Updated
Misted Double Glazing
A cloudy haze or trapped water sits between the two panes, and cleaning the glass makes no difference.
Find a window fitter
What's happening?
A double glazed window is a sealed unit: two panes bonded to a spacer bar around the edge, with a desiccant inside the spacer that absorbs the small amount of moisture sealed in at the factory. When the edge seal fails, outside air carrying water vapour is drawn in and out of the cavity as the unit warms and cools. The desiccant saturates, and moisture then condenses on the inner faces of the glass where nobody can reach it. That is the haze, the streaks and the pooled water at the bottom of the pane. The frame is usually fine. It is the glass unit inside the frame that has failed, which is why a fitter measures and swaps the unit rather than replacing the whole window.
What usually causes it?
- Age-related breakdown of the polysulphide or hot-melt butyl edge seal, typically after 15 to 25 years
- Standing water in the frame drainage channels sitting against the unit edge because the slots are blocked
- South-facing units cycling through large daily temperature swings, which flexes the seal
- A unit fitted without setting blocks, so the glass bears directly on the frame and stresses the corners
- Physical distortion of the frame — a dropped sash or a bowed transom pulling on the sealed edge
- Solar-control or thick safety glass fitted into a rebate too shallow for it, leaving the seal proud of the drainage line
- Pressure-washing or steam-cleaning directed at the glass edge, which drives water past a tired seal
Is it dangerous?
No. A misted unit is a performance and appearance fault, not a safety one. The glass is still glazed into the frame and still safe to touch and to open. The real cost is thermal: once the cavity is wet, the insulating gas fill has gone, so the window loses much of the U-value it was fitted for and the glass runs colder — which is why misted units often become the coldest surface in the room and start collecting room condensation as well.
Can I fix it myself?
You cannot repair a failed seal. Drilling the unit and fitting vents is sold as a cure but it converts a sealed unit into a vented one: the haze may clear, the insulating cavity does not come back, and the drilled unit no longer performs as the sealed unit it replaced. What is worth doing yourself is clearing the drainage slots along the bottom of the outer frame with a thin sliver of plastic, so water is not left standing against the seal of the units that have not failed yet.
When should you call a window fitter?
Call a window fitter when you want the misting gone. They measure the unit in situ, order the replacement to size and specification, and refit it into your existing frame. Worth calling sooner if the window is under a manufacturer or installer guarantee, as sealed-unit failure is the most commonly covered claim, and sooner again if the frame itself is moving — replacing a unit into a distorted frame just fails the new one.
What will the window fitter actually do?
- Measure the sealed unit's width, height and overall thickness, and record the glass make-up (toughened, laminated, low-emissivity, spacer colour)
- Check whether the location is a critical location under Approved Document K, so the replacement is ordered as safety glass where required
- Remove the glazing beads or drained-and-vented gaskets and lift the failed unit out
- Clear and re-form the drainage path in the frame, and fit new setting and location blocks so the new unit is not bearing on the frame
- Glaze the new unit in, re-bead it, and confirm the sash still closes onto its seal without binding
- Advise which of the remaining units on the same elevation are showing early seal failure
Frequently asked questions
We’re happy to answer all your questions.
Need a window fitter?
Compare local window fitters on Tradehand — coverage areas, accreditations and phone numbers, no middleman.