Spacer & desiccant (sealed-unit edge)

In a double-glazed sealed unit the two panes are held apart at the edge by a spacer bar, and that spacer is filled with a desiccant (a drying agent) that soaks up any moisture in the sealed air or gas gap. With the edge seal, they keep the cavity permanently dry and clear. When the seal degrades — UV, the UAE's big day-night heat cycling, and age all attack it — the desiccant eventually saturates and can no longer keep up, so moisture condenses between the panes and you get the permanent internal fog you can't wipe off. That's the reason foggy double glazing is fixed by replacing the whole sealed unit, not cleaning it: the spacer, desiccant and seal are one factory-sealed assembly you can't open and re-dry. A warm-edge spacer also cuts edge heat-transfer and condensation. Fitting is an aluminium and glazing specialist's job.