How to tell if your double glazing has failed, in five simple tests
By Glazing London,
Double glazing rarely fails with a bang. It fails quietly: a faint haze in one corner, a pane that never seems to clear, a window seat that is always a little colder than the rest of the room. These five checks take ten minutes and tell you whether the sealed unit has gone, or whether the problem is something cheaper.
The short answer
If you see mist, droplets or a white film between the two panes, and it does not wipe off from either side, the unit has failed. That is the only certain sign. Everything else below helps you catch a unit that is on its way, or tells you that the problem is actually condensation on the room side, a worn seal or a frame that needs adjusting.
Test 1: the wipe test
Wait for a cold morning, when moisture shows best. Wipe the inside face of the glass with a dry cloth, then the outside face.
- It wipes off inside: room condensation. Your unit is fine, the room is damp. Our window condensation guide explains what to do.
- It wipes off outside: dew on a well insulated pane, a good sign. More in condensation on the outside of windows.
- It does not wipe off from either side: the moisture is trapped between the panes. The seal has gone.
Test 2: the lighter reflection test
Hold a lighter or a phone torch close to the glass at night and look at the reflections. A working double glazed unit shows two flames, one for each pane. If the flames have a slightly different colour, one pane has a low-e coating, which is normal.
This test does not prove a unit has failed, but it tells you what you actually have: single glazing shows one flame, and a triple glazed unit shows three. It is worth doing before you order anything.
Test 3: the hand test for cold spots
On a cold day, put the back of your hand flat on the glass of each window in the room, one after another. Healthy units feel cool but even. A unit that has lost its gas fill and let damp air in often feels noticeably colder than its neighbours, especially along the bottom edge.
One window markedly colder than the others in the same wall is a good reason to look closely at it with Test 1.
Test 4: the rainbow and white-line check
Stand to one side of the window and look along the glass with daylight behind it. Look for:
- A rainbow or oily film in the middle of the pane. This can be the panes bowing towards each other as the unit loses pressure.
- A white or grey tide mark a few centimetres in from the spacer bar. This is left by moisture that has dried out between the panes, and it means mist will be back.
- A dark or crumbling line along the edge seal, visible on some older units where the bead has been removed.
Any of these on a unit that is ten years old or more points to a seal that is failing.
Test 5: the paper draught test
Close the window on a strip of paper and try to pull it out. Repeat at several points round the sash. If it slides out easily, the problem is not the glass but the gasket or the hinges: the window is not pulling tight against its seals. That is a cheaper fix than a new unit, and our guide to fixing a stiff or draughty uPVC window walks through it.
What the results mean
| What you found | What it means | What to do |
|---|---|---|
| Mist that will not wipe off either side | The sealed unit has failed | Replace the unit, keep the frame |
| Tide marks or rainbow film | The seal is failing | Plan a replacement before winter |
| One pane far colder than the rest | Gas fill lost, possibly damp inside | Check again with the wipe test |
| Paper slides out when shut | Worn gasket or hinges out of adjustment | Adjust or reseal, glass is fine |
| Water on the room side | Room humidity, not the unit | Ventilate, see the condensation guide |
How to send us what you found
The quickest way to a fixed price is a photo. Stand back and take one of the whole window, then a close-up of the problem, then one of the small print stamped on the metal spacer bar between the panes if you can see it. Measure the visible glass, width first. Send the photos on WhatsApp and we reply with a price for a new sealed unit made to fit your frame.
Worth knowing: most replacement units are made to order. While the new one is being made, the old unit stays in place and keeps the window secure, so there is no gap and nothing to board up.
If the tests point to a failed unit, our misted double glazing repair service replaces just the glass across Central and North London. If more than a few units have gone and the frames are tired too, read how long double glazing lasts before you decide between new units and new windows.
Failed double glazing, answered
What does blown double glazing mean?
A blown unit is a sealed double glazed unit whose edge seal has broken. Damp air gets into the gap between the two panes, the desiccant in the spacer bar fills up, and the unit starts to mist on the inside. Once it has blown, it no longer insulates as it should.
Can you fix misted double glazing without replacing the glass?
Drill-and-dry services clear the mist for a while, but they do not remake the factory seal, so the moisture usually comes back and the gas fill is already gone. Replacing the sealed unit in the existing frame is the repair that lasts. See our misted double glazing repair page.
Is failed double glazing dangerous?
No. A blown unit is not a safety problem, the glass is still whole. It costs you heat, light and a clear view, and in a bedroom it can add to condensation and mould. It is worth replacing, but it is not an emergency.
Do I need a new window if one pane has misted?
Almost never. If the frame still opens, closes and locks properly, only the sealed unit is replaced. It is made to the exact size of the old one and fitted in the same frame, usually in under an hour per window.