To fix a leaking wooden window, first inspect the window frame, sash, and seals for rot, gaps, or damaged weatherstripping. Remove loose or decayed wood and replace it with matching rot-resistant material. Apply exterior-grade sealant or caulk around the frame and sash joints. Reinstall or replace worn weatherstripping and ensure the drainage channels are clear. For persistent leaks, consider professional sash replacement or adding storm windows.

Wooden windows leak due to rot, which accelerates when moisture penetrates untreated wood. The U.S. Forest Service reports that wood decays when its moisture content exceeds 20%, with rot causing structural failure in up to 30% of older homes. Leaky windows also waste energy, as the Department of Energy notes that poor insulation from gaps can increase heating/cooling costs by 10-20%. Weatherstripping degrades faster in humid climates, with the EPA estimating that 40% of energy loss in homes stems from inefficient window seals. Exterior drainage issues compound leaks, as clogged gutters or downspouts can direct water into walls, worsening rot. Replacing damaged wood and sealing gaps reduces water intrusion by 90%, per ASTM International standards, while proper drainage systems lower flood risk by 15-25%, as cited by the Federal Emergency Management Agency. Regular maintenance, including annual inspections and sealant reapplication, extends window lifespan by up to 15 years, according to the National Association of Home Builders.