Finding water on a sill after a storm can be confusing because several different paths can lead moisture to the same spot. The key clue is timing: if the water appears during or shortly after rain, treat it as a possible water-entry problem rather than assuming indoor humidity is the cause.

1. Check the Exterior Caulk and Perimeter Seal

Cracked, missing or separated sealant around exterior trim can allow wind-driven rain into joints that were meant to stay protected. Do not simply cover every gap with more caulk—some window systems rely on intentional drainage paths that must remain open.

2. Look Above the Window, Not Just Beside It

Water can enter higher on the wall and travel behind cladding before appearing at the sill. Head flashing and the way the window integrates with the wall’s water-resistive barrier matter.

3. Inspect the Sill and Lower Corners

The bottom of a window opening should manage incidental water and direct it back outside. Damaged or poorly integrated sill flashing can allow moisture to move toward the interior.

4. Make Sure Weep or Drainage Openings Are Clear

Many window assemblies include drainage paths. Paint, debris or sealant placed in the wrong location can interfere with drainage. Follow the window manufacturer’s instructions before cleaning or altering any openings.

5. Separate Rain Leaks From Condensation

If water appears only during rain, especially on one side or at one corner, exterior water entry becomes more likely. If moisture forms across the glass during cold weather even without rain, indoor humidity and surface condensation may be the bigger factor.

6. Check Interior Trim for Hidden Damage

Soft wood, peeling paint, staining, swollen trim or musty odors can indicate that moisture has been present longer than the visible puddle suggests.

Our Practical Take

Take photos during the next rain event before wiping everything dry. Where the first drop appears—and whether wind direction changes the leak—can give a window or building-envelope professional useful diagnostic clues.

Related Reading

Compare this with our window flashing guide, drafty-window checklist, and more Windows resources.

Source

U.S. Department of Energy Building America Solution Center — Flashing Windows