Quick answer
A blown double-glazed unit shows as misting or haze trapped between the panes, caused by a failed edge seal letting humid air in over time. It isn't a sign of a faulty door slab - only the glazing cassette needs replacing, and modern warm-edge spacer units resist this far better than older aluminium-spacer designs.
Overview
'Blown' means the sealed unit's edge seal has failed, letting moisture into the airspace between the panes. Once blown, it can't be fixed - the unit must be replaced.
Symptoms
- Permanent misting between the panes.
- Watermarks that don't wipe off.
- Reduced clarity.
Common causes
- Age (10-20 years typical).
- Heat exposure.
- Poor original manufacture.
What 'blown' actually means
A double-glazed unit is two panes of glass bonded around their perimeter by a spacer bar and a sealed adhesive joint, with a pocket of air or argon gas trapped between them for insulation. When that perimeter seal degrades and fails, humid outside air finds its way into the gap, and the moisture condenses on the cold inner glass surfaces exactly like a fogged-up window on a cold morning - except here it's trapped inside the unit and can't be wiped away.
This misting is the clearest sign of a blown unit, and once it starts it's permanent - the seal doesn't heal itself, and the misting typically gets worse over subsequent cold, damp spells rather than clearing on its own. Occasionally a mineral residue or salt-like tide mark forms on the inner glass surface over time as moisture evaporates and re-condenses repeatedly.
Why seals fail
Edge seals fail through a combination of UV exposure, thermal cycling as the unit heats and cools through the day, and simple age - most manufacturers' warranties on sealed units run to ten years or more, but real-world failure can happen earlier in unusually exposed positions. South and west-facing doors that get the most direct sun and the most driven rain tend to show blown units somewhat sooner than sheltered, north-facing installations.
Older units built with aluminium spacer bars are particularly prone to this because the metal conducts heat and cold straight to the seal edge, accelerating the expansion and contraction that eventually breaks the seal down. This is largely why the industry moved to warm-edge spacer technology, which uses a less conductive material and holds up notably better over the same timeframe.
Warm-edge spacers versus older aluminium spacers
A warm-edge spacer bar uses a composite or stainless-steel-hybrid material rather than solid aluminium, which reduces the temperature differential at the very edge of the glass where the seal sits. Less thermal movement at that edge means less stress on the sealed joint over the unit's life, translating into a meaningfully lower failure rate over ten, fifteen, twenty years compared with an older aluminium-spacer unit.
If you're replacing a blown unit anyway, it's almost always worth specifying a current warm-edge spec rather than matching the original older design, since the cost difference is minimal but the expected service life is considerably longer - particularly relevant in a coastal climate where thermal cycling and salt exposure both work against the seal.
Argon fill and whether it's lost when a unit blows
Many modern sealed units are filled with argon gas rather than plain air, since argon is a better insulator and improves the unit's overall U-value. When the edge seal fails, the same breach that lets moisture in also lets the argon fill gradually leak out, meaning a blown unit isn't just cosmetically misted - it's also lost some of the insulating performance it was originally specified to provide.
This is one of the less visible costs of leaving a blown unit in place: even before misting becomes severe enough to be a real nuisance, the door may already be running at a noticeably worse U-value than when it was installed, quietly increasing heat loss through what's often the single largest glazed area in a front door.
Can a blown unit be repaired without replacement?
No - a failed edge seal can't be re-sealed or dried out effectively once moisture has entered the cavity between the panes. Some companies advertise a defogging or unit-venting service, drilling a small hole to allow the trapped moisture to evaporate, but this is a temporary cosmetic fix at best, doesn't restore the insulating gas fill, and typically needs repeating within a year or two as moisture re-enters through the same failed seal.
A full replacement sealed unit is the only way to properly resolve a blown unit, restoring both clarity and the intended thermal performance in one job rather than a repeated temporary measure.
Confirming it's actually blown before ordering
Misting that appears and clears with weather changes, or that's only visible first thing on a cold morning and gone within the hour, can sometimes be surface condensation on the room-side face of the inner pane rather than trapped moisture between the panes - this is a ventilation and humidity issue in the room, not a failed unit, and doesn't need glazing replacement at all.
The distinguishing test is whether the haze is on an internal glass surface you can wipe clean (surface condensation) or genuinely trapped and unreachable between the two panes (a blown unit). We always confirm this on site before quoting, since it's a straightforward visual check that avoids ordering a replacement unit that isn't actually needed.
U-values and choosing a replacement spec
Current composite doorsets are typically specified from around 1.0 W/m2K upward depending on glazing size and specification, and a replacement unit is a good opportunity to bring an older, lower-spec pane up to a more current thermal performance without touching the rest of the door. Larger glazed apertures particularly benefit from choosing the better available spec, since glass typically performs less well thermally than the insulated door slab surrounding it.
We'll talk through the available glazing specifications - including Low-E coatings, gas fill and spacer type - at the point of quoting, so the replacement isn't just a like-for-like fix but an improvement where it's cost-effective to do so.
The replacement process for a blown unit
As with a cracked pane, the internal beading is removed first, the old unit lifted clear, and the aperture cleaned before the new unit is set on fresh setting blocks and the beading refitted and resealed. Because there's no glass breakage risk with a blown unit (the glass itself is intact, it's the seal that's failed), removal is often slightly more straightforward than dealing with a cracked or shattered pane.
Most standard apertures are completed within an hour on site once the correct replacement unit specification has been confirmed and is in stock, with decorative or bespoke-sized units taking longer to source.
Multiple units failing around the same time
If a composite door was installed at the same time as surrounding windows, and those windows are now also showing blown units, it often points to a batch of units from the same era or supplier reaching the end of their expected seal life together rather than anything specific to the door itself. This is common enough in properties around fifteen to twenty-five years old that it's worth checking nearby glazing at the same time as booking a door unit replacement.
For holiday lets and managed properties, a single blown unit is often the trigger to check all glazing across a property in one visit, since booking multiple separate callouts as each unit fails individually is usually less efficient than addressing them together.
Spacer bar identification during diagnosis
Part of confirming a blown unit and specifying its replacement involves identifying the spacer bar type currently fitted, which is usually visible as a thin strip along the inner edge of the glass just inside the frame rebate. A dull grey metallic strip typically indicates an older aluminium spacer, while a black or dark-coloured strip usually indicates a warm-edge composite spacer of the kind fitted to current-specification units.
This visual check, alongside the approximate age of the door if known, helps set expectations for how much longer any other units in the same doorset might be expected to perform before showing similar seal failure - useful information for a customer deciding whether to replace a single blown unit now or budget for others following in the coming years.
We note the spacer type on the job sheet as standard practice, since it also informs which replacement specification makes sense - there's little benefit in fitting a like-for-like aluminium-spacer replacement next to units that are likely to fail again within a similar timeframe when a warm-edge upgrade costs only marginally more.
Argon top-up versus full replacement
It's sometimes asked whether a blown unit's lost argon fill can simply be topped up without replacing the whole sealed cassette. This isn't a viable repair - the argon escapes through the same failed edge seal that let moisture in, and re-gassing a unit with a breached seal would only see the new fill escape just as quickly, while doing nothing about the moisture and misting already present between the panes.
The only way to restore both clarity and correct gas fill is a genuinely new sealed unit, manufactured and gas-filled at the point of production rather than serviced in situ. This is standard across the glazing industry and isn't specific to composite doors - the same applies to any sealed double or triple-glazed unit, whether in a door, window or roof light.
Booking a blown unit replacement
Most blown unit jobs can be quoted from a couple of clear photographs of the misted pane taken from both inside and outside, along with rough aperture dimensions, which lets us confirm the fault and order the correct sealed unit ahead of a single fitting visit rather than needing a separate measuring appointment first. For decorative or unusual designs, a short measuring visit ahead of ordering avoids any risk of the wrong pattern or size arriving.
Once the replacement unit is on site, fitting typically takes under an hour for a standard aperture, with the door usable immediately afterwards and no drying time or mess involved, unlike a wet trade repair. We're happy to talk through warm-edge spacer and Low-E coating upgrades at the point of quoting if you're weighing up a straightforward like-for-like swap against a slightly higher-specification replacement.
Typical repair costs in Cornwall
| Work | Typical price | Notes |
|---|---|---|
| Standard blown unit replacement (clear, single glazed panel) | £130-£200 | Most common single-aperture front door job |
| Upgrade to warm-edge spacer + argon fill on replacement | £160-£260 | Recommended over matching older aluminium-spacer spec |
| Decorative/obscured blown unit replacement | £180-£300 | Pattern and size dependent |
| Multiple unit replacement (door plus adjacent sidelight) | £250-£420 | Cost-effective when booked as one visit |
Guide prices only — we confirm the cost after seeing the door.
When to call a professional
- Replacement unit needed.
Prevention
- Buy from established brands with unit guarantees.
