Direct answer: A leaking window is often caused by one of three different problems: interior condensation that only looks like a leak, water entering through the sash or frame because drainage or weatherstripping has failed, or water getting in around the opening from flashing or exterior details above the window.
Most likely: The most common homeowner-level causes are condensation on the glass or frame, clogged window weep holes, worn window weatherstripping, or a sash that is not closing and locking tightly.
Start by identifying when the water appears and exactly where it first shows up. Water on the sill during cold weather may be condensation. Water that appears only during wind-driven rain points more toward the sash, frame, or exterior opening details. The goal is to separate those branches before you buy parts or seal anything.
Don’t start with: Do not start by smearing caulk around everything you can reach. Blind caulking can trap water, hide the real path, and make later repairs harder.
Moisture forms on the inside surface, often in the morning or during cold weather, and may drip onto the sill.
Start here: Start with the condensation branch. Check indoor humidity, airflow, and whether the moisture is forming on the room side of the glass.
The sill, side jamb, or drywall gets wet during storms, especially with wind.
Start here: Start by finding the highest or earliest wet point. That helps separate a sash or drainage issue from water entering around the opening.
You see standing water in the lower channel of a sliding or hung window, sometimes spilling indoors.
Start here: Check the window weep holes and drainage path first before assuming the whole frame is leaking.
Drywall, trim, or paint near the window shows repeated moisture damage, but the active leak is hard to catch.
Start here: Look for the source path, not just the stain. Water may be entering above or beside the window opening and showing up lower inside.
If moisture forms on the room side of the glass or frame during cold weather, high indoor humidity and cool window surfaces can create drips that look like leakage.
Quick check: Wipe the moisture dry and watch whether it reforms evenly on the inside surface without rain.
Many windows are designed to collect small amounts of water and drain it back outside. If the drainage path is blocked, water can back up into the track and spill indoors.
Quick check: Inspect the lower track and exterior weep openings for dirt, paint, insect nests, or debris.
If the sash does not pull snugly against the frame, wind-driven rain can pass the seal and reach the interior.
Quick check: Close and lock the window, then check for visible gaps, flattened weatherstripping, or a latch that does not pull the sash in firmly.
If water shows up at the top of the trim, upper corners, or wall below the window, the source may be flashing, siding, or another exterior path rather than the operable window parts.
Quick check: During or right after rain, look for the highest wet area around the opening and whether the sash itself stays dry while the surrounding wall gets wet.
These two problems look similar but are fixed very differently. Starting here prevents unnecessary sealing and part replacement.
Next move: If the moisture only forms on interior surfaces and improves with lower humidity and better airflow, you are likely dealing with condensation rather than rain intrusion. If water appears during rain, especially at one corner, in the track, or from behind trim, continue to the next steps.
What to conclude: Condensation points to indoor humidity and surface temperature. Rain-timed water points to a drainage, seal, or opening-path problem.
The first wet point is more useful than the final stain. It helps separate a window assembly issue from water entering around the opening.
Next move: If the first wet point is in the bottom track, focus on drainage. If it is at the sash edge, focus on weatherstripping and closure. If it is above or behind trim, suspect the opening details rather than the window parts. If you cannot identify the first wet point, continue with the simple window checks before considering exterior opening problems.
What to conclude: Bottom-track water usually means drainage backup. Water at the sash edge suggests a seal problem. Water above the frame or behind trim often means the source is outside the window assembly.
Blocked drainage is a common, low-risk cause, especially when water collects in the lower channel before spilling inside.
Repair guide: How to Clean Window Weep Holes
A window that does not close squarely or seal tightly can let wind-driven rain pass the sash even when the frame itself is intact.
Next move: If re-closing the sash or replacing obviously damaged weatherstripping restores a tight seal and the leak stops in later rain, the problem was likely at the sash seal. If the sash seals well but water still appears from above, behind trim, or at the wall, the source is likely around the opening rather than the operable window parts.
Once drainage and sash-seal checks are ruled out, the remaining branch is often water entering around the opening from flashing, siding, or another exterior path. That is important to identify before cosmetic repairs.
A good result: If the evidence points away from the sash and toward the surrounding opening, you have narrowed the problem enough to avoid buying the wrong window parts.
If not: If you still cannot tell whether the source is the window or the wall, a pro water-intrusion inspection is the safest next step.
What to conclude: This branch usually means the repair may involve exterior detailing rather than a simple window part. The right fix depends on tracing the source path accurately.
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If moisture forms on the room side of the glass or frame during cold or humid conditions, it is often condensation. A true leak is more likely when water appears only during rain, especially wind-driven rain, or when it shows up from behind trim or at one specific corner.
Not until you know where the water starts. Blind caulking can trap water, block drainage, and hide the real source. First determine whether the problem is condensation, blocked weep holes, a poor sash seal, or water entering around the opening.
Some windows are designed to collect small amounts of water in the lower channel and drain it outside through weep holes. If those openings are clogged, water can back up and spill indoors. Clean the track and confirm the drainage path before assuming the whole window is bad.
Yes. If the sash does not seal tightly because the window weatherstripping is torn, flattened, missing, or hardened, wind-driven rain can pass the sash. This is more likely when the leak happens at the sash edge rather than from above the frame or behind trim.
Call for help if the source appears to be around the opening rather than the sash, if there is soft drywall or trim, if the frame is loose or out of square, if the leak is recurring and hard to trace, or if safe inspection would require high ladder or roof work.