Misted double glazing, fog, droplets or a milky bloom trapped between the panes, is the most common glass fault in London homes, and the most over-sold. You'll be told you need new windows. You almost certainly don't. Here's what's actually happened inside the unit, and the cheapest lasting way out.
Quick answer
Misted (or "blown") double glazing means the sealed unit's edge seal has failed and its desiccant is saturated, so condensation between the panes cannot be wiped away or permanently dried out. The fix is to replace the glass unit, not the frame: a like-for-like or upgraded sealed unit refitted into your existing frames, in London usually within 3-7 days of measuring.
Why double glazed units fail: the seal and the desiccant
Every sealed unit is a factory-made sandwich, and it fails at the edges, not the glass. Three parts do all the work:
Two barriers of butyl and polysulphide/hot-melt bond the panes to the spacer and keep the cavity airtight.
The hollow frame between the panes, filled with desiccant beads that keep the cavity air bone dry.
Argon (usually) fills the cavity to slow heat transfer. Once the seal goes, it leaks away for good.
Every day, the sun warms the unit and the air inside expands; every night it cools and contracts. The unit "breathes" against its seal thousands of times a year. After 15-25 years, sooner on hot south-facing elevations, sooner still if the unit was poorly glazed and sat in trapped water, the seal micro-cracks. Each breath now pulls in a little humid London air. The desiccant absorbs it silently for months, then saturates. The next intake of moist air has nowhere to go, and the first mist appears, usually a faint fog in cold morning light, low in the unit.
From there it only worsens: droplets, tide-marks, and eventually a permanent milky bloom where trapped moisture has reacted with the inner glass surfaces. Wondering whether what you're seeing is this, or ordinary surface condensation? Our window condensation guide covers how to tell the three types apart in seconds.
Why DIY drilling and "condensation removal" are a false economy
Drilling holes in a failed unit to dry it out treats the symptom, keeps the disease, and destroys what's left of the unit's performance. The method (drill one or two small holes, flush or ventilate the cavity, fit one-way valves) is heavily marketed because it's cheap to do. Here's what it can't do:
- It can't fix the seal. The perimeter seal is still failed, so humid air keeps entering. The mist typically returns within months to a couple of years, now with drill holes in your glass.
- It can't restore the desiccant. The saturated beads stay saturated; the unit has no buffer left against the next damp spell.
- It can't put the argon back. A drilled unit is permanently vented to the outside air. The insulating gas fill, a good chunk of why you bought double glazing, is gone, and the U-value degrades towards that of a plain air gap.
- It voids any remaining guarantee on the unit, and a visibly drilled unit raises questions at survey when you sell.
The maths that matters
Drilling costs perhaps half the price of a new unit, and buys you a temporarily clearer pane with permanently worse insulation. A replacement sealed unit costs a little more once, restores full performance, and comes with a fresh guarantee. Paying twice for the worse outcome is the definition of false economy.
Replace the unit, not the frame
If your frames open, close and lock properly, only the glass needs to change, and the price difference is dramatic. Window companies quote whole new windows for misted units because that's what they sell. But the sealed unit is a serviceable component, designed to lift out of uPVC, aluminium and timber frames alike:
| Replace sealed unit only | Replace whole window | |
|---|---|---|
| Typical relative cost | ~25-35% of a new window | 100% |
| Time on site | 30-60 minutes per window | Half a day+ per window |
| Disruption | None, no plaster, trims or cills touched | Reveals, trims, sealant, making good |
| Building Regs / FENSA | Not required for like-for-like unit swap | Certification required |
| When it's right | Frames sound, seals & hardware working | Frames warped, rotten, or units failing repeatedly |
The honest exception: if several units in one elevation have failed and the frames are 30-year-old warped uPVC with failing gaskets and hardware, repeated glass swaps become good money after bad. That's when we'd show you the numbers for new windows instead. We fit both, so we have no incentive to steer you either way.
The frame is the suit; the unit is the shirt. You don't buy a new suit because the shirt wore out.
The upgrade opportunity: spacer bars and low-E glass
A failed unit is a chance to fit better glass than the original, for a small premium, since you're paying for manufacture and fitting anyway. A unit made today can beat one made in 2005 comfortably:
- Warm-edge spacer bars replace old aluminium spacers, cutting heat loss at the unit's edge and reducing that tell-tale line of condensation around the perimeter of the glass.
- Modern soft-coat low-E glass reflects room heat back inside far better than early coatings: a like-for-like-sized unit can drop from ~2.8 to ~1.2 W/m²K.
- Argon fill comes standard on our units, restoring the insulation your failed unit lost years ago.
- Acoustic or laminated glass can be specified for street-facing rooms: same frame, quieter and more secure glass.
- Toughened glass where regulations now require it (doors, low-level glazing). Older units often pre-date these rules.
Key takeaway
Never replace a failed unit with 2005-spec glass. For a few pounds more per unit, warm-edge spacers and modern low-E glass mean the "repair" leaves your window performing better than the day it was installed.
What we do about misted units in London
Our misted double glazing repair service is a measured, glass-only replacement, one short visit to measure, one to fit. The process, start to finish:
- Photo quote. WhatsApp a photo of the misted window to 07399 253464 and we'll give you a fixed price, usually within minutes. No call-out fee, ever.
- Measure. We survey the exact unit thickness, spacer size and glass spec (free across Central & North London), and check the frames and gaskets while we're there.
- Manufacture. Your units are made to measure, typically 3-7 days, with warm-edge spacers, low-E glass and argon as standard.
- Fit. Old units out, new units in, gaskets and beads re-seated, usually 30-60 minutes per window, no mess, guarantee in your hand before we leave.
Multiple misted units? We price per-unit so doing them in one visit costs less than piecemeal. And if a failed pane has cracked or fallen out entirely, our 24/7 emergency glazing team can make it safe the same day. Prefer to start with a proper look? Book a free survey and quote.
Misted double glazing FAQs
Can misted double glazing be repaired without replacing the glass?
Do I need a whole new window if my double glazing is misted?
Why has my double glazing misted up?
How much does it cost to replace a misted unit in London?
Can I leave a misted double glazed unit as it is?
Is misted double glazing covered by guarantee or insurance?
Misted unit? Send a photo, get a price.
Glass-only replacement across Central & North London, measured free, made in days, fitted in under an hour per window, guaranteed.