Misted double glazing: fix the unit, not the window

Where is the moisture, exactly

Glass has three faces to check: the room side, the outside, and the gap in between. Moisture you can wipe off from indoors is condensation, a ventilation story rather than a glazing one. Dew on the outside face on cold mornings means the glass is keeping heat in so well that the outer pane stays cold; file under compliment.

Mist you cannot reach from either side, trapped between the panes, is the one this guide is about.

What actually failed

A double-glazed unit is a sandwich: two panes, a spacer bar packed with desiccant, and a sealed edge holding dry air or argon in the gap. Years of sun and rain flex that seal until damp air starts creeping in, and once the desiccant is saturated, moisture shows up on the glass inside the cavity. Passing mist arrives first; a permanent tide mark means the desiccant gave up some time ago.

Replace the unit, not the window

Any decent glazier can measure the old unit, order a match and swap it, often in under an hour per window, with no plastering, no redecorating and no scaffold. The frame, handles and locks you already own carry on as before.

Failed units rarely fail alone, because every window on the same elevation has weathered the same seasons, and south-facing ones go first. Have the glazier check the lot while the van is outside; a batch of units in one visit prices far better than three separate call-outs.

The drill-and-dry question

Demisting services drill into the unit, dry or flush the cavity, then fit small vents or plugs. The mist goes and the window looks right again, which is real value if you are tidying a house for sale. What the treatment cannot do is reseal the unit or refill the argon, so the insulation stays compromised and the fog often returns in time. Cosmetic surgery at a cosmetic price, not a repair; weigh it against the cost of a new unit before committing.

When a whole window is the right call

Sometimes the frame really is the problem: rotten timber, cracked uPVC welds, hinges and locks beyond saving, or single glazing with no unit to swap. Replacing whole windows is notifiable work under building regulations, which is usually handled by a FENSA or Certass registered installer, and the certificate matters when you sell. Units in doors and low-level glazing must be safety glass; a competent fitter specs that without being asked.

Whole-window upgrades also move your energy performance, which is where the EPC guide takes over. And while the glazier is at the window, it is a good moment for the wider fabric: stiff hinges and handles cost pennies to sort alongside, and the security walk-round covers the lock standards insurers ask about.