Flextral’s standard HVAC ducting (series DU90 and DU91) are of welded construction.  Welded seems allow our duct to be used in a bi-directional manner.  Meaning, it doesn’t matter which end is connected to the HVAC unit and which is connected to the vent.  In a sewn duct construction, the duct must be installed directionally specific.  There is a slight tab hanging off the end of the seam, which is only meant for uni-directional flow.  Bi-directional flow will cause turbulence in the air flow and result in an inefficient setup.  Additionally, welded seams are a stronger joint over sewn.