· by Michael Blair, Nashville's Window Cleaning
Most windows installed in Middle Tennessee over the last few decades are insulated glass: two panes held apart by a spacer around the edge, sealed, with a desiccant inside the spacer to keep the air between them dry. When that seal fails, humid air gets in, the desiccant fills up, and moisture condenses between the panes on cool mornings. Over time it leaves a milky haze that stays even when the glass is dry. It is one of the most common things people ask a window cleaner to fix, and one of the few that cleaning cannot touch.
Which side of the glass the water is on
Late September and October bring the first nights cool enough to put condensation on glass, and it is easy to lump every foggy window together. Wipe the inside surface with a dry cloth. If it clears, the moisture was on the room side. Go outside and do the same on the exterior face. If the fog is still there after both, it is between the panes.
A failed unit often shows a pattern: droplets or a white bloom along the bottom edge, a haze that comes and goes with the temperature, or a faint rainbow film. On a sunny afternoon it may disappear entirely and then return the next cold morning, which is why people sometimes think it is fixed.
Why the seal gives out
The edge seal is flexible, and it goes through a heating and cooling cycle every day. South and west-facing glass in this climate cycles hardest, and those are usually the first units to fail. Water sitting against the bottom edge of the sash, from sprinkler overspray, a blocked weep hole or a sill that slopes the wrong way, breaks the seal down faster.
Pressure washing at close range can drive water past the window frame and stress the seal as well, which is one reason our crews clean glass by hand or with a water-fed pole rather than a pressure washer. Age is the rest of it. A seal that has lasted a long time in a Nashville summer has done its job.
Replacing the glass, not the window
In most windows the sealed glass unit can be replaced on its own, leaving the frame and the sash in place. A glass shop measures the unit, orders a new one to size, and swaps it in. On some vinyl and clad windows it is easier to replace the whole sash, which is still far less work than a new window. Check the original manufacturer’s warranty first, because many cover seal failure for longer than the rest of the window and name the conditions in the paperwork.
Defogging services drill small holes in the glass, dry the space, and seal the holes with a vent. They can clear the view on some units, but the gas fill and the original seal do not come back, and results vary. The honest answer is that a failed seal is repaired by new glass.
- Which window and which pane, from outside
- The glass size, measured to the frame
- Any mark etched in a corner of the glass, which can show the maker and whether it is tempered
- Any printing on the metal spacer between the panes
- Roughly how old the windows are and who installed them
Morning condensation on the room side
Moisture on the inside surface is a different problem. It means the air in the room is humid and the glass is cold. An occasional fogged window on the first chilly morning is normal. Windows that stream every morning, with water pooling on the sills, point to too much humidity in the house, from a damp crawl space, unvented bathrooms, drying laundry indoors or a humidifier set too high.
That water sits in the corners of the sash and feeds mold on the frame and rot on wood sills. A window cleaning takes the film off, but the fix is lowering the indoor humidity: running bathroom fans, venting the dryer outside, and dealing with the moisture under the house.