Misted or Blown Double Glazing, Repair or Replace

What has gone wrong inside the glass

A sealed unit is two panes held apart by a spacer bar filled with a desiccant, with a perimeter seal keeping moisture out and argon in. Over years of heating and cooling, that seal flexes and eventually admits damp air. The desiccant absorbs it until it is saturated, and then the moisture appears as misting on the inside faces of the glass. Once that happens the unit is no longer insulating properly either, because the argon has gone with it.

This is why misting always appears on the shaded, weather facing side of the house first. The temperature swing across the seal there is the largest.

Does drilling and drying work

You will find companies who drill the pane, pump the moisture out and fit a small vent. It clears the view for a while. What it does not do is restore the seal or the gas fill, so the unit stays thermally compromised and the misting frequently returns within a season or two. We do not offer it, because we would rather not do a job twice.

When a glass only swap is right

If the frame is square, the hardware works, the seals are supple and the misting is limited to one or two units, replacing the sealed unit alone is the correct and economical answer. It takes under an hour per unit, the frame stays where it is and you get the full thermal performance back with a warm edge spacer and a new argon fill.

• Frame under roughly fifteen years old and structurally sound
• Handles and hinges operating smoothly
• Rubber gaskets still soft rather than hard and shiny
• One or two failed units rather than a whole elevation

When the window should be replaced

Once several units on the same elevation have gone, the frames are the same age and the same weather has been working on all of them. Add tired hardware and perished gaskets and the sensible money goes on the window rather than the glass. Anything with a rotten timber frame or discoloured brittle uPVC is a replacement regardless of how many units have misted.