What causes a sealed unit to fail, and how do I spot it?

A sealed unit “fails” when the airtight seal around its perimeter breaks down, allowing moisture into the cavity between the panes. The tell-tale sign is misting or condensation trapped between the glass that you can’t wipe away — often worse in the mornings or in changeable weather — sometimes with a hazy film or visible water droplets on the inside faces of the panes.

Common causes include age-related breakdown of the perimeter seal, standing water sitting in the frame because drainage holes are blocked, units installed without proper packing or drainage, and prolonged exposure to heat and UV. Once a unit has misted, the seal can’t be reliably repaired — the fix is a replacement sealed unit, which usually doesn’t mean a new frame – see our comprehensive how to guide on how to change a sealed unit.