When the glass unit mists up, this means that the seal has failed and is allowing moisture inside the glass unit. Unfortunately you can’t just wipe this away as the moisture is inside the unit and can’t be accessed.

Contrary to popular belief, even the best double-glazed windows and doors aren’t designed to last forever. There simply isn’t the kind of technology available right now to produce affordable home windows and doors that are genuinely infallible. Of course, invest in quality products and you’ll get at least a decade or two out of them, but given the punishment inflicted by the Great British elements, you can’t expect them to last forever!