Why Is My Window Sill Rotting From Inside?

Condensation is a common culprit. Warm, humid indoor air meets the cold glass and frame, and water drips onto the sill. If the sill is painted or sealed on top but not underneath, that water soaks into the end grain and stays trapped against the framing.

A second path is a failing exterior seal. Rain can push past cracked caulk or worn weatherstripping, then run down the inside of the frame. The sill absorbs that water slowly, and rot starts from the hidden side, which is why the damage appears to come from inside.

• Check for dark streaks or soft spots along the sill's underside
• Look for peeling paint or bubbling finish on the sill surface
• Feel for dampness on the sill after a rainstorm
• Inspect the caulk line where the window meets the siding
• Notice if condensation forms heavily on the glass each morning
• Look for water stains on the wall below the window

Start with a careful visual check. Look at the exterior caulk, the flashing above the window, and the siding around the frame. Also check the interior for gaps where air or water could pass. A slow leak from a pipe or roof can travel along framing and reach the window sill.

If you cannot find the source, a moisture meter can help. The pros use these tools to trace dampness through the wall. You can also watch the sill during a heavy rain to see where water appears first.

• Run your hand along the sill's underside for dampness
• Check the window's weep holes for blockage
• Look for gaps in the interior caulk or trim
• Inspect the roof edge and gutter above the window
• Feel the wall around the window for cool or damp spots
• Note any musty odor near the window

Keep the exterior caulk and weatherstripping in good condition. Reapply sealant when it cracks or pulls away. Also make sure the window's weep holes stay clear so water can drain out instead of pooling on the sill.

Control indoor humidity, especially in cold weather. Use exhaust fans in kitchens and bathrooms, and consider a dehumidifier if you see constant condensation. If you live in Rogers, keep a note of where your main water shutoff valve is, in case a pipe leak is the cause.

Check the sill a few times a year, especially after heavy rain or big temperature swings. A quick feel for dampness and a look at the paint can catch problems before rot takes hold.