Condensation in sealed units
After about ten years, condensation can form between the glass panels of sealed double glazing units. This is because the dry gas between the panes becomes contaminated with moist outside air and replacement is the only remedy.
Condesation in secondary units
If secondary double glazing units develop condensation, open them and wipe them with a chamois leather. Don’t insert silica gel crystals to absorb moisture, because they release the moisture again when the sun warms them.
Persistent condesation can often be reduced by drilling three or four holes from the outside into the space between the panes. This admits cold air , which holds very little moisture.