resistant to physical abuse than most other types of ducts..
2) Flex is the easiest to install and easiest to seal. You only have
to seal the two ends. It has higher resistance to air flow than
ductboard or sheet metal. It is the quietest of all ducts. It is
prone to damage from abuse. It is easy to kink, which reduces air
flow. The insulation around it is fluffed up and is better insulated
than wrapped sheet metal in most installations.
3) Duct board is more resistant to abuse than flex, but less resistant
to abuse than sheet metal. Ductboard is Thermally superior to wrapped
sheet metal in most installations. 1-1/2" Ductboard is R-6. It takes
2-1/4" duct wrap over sheet metal for R-6. If the duct wrap is pulled
tight over sheet metal duct for good appearance, the R-factor is
reduced to as little as R-2 or R-3. Ductboard with tight corners leaks
NOTE: Regular duct tape does not meet code for either ductboard or
flex. Tape for ductboard should be rated UL181 A-P or A-H. Tape for
flex should be rated UL181 B-FX. The rating should be written
continuously on the outside of the tape along with the model of the
tape, the brand and the last test date that it passed UL..
Note that if you install sheet metal, you should SEAL EVERY SEAM. With
round metal duct, in an elbow, that is 3 joints in the middle plus two
at the ends where it attaches to the round duct. Every longitudinal
seam should be sealed as well. Use mastic, not duct tape. If the
sheet metal is not well sealed, the cold supply air will leak out into
the wrap, making the outer vapor barrier of the wrap cold, making it
more likely to sweat and less effective as insulation.
So you see, it is not that straight forward. There are tradoffs for
any installation that must be considered.
Also, if you don't size the ducts properly, you will not get the right
amount of air flow, reducing the performance of the system. Many
contractors have a hard time with that. Look for a contractor with
NATE certified technicians and installers. You stand a better chance
of getting a good job that way.