Fix a basement window that leaks

Steps

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

• 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.
• Good: You know rain vs condensation between panes vs interior condensation.
• Bad: Unclear—wait for rain or run a hose to observe; for condensation, look closely at where the moisture sits.

Window well path

Goal: Fix drainage when water pools in the window well during rain.

• Inspect the window well from outside. If water pools during or after rain, the well is flooding—fix drainage before resealing the window.
• Check whether the well has a cover. A missing or broken cover lets rain and debris in. Add or replace the cover.
• Confirm that downspouts do not empty into or near the well. Downspouts should direct water at least 4–6 feet from the foundation. See fix-downspout-overflows.
• If the ground slopes toward the well, add soil to create a slope away from the house (at least 1 inch per foot for 5–10 feet).
• For persistent pooling, a drain at the bottom of the well may be needed—call a contractor if you are not comfortable with excavation.
• Good: Water drains away from the well. The window stays dry after rain.
• Bad: Water still pools—call a contractor for drainage work.

Window seal path

Goal: Find where rain enters the window and fix the seal when the well is dry.

Inspect exterior caulk and flashing

• Examine the caulk where the window frame meets the foundation or siding. 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.
• Good: You see gaps in caulk or damaged flashing.
• Bad: 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 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 contractor or window professional if:

• The leak comes from foundation cracks.
• The flashing or frame is structurally damaged.
• You need to regrade or install a drain at the window well.
• 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 basement that floods, Fix a window that leaks, Fix a window seal that leaks, Fix a downspout that overflows, or Fix a water stain on the ceiling.

Verification

• Rain leak — window well: No water pools in the well after rain; cover is in place; downspouts direct water away.
• Rain leak — window seal: 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.