A wet sill after a rainstorm, or water sitting on the glass on a cold morning, sends most people to the same worried conclusion: the window is leaking and needs replacing. Sometimes that is true, but often it is not. “Leaking” windows have several very different causes, the fix depends entirely on which one you have, and some leaks are not leaks at all. Here is how to read the signs, tell a genuine leak from condensation, and match the right fix to the real problem before you spend money on the wrong one.
First, look at where and when the water shows up
The location and timing of the water tell you most of what you need to know. Water at the top or corners often points to failed flashing above the window; along the sides or perimeter suggests the exterior seal; at the bottom or sill can mean blocked drainage. Moisture trapped between the panes is a different problem entirely, and water on the inside surface of the glass is usually condensation rather than a leak. Timing helps too: water that appears during or right after rain is getting in from outside, while water that forms on cold mornings with no rain is almost always condensation. Pin those two things down first.
The common causes of a real leak
When water genuinely gets in from outside, it is usually one of these:
Failed exterior caulk or sealant. The bead around the outside of the frame cracks, shrinks or pulls away over time, opening a path for rain.
Damaged or missing flashing. The flashing that directs water away above and around the window sits behind the siding; if it was poorly installed or has failed, water gets in behind and shows up inside. This is often an installation issue.
Blocked weep holes. Sliding and casement frames have small drainage holes; when they clog with dirt, water backs up in the frame and spills over inside.
Worn weatherstripping or a cracked pane. Perished gaskets let water past the sash, and a crack in the glass is an obvious path for it.
A rotted or damaged frame or sill. Once the frame itself has broken down, water finds the gaps.
When it is not a leak at all
Two of the most common “leaks” are nothing of the sort, and naming them correctly saves a lot of wasted effort. The first is condensation on the inside of the glass: warm, humid indoor air meeting a cold pane, which beads into water. That is not water getting in, it is humidity, and the answers are lowering indoor humidity, improving ventilation, and fitting warmer glass such as Low-E or triple-pane that keeps the inner surface less cold. The second is moisture or fog trapped between the panes, which means the seal of the unit has failed, the failed-seal problem. That cannot be wiped away and is not a leak from outside; the sealed unit is replaced. Chasing either of these as a leak leads to the wrong repair.
How each one is fixed
Once you know the cause, the fix follows. A failed perimeter seal is re-caulked and resealed on the outside. Flashing problems usually need a proper repair or replacement and often a contractor, because the siding has to be addressed too. Blocked weep holes simply need clearing. Worn weatherstripping is replaced, a cracked pane is replaced, and a failed sealed unit is swapped out, frequently while keeping the existing frame. Condensation is managed by reducing indoor humidity and, where it is persistent, upgrading to warmer glass. And where the frame itself has rotted, replacing the window is the honest answer rather than patching around it.
When to call a professional
Some leaks are worth handing over rather than fighting. If a leak keeps returning after you have resealed it, if you suspect the flashing or the original installation, if a sealed unit has failed, or if water is showing up some distance from the window, where it may be travelling behind the wall, a proper diagnosis is the cheaper path in the end. It is the kind of thing the team sorts out as part of residential window work. Getting the diagnosis right the first time avoids paying for repairs that were never going to fix the real cause.
FAQs
Ready to get help with your glass issue?
Dealing with a window that leaks, fogs or will not stay sealed? Request a free quote for a proper diagnosis. The team handles residential window repair and replacement across Alberta. Call 310-GLAS (4527), text 780-652-1296, or book online.