Home | System | Performance | Water Tightness Water Tightness -350 Pa 1. IWIN-S Aluminum Sliding Door/Window features a Step Sill Structure which is a Patented technology by YKK AP.YKK AP ‘s Patented Step System Structure uses a raised two rail design to ensure superior drainage which ensures a stable Water Tightness of 350Pa. Superior Drainage 2. IWIN-S Aluminum Doors and Windows features 90 Degree Corner JointsIWIN-S Aluminum Doors and Windows features 90 Degree joints with a Sealer Pad, which ensures a stable water tightness of 350 Pa Stable Sealing Whereas in Normal Doors and Windows, Mitre Joint Frame corners results in a low water tightness due to unstable sealing