Why Is There Fog Between the Panes of My Dual Pane Windows
Milky fog that sits between two panes and will not wipe off from either side is the first sign of a dead edge seal. The notes kept at window replacement san bernardino civil war web on this problem come from sash and frame work in San Bernardino, CA, where a failed insulated glass unit can often be swapped without buying a whole new window. The fog is the visible end of a seal problem that started earlier.
What the fog is
An insulated glass unit is two sheets of glass held apart by a spacer around the perimeter. The spacer is hollow and filled with desiccant, which pulls moisture out of the trapped air. When the edge seal cracks or pulls away, humid air gets in and the desiccant slowly fills. Once it is saturated, the water has nowhere to go and condenses on the inner glass faces as haze, droplets or mineral streaks.
Why seals give out
The sealed air space expands in the afternoon and contracts overnight. Every cycle pushes and pulls on the sealant at the edge, and over years it fatigues. Strong direct sun on south and west walls raises the glass temperature further and widens the swing. In San Bernardino County the long hot stretch of summer puts a double pane unit on a west elevation through a hard test, so the first failures often show up on those walls.
Wiping does nothing
The film is inside the unit, so a cloth on either face changes nothing. Condensation on the room side surface is a separate matter. That moisture comes from indoor humidity meeting cold glass on a winter morning, and it wipes away. A sealed unit with a good seal can still sweat on the room side. Only moisture trapped between the panes points to the seal.
What defogging can and cannot do
Some services drill the glass, dry the space and fit a vent or a plug. That can clear the visible moisture. It does not restore the original gas fill or the factory seal, so the unit is no longer the sealed assembly it was. Fog also means the unit has stopped working as built, so treating it as only cosmetic misreads what the glass is doing.
Replace the unit, not always the window
When the frame is sound, a new insulated glass unit can often be installed in the existing sash or frame. Check the frame first. Probe the sill of a wood window for soft spots, look for warped corners on a vinyl window, and look for corrosion at the joints of an aluminum window. A double hung, slider or casement with a straight, tight sash usually takes a new unit cut to the measured opening. A sash that is rotted, bowed or racked is a different job, and then a full unit makes more sense. Measure the glass thickness and the overall unit size before ordering, since the new unit has to seat in the same rebate.