Why Windows Leak
Windows leak when the seal between the window unit and the surrounding wall fails. Common causes: failed exterior caulk, damaged or missing head flashing, improper installation without a sill pan, cracked or missing sill flashing, and deteriorated glazing compound around glass panes. In older windows, the wood sill itself may have deteriorated enough to allow water entry. Wind-driven rain during severe storms can exceed the capacity of even properly installed windows.
Signs of Window-Related Water Damage
- Staining on the wall below the window — often triangular, following the window corners
- Soft or spongy drywall below or beside windows
- Rotting wood sill or interior window trim
- Efflorescence on masonry below windows
- Musty odor in rooms with older windows during or after rain
- Peeling paint on interior windowsill
Repairing Window Leaks
Diagnosis first: determine whether the leak is at the exterior caulk joint, the flashing, the glass-to-frame seal, or the frame-to-wall connection. In most cases, re-caulking the exterior perimeter with high-quality silicone caulk, adding or replacing head flashing, and installing sill pan flashing resolves the leak. Full window replacement is required only when the window frame itself has deteriorated beyond repair. After waterproofing, inspect and replace any damaged interior drywall and framing.
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