If your double-pane window looks foggy, no cleaner on earth will fix it

Double-pane windows are insulated glass units: two panes separated by a sealed gap, often filled with argon gas, with a desiccant-packed spacer around the edge to absorb the trace moisture sealed inside. The design works well for years, but the glass expands in the heat and contracts at night, and that daily cycle slowly works the perimeter seal. When it starts to fail, humid outside air infiltrates the gap.

Once the desiccant is saturated, moisture condenses directly on the cold inner faces of the glass — and that's the fog you see. Because it's between the panes, not on them, no squeegee, cleaner, or cleaning service can reach it. The only real fix is replacing the insulated glass unit or the whole window. Cleaning the outside more vigorously won't change anything.