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wrote:
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.

1. It should improve the flow at least a little, the corrugations in
the flexible duct interfere with smooth airflow to some extent.
Whether it will be enough to solve your HVAC problems in that room is
hard to say. There are "duct boosters" which are little electric fans
that fit into the duct to increase airflow, but they are not the most
elegant solution, cost, and installation is much more complicated than
mere ductwork. If you need more heat/cool you might consider using a
bigger duct all the way from the plenum, or two ducts.

2. I'm surprised you can't find a book on it, but barring that, try
this: go to a building supply store and look over the various ductwork
pieces, play with them a little, see how they fit together, and think
about how to lay out your ductwork. There is a big variety of pieces
to work with: round duct, rectangular duct, pieces to transition from
one to the other, elbows, elbows that swivel around to any angle, etc.
etc. There is not much you can't do with a tape measure, sharpie pen,
and a pair of tinsnips. And duct tape. Get the shiny metallic looking
duct tape, not the gray cloth stuff. I haven't tried mastic but that
sounds like an awful bother when the tape works fine. -- H