The short answer

A stain on a window sill is the end of a path, not proof that the sill is the source. Compare the timing with rain, indoor humidity, temperature, and plumbing use. Inspect the frame, trim, wall above, exterior cladding, flashing clues, gutters, and nearby roof or pipe routes without destroying the evidence.

Build a source log

Clue Possible source What to record
Beads on interior glass Condensation Room humidity, temperature, and airflow.
Stain after rain Exterior entry or flashing Wind direction and roof runoff.
Stain above the window Wall, roof, pipe, or cladding path Higher wet marks and ceiling condition.
Stain near a bathroom or kitchen Plumbing or humid air Appliance use and adjacent wall condition.
Soft trim or bubbling paint Repeated moisture Extent and whether it returns after drying.

Photograph the fresh wetting and stain, then dry accessible surfaces promptly. Do not delay leak repair to complete a log, or seal and repaint before the moisture source is corrected. Keep furniture and electrical equipment away from active water. If the wall is soft, the stain grows, or mold-like growth is broad, arrange professional assessment.

Use the location of fresh water to narrow the next check

Water beading on the room-facing glass during cold weather supports a condensation explanation. Andersen explains that moist room air can condense on cold glass without a glass failure. Check humidity and ventilation, and allow room air to circulate around the glass. That observation does not exclude a separate rain leak at the frame.

For a window with a designed drainage system, inspect accessible exterior weep openings according to its maintenance instructions. Andersen’s care guide explains that those openings carry collected water out. Do not caulk them closed or drill new ones based on a different window’s design. Rain-correlated water appearing above or behind trim still requires investigation of the surrounding wall and flashing.

Keep separate records for rain events and dry, cold mornings. Photograph fresh water before wiping it away, then dry accessible surfaces promptly. A controlled water test or trim removal may be useful for a professional, but indiscriminate hose spraying can introduce more water without identifying its entry point. Ask the investigator which path the evidence supports before authorizing a window replacement.

Compare repair scopes

An estimate should separate condensation control, exterior sealant, flashing, cladding, trim, drywall, insulation, plumbing, and paint. A replacement window is not automatically the right response to a failed flashing detail, and interior caulk is not an exterior water-management system.

Limits

This guide cannot diagnose a concealed leak from a stain. Water entering an occupied wall, ceiling, or electrical area deserves prompt help.