Why Are My Windows Misting Up?

Condensation on the inside face of the inner pane is a room-humidity issue — you can wipe it off. Misting between the two panes cannot be wiped off because it's inside the sealed unit. If cleaning both faces of the glass doesn't clear it, the sealed unit has failed and the mist is inside.

Sealed units are two panes of glass separated by a spacer bar and bonded around the edge with butyl and silicone sealant. When the primary or secondary seal fails (usually after 15–25 years), moist air enters the sealed cavity. The desiccant inside the spacer absorbs it until it's saturated, then moisture starts condensing on the inside faces — that's the mist you see.

The short answer is no. 'Defogging' services drill the unit, dry it out and reseal — a temporary fix that usually fails again within 2–3 years and voids any manufacturer warranty. A replacement sealed unit is the proper repair and lasts 20+ years.

You don't need to replace the whole window. A misted sealed unit can be swapped for a new one while the frame stays in place — usually a 30-minute job per window. Costs typically £80–£200 per unit depending on size and glass specification. Take the opportunity to upgrade to warm-edge spacers and low-E glass at the same time.

A modern replacement sealed unit with warm-edge spacers and quality sealant should last 20–25 years. If a unit fails within 10 years of installation, it's a manufacturing defect and should be covered by a FENSA-registered installer's insurance-backed guarantee.