Metal ducts are more rigid due to the nature of steel making them ideal to build an entire hvac system.
Flexible vs sheet metal ducts.
So it may be a wash.
Rigid metal is installed then mastic is applied to air seal then insulation is applied on the outside so there is less chance of fibers getting into your air stream.
Even when doing this the flex duct creates much more resistance.
If any duct product should be banned it s duct board.
I still don t believe in duct board.
While sheet metal ductwork tends to be a better long term choice flexible ductwork has the advantage of relatively low upfront cost and versatility.
Alternatives to flex duct.
I do agree with what ponytl said about possibly avoiding some duct loss with flex but possibly having duct loss due to flex.
Flexible ducts are also the worst type of ductwork for airflow.
In comparison to metal duct flex duct is easier and faster to install.
Flexible ducts are made out of corrugated plastic often wrapped in insulation.
Flexible ducts are good for air quality because you are not breathing in fiberglass insulation lined ductwork like the fiber air ducts and they are mold and rust resistant unlike galvanized sheet metal ductwork.
Rigid metal is not easily damaged by rodents and uv light.
This is because they re more versatile and flexible.
Rigid metal is more likely to be installed without constrictions whereas flex duct can easily be pinched down.
Btw we did replace the rusted metal duct with new metal.
The inner liner ribs restrict airflow more than sheet metal ductwork but not significantly.
Air flow in a flex duct tends to tumble probably because of the rib like intrusion every inch or so.
Flex must be oversized at least one duct size.
Flexible ducts are fairly easy to install and will often cost less than the rigid ductwork.
Rigid metal is straight and smooth walled you have to work to crush or kink rigid metal.
There s duct board rigid fiberglass with a foil facing but in my opinion that s worse than flex.
While they tend not to last as long as sheet metal they are very cost effective.
Most of what s being installed in new houses now is flexible ducts because they cost so much less than rigid sheet metal ductwork.
Well if we re not going to use flex what s left.
Their shape and size also allows them to be installed in locations that would not accept full sized sheet metal ductwork.
The installation of hard ducts is also more difficult because.
Flex can be good to help dampen velocity noises and it is relatively air tight but imo mastic on the old sheetmetal ducts covered by a good foil backed insulation is without question the best way to go old ducts or not.
Sheet metal ducts are less likely to incur dangerous mold and growths because their surfaces are non porous and smooth.
Air flow in a steel duct tends to travel smoothly in a coil like motion.
