On hard metal pipes the air moves swiftly through the pipes with little to no hindrance the pipe walls are smooth galvanized metal with adjustable elbows as needed. The minor drawback of this type of pipe is the metal you must hot or cool the pipe first before tempering the room that it’s being sent to. The good thing about this type of ductwork it never comes apart all the joints are screwed together and all seams are now sealed so no air leakage. Finally when its time to have your duct work cleaned, they guys bring in 2 hoses a large volume vacuum hose and a large volume air compressor hose with these hooked on ether end each duct the volume of air used to clean the pipe run handles is with no problem.
Now with insulated flex pipes the air moves swiftly through the pipes with little to no hindrance the pipe walls are made of a spiraling piece of wire in beaded in the poly tubes pipe wall than covered with insulation and a 2nd poly to keep the cavity air space that the pipe is passing through. Sorry to say the inexperienced installers use this ductwork a lot and in places that it was not intended to be used. Granted this pipe is very flexible but bending the pipe in a hard 90° or even 180° and/or squeezing it down to fit a 3.5 inch space cavity. When the inside pipe measures 6” and the out side measures 8 1/2. A 6” round duct has 28.26 Cubic inch of open area when you crush it down to 3 1/2” on the exterior; the interior only has 5 cubic inches of open area. The bad thing about this ductwork is if too much air pressure and vacuum is used in the pipe run and the connections come apart and contracts to the vacuum side of the cleaning.
When you need this type of ductwork cleaned, many things can go wrong 1 the joints can come apart. The compressor hose can puncture the inner poly wall of the pipe, or if there is a restriction in the pipe and there is debris in the pipe it could clog. Remember these entire ducts are running in concealed spaces between your ceiling and 2nd floor or your basement ceiling and your main floor. If they disconnect there is no way to re attach them with out remove 1 of your finished surfaces.