Fix a window that leaks

Steps

Goal: Confirm whether the leak is from rain or condensation, then apply the correct fix.

Confirm the source

• Check where the water appears: sill, frame, glass, or between the panes.
• If water runs down the glass or pools on the sill during or after rain, it is a rain leak.
• If moisture is trapped between the two panes of glass (foggy, cannot wipe off), the sealed unit has failed.
• If water beads on the interior surface of the glass and you can wipe it off, it is interior condensation from humidity.
• Symptom confirmed: You know rain vs condensation between panes vs interior condensation.
• No problem: Unclear—wait for rain or run a hose to observe; for condensation, look closely at where the moisture sits.

Rain leak

Goal: Find where rain enters and fix the seal.

Inspect exterior caulk and flashing

• Examine the caulk where the window frame meets the siding or brick. Cracked, peeling, or missing caulk lets water in.
• Check the flashing above the window. Bent, loose, or missing flashing directs water behind the frame.
• If the window has weep holes at the bottom of the frame, check that they are not blocked.
• Symptom confirmed: You see gaps in caulk or damaged flashing.
• No problem: Caulk and flashing look intact—check weatherstripping next.

Replace weatherstripping

• Check the weatherstripping where the sash meets the frame. Worn, compressed, or missing strips let rain and drafts in.
• Remove the old strip and install new foam or vinyl weatherstripping that matches the channel. You should see a snug seal when the window is closed.

Recaulk the exterior

• Scrape out old caulk with a putty knife. Clean the surface with rubbing alcohol and let it dry.
• Apply a continuous bead of exterior-grade silicone or urethane caulk. Tool the bead with a wet finger or caulk tool for a smooth seal.
• Let the caulk cure per the product label. Test with a hose (low pressure) or wait for rain. You should see no water entering.

Condensation between panes

Goal: Recognize that the sealed unit has failed and must be replaced.

• Moisture between the two panes means the sealed unit seal is broken. Recaulking will not fix it.
• Order a replacement unit by size and thickness (check the spacer bar for dimensions). Replace the unit or the whole window.
• If the window is on an upper floor or you are not comfortable with the work, call a window professional. You should see the unit dimensions before ordering.

Interior condensation

Goal: Reduce humidity or improve insulation so the interior glass stays warmer.

• Water beading on the inside surface is caused by high humidity and cold glass. Improve ventilation (run a bathroom fan, open a window briefly), use a dehumidifier, or add storm windows.
• You should see less condensation after reducing humidity or improving insulation.

When to get help

Call a window professional if:

• The window is on an upper floor and requires a ladder.
• The flashing or frame is structurally damaged.
• You have recaulked and replaced weatherstripping but the leak continues.
• You need to replace a failed sealed unit and are not comfortable with the work.

For related fixes, see Fix a leaking faucet, Fix a drawer that sticks, Shut off the main water supply, or How to unclog a drain.

Verification

• Rain leak: No water enters during or after rain; exterior caulk is continuous with no gaps; weatherstripping seals the sash.
• Condensation between panes: Replacement unit is ordered or installed; no moisture between the panes.
• Interior condensation: Humidity is reduced or insulation improved; less or no condensation on the interior glass.