Round spiral lock-seam duct is the most efficient way to transport air in HVAC systems. The round profile has greater flow efficiencies than rectangular ducts. Round ducts are inherently stronger than rectangular profiles, they can be lighter metal gauges and can be installed in longer spans. The 4-ply spiral lock-seams of “spiral duct” create a product that is stronger than traditional welded-seam round ducts of the same gauge. Spiral lock-seams have practically no leakage, and are the only mechanical seams exempted from sealing under energy standards such as ASHRAE Standard 90.1 – 2013.