Replace flex duct with rigid duct?

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.
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First question: Is the system large enough to handle the additional 240 sf of space?
Flexible ductwork is typically used to "runout" a rigid line (my Mechanical code states it can be no longer than 8' long) ... your question about connecting to the "last piece" is where this piece of flexible ductwork would be used (if at all). If your ductwork is under your house, then use all rigid ... and yes, you should have enough play in the ductwork to get everything pieced togther.
No, I doubt that you'll get noticeably better airflow moving from flex to rigid, but that depends entirely on the installation. You may find that this will certainly increase your airflow, but if the duct wasn't tapped of in the correct place, or if the system is sized to handle the additional ductwork, or if there are other mitigating factors, then all bets are off. Mastic will certainly seal those seams very well (assuming it's installed correctly, that is) but it's not going to solve any problems.
As far as determining the size of ducts required ... we can't see it from here, so we can't really comment on it. Taking into consideration your description of the work done so far, however, I believe I'd cut my losses and try to find a licensed, competent contractor who uses skilled laborers (not whoever they can pick up at the labor pool each day) to install their equipment and ducts.

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Scott McDaniel wrote:

Certainly a fair question. The current system was installed after the addition was built.

Other ducts seem to blow markedly stronger with the same length run. But of course, the original system wasn't designed to have this duct extended. This extension is a straight extension of an existing duct. I would use mastic to avoid creating new problems, not to solve existing ones. Is it tricky to install mastic correctly? The manufacturer instructions seemed fairly clear.

Absolutely. I didn't ask any questions about sizing.

It's a nice idea. But I could probably read the Manual J or whatever it's called and learn how to do all the computations myself before I could find a competent contractor in my area willing to take on a job this small. And I could probably do the job myself in less time than it would take me to interview contractors to try to find someone, to say nothing of waiting home for the ones that don't bother to show up. It's unfortunate that this is the way things work. The guy who did the previous work did the work himself and was licensed and at least moderately competent.

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snipped-for-privacy@cam.cornell.edu wrote:

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
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Are the rectangular duct pieces supposed to actually stay together, or is the duct tape supposed to be all that keeps them from falling apart? The ones I saw at Home Depot wouldn't stay together.

I have some leaks I patched with the aluminum tape where I believe the tape is failing, so I'm not so sure it works fine.
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