To fix a leaking window sill, first inspect for cracks, gaps, or damaged components. Clean the area with a wire brush and soapy water, then apply a high-quality silicone or polyurethane caulk to seal gaps. Reinforce loose interior trim with wood screws or brackets. For exterior leaks, check flashing and replace if corroded. If the sill is rotting, remove it and install a new one with proper drainage. Regular maintenance reduces recurrence.

Leakage often occurs due to worn seals, structural damage, or improper installation. According to the U.S. Department of Energy, 10% of home energy loss stems from inefficient windows, with leaks contributing to moisture damage and mold growth. A 2021 study by the National Association of Home Builders found that 30% of window-related repairs involve sill leaks, costing homeowners an average of $300-$800 per fix. Silicone caulk, when applied correctly, lasts 5-10 years, while flashing replacements typically prevent leaks for 15-20 years. Neglecting repairs can lead to wood rot, which averages $5,000 in restoration costs. Proactive maintenance, including annual inspections and timely sealing, reduces long-term expenses and enhances energy efficiency by 15-20%, as noted by the EPA.