It starts as a faint haze in one corner of the glass. A few months later there's a permanent fog between the panes that no amount of cleaning will shift, because the moisture is inside the sealed unit, where your cloth can't reach. We'd put misted double glazing among the most common window faults in Greater Manchester, and comfortably the most misunderstood. It almost never means you need a new window.
So why do units mist up, what does it say about the window's health, and what are your options? Let's take it from the cheapest fix upwards.
Why double glazing mists up
A double-glazed unit is two panes of glass sealed around a spacer bar, with the cavity between them filled with air or argon. Inside that spacer bar sits a desiccant, tiny beads whose only job is soaking up any trace of moisture so the cavity stays bone dry.
Years of sunlight, temperature swings and frame movement slowly fatigue the perimeter seal. Once it cracks, humid air gets drawn into the cavity every time the temperature drops, a process called thermal pumping. The desiccant drinks up that moisture until it's saturated. Then one cold morning the water vapour has nowhere left to go and it condenses on the glass. That's your mist, and by the time you can see it the seal has usually been failing for months.
Does misting mean my windows are finished?
No, and this is exactly where homeowners get oversold. The sealed unit is a replaceable component that sits inside your frame, held in place by beads. If the frame is sound and square, a glazier can lift out the failed unit and fit a brand-new one, same day for standard sizes, without touching the frame, the plaster or your décor.
- Frame in good condition, one or two misted units: replace the glass only. In most homes under 20 years old, that's the right call.
- Frames discoloured, warped or draughty, multiple units failing: the seals failed because the frames are moving, and new glass in a failing frame buys a few years at most. Replacement is better value. Our guide to signs you need new windows will help you judge.
What misted unit replacement costs
| Job | Typical cost (supplied & fitted) | Timescale |
|---|---|---|
| Standard casement unit (up to 1m²) | £80-£150 | Same week |
| Large picture window unit | £150-£300 | 1-2 weeks (made to measure) |
| Obscure/patterned bathroom glass | £100-£180 | 1-2 weeks |
| Whole new window (for comparison) | £450-£900 | 2-3 weeks |
We manufacture sealed units at our own Trafford Park factory, which tends to make us quicker and keener on price than firms who order glass in. It also means we can match obscure patterns, Georgian bars, and acoustic or solar-control specs. Our glass replacement service page has the detail.
Can misted glass be repaired without replacement?
You may have come across drill-and-dry services, which bore a small hole, flush the cavity and fit a valve. Does it work? Temporarily, sometimes. The visible fog can clear, but nothing restores the argon fill or the desiccant, so the unit's insulation never recovers and the misting tends to come back. A new unit costs little more, so we don't recommend the drill route. Nor do most glazing trade bodies.
Upgrade while you're at it
There's a silver lining here. A failed unit is a free chance to upgrade the glass without paying for new frames. Swap a 1990s clear unit for a modern low-E, argon-filled one with a warm-edge spacer and heat loss through that pane drops by 30-40%, the same thinking behind our guide on how new glazing cuts energy bills. You can also step up to acoustic glass for road-facing rooms, or toughened safety glass where regulations require it.
Making seals last longer
- Keep the drainage slots at the bottom of frames clear, so water never pools against the unit edge.
- Re-seal any cracked or shrunken silicone where the glass beads meet the frame.
- Sort dropped hinges promptly. A sash that slams or flexes puts stress straight into the unit seal.
- Book the occasional service visit; our repair and maintenance team checks seals, hardware and drainage in a single call.
Misted windows when you're selling a house
Misted units have a habit of surfacing at the worst possible moment, which is the buyer's survey. Surveyors flag failed sealed units as a matter of routine, and buyers use them to negotiate, often asking for far more off the price than the repair would ever cost, on the logic that 'the windows are failing'. Planning to sell within a year or two? Replacing misted units beforehand is about the cheapest pre-sale maintenance going. A few hundred pounds of glass can defuse a four-figure renegotiation, and the house photographs better without fogged panes.
Landlords should think along the same lines. Misted units drag down EPC performance, generate tenant complaints every winter, and replacing them counts as straightforward deductible maintenance.
Why buying from the manufacturer matters
Most glazing firms order replacement units from a trade counter, add their margin, and fit them whenever the glass turns up. Prime Fabrication Windows, Doors & Glazing makes sealed units at our own Trafford Park factory, and the shorter chain works in your favour at every step. Measurement to fitting in days rather than weeks. Direct control over the spec, from low-E coating and argon fill to warm-edge spacers, acoustic and patterned glass. Keener pricing with no middle margin, and one company answerable if anything is ever wrong. It's also why we can match awkward sizes, Georgian bars and obscure patterns that stump firms working from stock sheets.
Frequently asked questions
Why is there condensation between my double glazing panes?
The perimeter seal of the sealed unit has failed, letting moist air into the cavity, and the desiccant inside the spacer bar is saturated. It's a fault in the glass unit rather than anything cleaning can reach. The unit needs replacing.
Can I claim misted windows on my guarantee?
Quite often, yes. Sealed units typically carry 5-10 year guarantees, so dig out your installation paperwork. All Prime Fabrication Windows, Doors & Glazing units come with a 10-year guarantee that covers seal failure.
How much does it cost to fix misted double glazing?
Replacing a standard misted unit costs £80-£150 fitted, a fraction of a new window. Large or specialist glass costs more. We'll quote from a WhatsApp photo before anyone visits.
How long does a replacement sealed unit last?
A quality unit with a warm-edge spacer, properly fitted and drained, should give you 15-25 years. Cheap units in poorly drained frames can fail inside 5, which is one reason the lowest quote is rarely the cheapest.
Is misted glass an emergency?
Not urgent in the way broken glass is, but it is a live insulation failure. The cavity has lost its argon and that pane leaks noticeably more heat. Cracked or smashed glass, on the other hand, is a same-day job for our 24/7 emergency glazing team.
Misted unit? Send us a photo
WhatsApp us a picture of the window with rough sizes and we'll price a like-for-like or upgraded replacement unit, made in our own Manchester factory and normally fitted within the week.
Call 07533 830717, WhatsApp us a photo for a fast quote, or request a free quote online.