To fix a leaking window sill, start by inspecting the sill for cracks, rot, or damaged weather stripping. Remove loose or crumbling caulk with a putty knife, then clean the area with a damp cloth and let it dry. Apply fresh silicone caulk along the sill’s edges and around the window frame, pressing it firmly as it sets. For severe gaps, use expanding foam insulation to seal gaps before applying caulk. Finally, replace worn weather stripping and seal any gaps with adhesive-backed foam tape. Regular maintenance every 1-2 years ensures long-term protection.

Leakage at window sills often stems from worn or missing caulk, rot, or poor insulation. According to the U.S. Environmental Protection Agency (EPA), homes lose 10-15% of heating and cooling energy through inefficient windows, with sills being a common weak point. A 2021 study by the DIY Network found that 30% of window leaks are caused by damaged sills or seals. Silicone caulk is durable and resists moisture, while expanding foam fills gaps that caulk cannot. Replacing weather stripping, which degrades over time, is critical, as the EPA notes that 40% of air leaks occur around window frames. Combining these steps addresses both structural damage and insulation gaps, reducing energy waste and moisture-related issues like mold. Data from the National Association of Home Builders (NAHB) supports this approach, showing that proper sill maintenance can lower heating bills by up to 20%. By prioritizing timely repairs and using weather-resistant materials, homeowners can prevent costly water damage and improve energy efficiency.