Misted double glazed window units are caused when the seal between the two glass panels get damaged, which can happen over time, this then allows the argon gas that fills the gap between the panels to escape, and as the gas can get out then so can air get in, which means that moisture can then also get it, which is what causes the condensation between your double glazing sealed window units.
If you would like to read more about condensation in windows, please
click here to download the Glass and Glazing Federation’s brochure