Short answer: water leaks are prevented by coordinating the wall opening, sill and flashing, frame drainage, gaskets, sealants, anchors, glass, and installation workmanship. Adding more sealant over an existing leak may hide the symptom temporarily, but it will not correct blocked weep paths, failed flashing, poor sill slope, frame movement, or water entering from another part of the wall.
For designers, details should show how water is collected and returned outdoors, including sill transitions and perimeter joints. For owners, warning signs include staining, bubbling paint, mold odor, wet tracks, deteriorated sealant, and leakage that appears only during wind-driven rain. For contractors, the priority is to keep drainage paths open, use compatible sealants and backer materials, anchor without distorting the frame, and protect completed work from damage.
An existing leak should be diagnosed before repair. Water may enter above or beside the window and appear at the sill, so the inspection should include the surrounding wall, façade joints, roof or balcony edges, flashing, glass seals, frame corners, tracks, and weep outlets. The correct repair could involve cleaning drainage, renewing a joint, correcting flashing, resetting glass, or replacing a damaged component.
Photograph the leak during rain if safe, note the wind direction, and avoid permanently blocking drainage holes with sealant. Send OTEC photos, videos, dimensions, and the project location so the likely cause and appropriate inspection scope can be discussed before replacement is recommended.
