I have a small (20 x 12) addition which is poorly served by my HVAC
system. I tried to improve things by having a duct extended and the
contractor used insulated flexible duct which I think is pinched in at
least one place and possibly compressed by surrounding insulation. The
6" round flex duct has exactly the same cross sectional area as the
duct that it is extending.

I recently read that insulated 6" flex duct has an equivalent duct
length of two times it's actual length. And of course there's the
issue of the pinching. So I was thinking I should replace this duct
with rigid duct.

I have two questions. The first question is: am I wasting my time to
do this? Can I expect to improve the air flow by switching 12' of flex
duct to 12' of rigid duct and of course, eliminating any pinching?

The second question is: where can I find information about the proper
way to install ducting? I have been unable to find any books or web
sites that explain how to do this. Only thing I've managed to turn up
so far is the claim that "duct mastic" is much better than any kind of
tape and should be used to seal seams and joints. It seems like there
might be a problem with fitting in the last piece of rigid duct. (Or
is the ductwork flexible enough that I can bend it open to make room
to fit in the last section?) This duct runs through an insulated
floor. Rectangular ducting would be easier to fit in and insulate
around if I can figure out how to install it and mate it to round duct
ports at one or possibly both ends.