Technical guide
Why double glazed units mist up
A double glazed unit mists when the edge seal fails and moist air enters the cavity. The desiccant in the spacer bar saturates, and condensation forms between the panes. The unit cannot be resealed and must be replaced.
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How the seal fails
The primary butyl seal and the secondary polysulphide or silicone seal degrade under thermal cycling, UV exposure and, most damagingly, standing water in a blocked frame drainage channel.
Why 'defogging' is not a repair
Drilling and drying clears the visible mist for a period, but the cavity remains unsealed, the argon has gone and the thermal performance is permanently reduced. The misting returns.
Units or whole windows
Replace units where the frames are sound and square and the hardware works — it is typically a quarter to a third of the cost of new windows with no making good. Replace windows where the frames are distorted or the thermal break has failed.
Preventing recurrence
Clear the drainage slots at the bottom of the frame. Units sitting in standing water fail far sooner than any other cause, and the fix costs nothing.
Upgrade at the same time
A unit swap is the cheapest chance to add acoustic laminate, solar control or toughened glass where a critical location was previously glazed in annealed float.
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