Flex must be oversized at least one duct size.
Flexible vs sheet metal ducts.
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.
Well if we re not going to use flex what s left.
I do agree with what ponytl said about possibly avoiding some duct loss with flex but possibly having duct loss due to flex.
In comparison to metal duct flex duct is easier and faster to install.
Alternatives to flex duct.
Air flow in a steel duct tends to travel smoothly in a coil like motion.
The installation of hard ducts is also more difficult because.
Hard duct is probably the best but most won t pay the difference for it.
Rigid metal is straight and smooth walled you have to work to crush or kink rigid metal.
So it may be a wash.
This is because they re more versatile and flexible.
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.
Flexible ducts are also the worst type of ductwork for airflow.
That leaves rigid sheet metal often called hard pipe.
Rigid metal is more likely to be installed without constrictions whereas flex duct can easily be pinched down.
Sheet metal ducts are less likely to incur dangerous mold and growths because their surfaces are non porous and smooth.
If any duct product should be banned it s duct board.
Flexible ducts are made out of corrugated plastic often wrapped in insulation.
Flexible ducts are fairly easy to install and will often cost less than the rigid ductwork.
Metal ducts are more rigid due to the nature of steel making them ideal to build an entire hvac system.
Btw we did replace the rusted metal duct with new metal.
I still don t believe in duct board.
Even when doing this the flex duct creates much more resistance.
Their shape and size also allows them to be installed in locations that would not accept full sized sheet metal ductwork.
There s duct board rigid fiberglass with a foil facing but in my opinion that s worse than flex.
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.
Air flow in a flex duct tends to tumble probably because of the rib like intrusion every inch or so.
Rigid metal is not easily damaged by rodents and uv light.
Most of what s being installed in new houses now is flexible ducts because they cost so much less than rigid sheet metal ductwork.
While they tend not to last as long as sheet metal they are very cost effective.
