Misted Double Glazing: Repair the Unit or the Window?

A double-glazed unit is two panes of glass bonded around a spacer bar, with the gap between them filled with dry air or gas and a small amount of desiccant packed into the spacer to mop up any trace moisture. An edge seal around the perimeter is what keeps the whole thing airtight.

That seal does not last forever. Years of sun, frost and the glass expanding and contracting with the weather gradually work it loose. Once it lets go, humid air from outside finds its way in, the desiccant becomes saturated, and condensation starts forming on the inside faces of the glass — the one place you can never reach with a cloth.

This is the point worth being clear about: a misted sealed unit is a failed unit, not a failed frame. The seal between the two panes of glass has broken down and let moisture in, and that is a glass problem, contained entirely within the unit itself. It says nothing about the condition of the timber frame holding it.

We still check the frame properly before recommending anything, because the two issues can coincide in an older window. But a sound frame around a misted unit is extremely common, and treating the glass and the frame as separate questions is what keeps the repair proportionate to the actual fault.

Where the frame is sound, the fix is to replace the sealed unit itself and leave the frame exactly as it is. We take accurate measurements and the glass specification, confirm the replacement unit with a specialist glazing partner, and re-fit it into the existing timber once it arrives. The beading comes off, the failed unit comes out, the new one goes in, and the beading goes back.

Nothing about your existing joinery needs to change for this to work. If the frame is painted, moulded to match the rest of the house, or simply a profile you do not want to lose, a glass-only swap keeps all of that intact while sorting the actual fault.

A different problem altogether is a frame that has deteriorated in its own right. Rot in the timber, joints that have opened up, a sash or casement that has warped out of square, or hardware that no longer seats and seals properly — these are frame failures, and no amount of new glass fixes them. Swapping the unit into a frame that is already failing just delays a job that needs doing properly.

In that situation the honest advice is a new window, not a repaired one. It is also, practically speaking, a different trade of work: a frame replacement involves removing and refitting the whole opening, not just the glass within it.

This split between glass and frame also matters for Building Regulations. Replacing a sealed unit within an existing frame is not notifiable building work — the frame and the opening it sits in are unchanged, so the thermal envelope of the house is not being altered in a way the regulations track.

Replacing a whole window is a different matter. That is notifiable work, which is why window installation is normally handled by an installer registered with a competent person scheme such as FENSA or Certass, who self-certify the work as compliant on completion. If your window needs that level of work, we will say so and point you towards a registered specialist for it.

One thing worth checking before any replacement glass is ordered is whether the opening counts as a critical location under Approved Document K — broadly, glazing near doors and at low level, where someone could fall against it. In those locations, safety glass such as toughened glass to BS EN 12150 is typically the expected specification.

We raise this at the measuring visit rather than assume the existing glass was specified correctly when the window was first fitted, particularly in older properties. It is a specification check, not a certification we hold ourselves — the glass itself is sourced and manufactured through a specialist glazing partner to whatever standard the location requires.

To be clear about where each part of the job sits: sealed units and glass come through a specialist glazing partner, made up to your window's exact sizing and specification. Our work is the carpentry either side of that — surveying the opening accurately, removing the failed unit without damaging the surrounding frame, and re-fitting the new glass cleanly so the beading and finish look original.

We repair and re-glaze existing frames. We do not replace whole windows. If a survey shows the frame itself has failed, we will tell you directly and refer you to a FENSA or Certass registered window installer for that part of the work, rather than take on a job outside what we do.

A misting unit is easiest to deal with before it gets worse. Early signs include a faint haze that only shows at certain light angles or times of day, a slight blurring towards the edges of the pane, or condensation that appears between the panes on cold mornings and does not clear once the room warms up. Left alone, the haze typically spreads and the trapped moisture can leave a mineral residue on the inside faces of the glass that no amount of cleaning will shift.

None of this is urgent in the way a draught or a forced door is, but it is worth getting measured and quoted before the unit fully clouds over. Catching it early also makes it easier to confirm the frame is still in good order while there is still clear glass to see through.