Double glazed unit replacement

Misting means the seal has failed — it cannot be fixed

Once moisture is inside the cavity, the desiccant in the spacer bar is saturated and the unit is finished. Drilling and 'defogging' clears the visible mist for a while but leaves an unsealed cavity that will mist again and has lost its gas fill and much of its thermal performance.

The honest answer is replacement of the unit. The frames, hardware and seals normally stay, which is why this is a fraction of the cost of new windows.

Replace the unit or replace the window?

If the frames are sound, square and the hardware works, replacing the units is far better value — typically a quarter to a third of the cost of full replacement, with no making good and no disruption to the fabric.

Replace the whole window when the frames are rotten or distorted, when the thermal break has failed, when the opening lights no longer seal, or when the required U-value cannot be reached with a glass change alone.

Upgrade while the frame is open

A unit swap is the cheapest opportunity you will get to change the glass performance. For a modest uplift you can move to acoustic laminate on a road-facing elevation, solar control on a south face that overheats, or toughened where a Part K critical location was previously glazed in annealed float.

We survey for all three as a matter of course and flag them rather than quietly replacing like for like.