flexible duct versus rigid duct

jkmcOctober 9, 2008

Any comments on pros of one type of duct over the other? I am moving my entire system to the attic. The rigid duct costs about $700 more.

Also, one contractor want to mount the furnace on all thread rods connected to the roof rafters. Another contractors wants to mount on the ceiling joists. Any comments on the pros and cons of each method?

Finally one contractor said I could not put a 90% plus furnace in the attic because the condensate would freeze while the other contractor said this would not be a problem.


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I used all insulated, rigid duct and don't regret it. Flex duct is the cheap way.

I set my air handler on the attic plywood floor above my bedroom on rubber pads and I can hardly hear it. Suspended from the joists is supposed to stop the direct transfer of vibration.

    Bookmark   October 9, 2008 at 5:46PM
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I'm pretty happy with my solid ducts. Very quiet.

    Bookmark   October 9, 2008 at 6:22PM
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Frozen condensate resulting in probable fractured furnace condensate collectors, and traps is a concern in cold climates. If your located in a climate that the air in the attic may be at freezing temps, then a 80% furnace should be considered.

While some attempt to box in areas and wrap with heat tape, not always the best idea

    Bookmark   October 12, 2008 at 6:45PM
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Nothing wrong with metal trunk lines and flexible ducts leading to the supplies for most cases. However, if not cost prohibitive, indeed, I would recommend all rigid ductwork.

    Bookmark   October 12, 2008 at 9:02PM
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The recommendation we tried to implement was to have rigid duct to within 5 feet of register boots. Flex only for those last connections. We ended up with more in several places. Still seems to work fine.

Our furnace is 80% (dual fuel backup to HP) and is set on plywood on the ceiling joists. My only gripe about this is that there's at least one electrical junction box that was pulled loose and dropped into the joist bay beneath the air handler during installation.

I can't get to that or really do anything much about the insulation under the air handler. Kinda sucks b/c the hallway below really needs a ceiling fixture, and there's already a blanked off switch box in the hall from a whole-house fan that we removed when the air handler went in.

    Bookmark   October 13, 2008 at 1:22PM
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I prefer the rigid main/trunk duct with flex to the duct register boxes. Remember, when cutting and designing duct, the rigid can be cut in any dimension, eg. 12 1/2 x 12 1/2 or 9 x 7 or any combination. The tool for this is a 'ductalator' a ductwork slide rule. By placing the knowns it will provide the above combinations that will deliver the maximum amount of air flow. FLex duct cannot do this as it only comes in fixed diameters, eg. 6" 7" etc. So you see the rigid is more precise if done properly. To help save on labor the flex can then be used to run from the main to the duct box. As the runs are usually short the affect on the total system efficiency is usually minimal. As for hanging or placing on the ceiling joists consider: if hanging from the rafters the weight is pulling DOWN pulling the rafters inwards which is also the force they were designed to resist. if placing on the joists that is only a board that is run between the rafters and has no real support so to speak. Here in FL they want the a/h to be supported from the rafters. Can't help you with the gas furnace issue. Sorry.. Good luck..

    Bookmark   October 17, 2008 at 3:14PM
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I just moved into a custom-build home. It has hot water radiant floor heating; the air conditioning uses rigid ducts with registers in the ceiling.

I clearly specified that the a/c should be quiet to the extent that it would not be noticeable, and that it should be draft free; it is neither. The noise level is about what one would expect in a tract home, which this is not!! Almost certainly the noise is the result of excessive air velocity resulting from ducts that should be larger. Probably the blower cannot be slowed down since the air discharge temperature is about 51 degrees and, reducing the blower speed enough to cause a significant reduction in noise would reduce the efficiency.

At this point, I am not certain what the solution is, but probably it will not be cheap. I'm guessing that it will be necessary to tear into the gypsum board ceilings and increase the duct size for the last few feet (5 feet perhaps?) and put in larger registers. Also, replacing the rectangular registers with square ones having adjustable curved fins which discharge the air in 4 directions instead of 2 directions should considerably reduce drafts.

If this were done, would it be quieter if the last few feet of duct before each register were flexible instead of rigid? Replacing ducts all the way from the air handlers (there are 2 air handlers and 2 condensing units because the upstairs and downstairs are cooled separately) would be virtually impossible because of space limitations.

Can others offer suggestions? I'd like to hear them.

    Bookmark   May 27, 2009 at 2:55AM
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"As for hanging or placing on the ceiling joists consider: if hanging from the rafters the weight is pulling DOWN pulling the rafters inwards which is also the force they were designed to resist. if placing on the joists that is only a board that is run between the rafters and has no real support so to speak."

This needs evaluation on an individual basis. Joist loading will increase compressive strain possibly offset by walls. Rafter loading increases shear on fasteners unless hung equally either side of a structural ridge which most houses don't have. Given concerns with freezing of condensate, snow loads could further complicate the issue.

    Bookmark   May 27, 2009 at 3:27AM
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The thing about flex duct that can be so misleading is the fact that most of us have seen really crappy installs that use alot of flex strung out all over an attic. There is nothing wrong with flex duct as long as it's installed correctly. It need to be pulled tight as to make the inner skin as smooth as possible, resulting in less air friction. Sure, it doesn't last as long as board or metal, but overall is cheaper to install. I personally like a nice trunk (board) across the whole house and then i branch off the the insulated boxes with flex, usually less than 10" runs. It won't matter what kind of material your duct system is made of... if the system isn't sized correctly.

Hope this helps

    Bookmark   May 27, 2009 at 9:00AM
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Flex duct has higher flow resistance that must be accounted for in the duct design.

Ductboard is another good alternative.

You want the lowest mass duct work in unconditioned space.
The more mass is present, the higher the heat capacity of the material (amount of heat needed to increase (or decrease) the duct material temperature.

When the system is not circulating, the ambient around the ducts starts to heat (or cool) them.
When the system comes back on it heats (or cools) the duct material and adds to the system loading.
The more mass the ducts have the more energy the absorb changing temperature.

Insulation slows the movement of heat, but does not stop it.
3 inch ductboard makes a well insulated low mass system for unconditioned space.

    Bookmark   May 27, 2009 at 11:40AM
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Our home is 40 years old and I diyed the HVAC had it inspected,and was told that yes rigid duct is far superior,I used 16" flex for the trunk line,inspec said thats OK but over time the tape on the take offs will lose thier seal due to temp changes the system runs great and I can barely hear it, was I wrong doing it this way?

    Bookmark   February 22, 2011 at 7:30AM
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agreed with edward. my fiend, working in a local construction firm, also suggested me rigid duct over flexible duct as its more durable

Here is a link that might be useful: flexible duct

    Bookmark   June 27, 2011 at 3:26AM
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