Flat oval ducts are expected to leak half of what rectangular ducts do. They have no longitudinal seams to seal. Spiral lock-seams have negligible leakage (various tests carried out in accordance with AMCA 511) which have earned them their sealing exemption in ASHRAE Standard 90.1. More significantly, they usually have less than half as many duct joints as rectangular duct (10’ or greater lengths versus 56” lengths). Most duct leakage occurs at the transverse joints; half the joints mean half the leakage.