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Double glazing failure isn’t always obvious, but it’s also not hard to identify once you know what to look for. The most common signs are visible without any tools, and working through them systematically will give you a confident answer in a few minutes rather than a vague feeling that something might be wrong.
This guide explains what failure actually means physically, walks through the key checks in order of reliability, distinguishes between genuine failure signs and normal window behaviour that often gets misidentified, and explains what your options are once you’ve made the diagnosis.
A double-glazed window works because of the sealed cavity between the two glass panes. That cavity — typically filled with air or argon gas — creates thermal resistance that a single pane of glass cannot provide. The cavity is hermetically sealed at the edge of the unit by a spacer bar and a perimeter sealant, typically silicone or polysulphide.
When the seal fails, that hermetic barrier is compromised. Atmospheric air — carrying moisture — enters the cavity. The desiccant packed into the spacer bar, which is there specifically to absorb any residual moisture, becomes saturated. Once saturated, it can no longer prevent moisture from condensing inside the unit when temperatures drop. The result is visible: fogging, condensation, or mineral deposits between the panes.
This is the core failure mode of double glazing, and it’s irreversible. Once the seal has failed and moisture has entered, the unit’s thermal performance is permanently reduced. It cannot be dried out or resealed effectively from the outside.
If you’re planning to replace failed units or upgrade multiple windows, timing the work properly can make a noticeable difference to cost, disruption, and overall project flow. This guide on how to schedule a double glazing renovation explains how to approach the process so it aligns with other home improvements and avoids unnecessary delays.
This is the most reliable check. Look at the glass carefully — not the surface, but between the two panes.
What you’re looking for is any sign of moisture, fogging, haze, streaking, or white mineral deposits in the cavity between the panes. These appear in various forms:
A uniform haze or fog that obscures the view through the glass, similar to a frosted appearance but only between the panes. A streaky pattern where condensation has run down inside the cavity and left mineral residue. A clear puddle or moisture droplets visible in the cavity — less common, but a definitive sign. White or grey residue that looks like dried water marks inside the glass, which doesn’t wipe off because it’s in the cavity, not on the surface.
The condition can be worse at certain times of day and in certain weather conditions. Fogging between panes is often more visible in the morning or after a cool night, when the cavity temperature drops and condensation forms. On a warm day, the moisture may partially evaporate and the glass can look clearer.
Confidence level: Definitive. Fogging or deposits between the panes is a confirmed seal failure. There’s no other explanation for moisture inside a sealed unit.
Before concluding you have a failed unit, it’s important to distinguish internal cavity fogging from surface condensation — because the two look superficially similar and one indicates a real problem while the other typically doesn’t.
Surface condensation appears on the interior face of the glass — the side facing into the room — and wipes off when you touch it. It’s caused by warm, humid room air coming into contact with a cold glass surface. You’ll often see it in bathrooms or kitchens on cold mornings, or in poorly ventilated rooms. It’s a sign of high room humidity or inadequate ventilation, not a double-glazing failure.
To confirm: wipe the glass with your finger or a cloth. If the fog wipes away, it’s surface condensation on the room-side face — the glass is performing normally and the issue is room humidity. If it doesn’t wipe off, it’s in the cavity — and the unit has failed.
Similarly, condensation on the exterior face of the glass is actually a sign of a well-performing window — the external glass surface is cool because the unit is preventing heat from escaping, and atmospheric moisture condenses on it just as it would on any cold surface. This is not a failure.
A functional double-glazed window that’s properly closed and latched should be essentially draught-free. If you can feel a cool draught around a closed window — run your hand slowly around the perimeter of the frame while the window is fully closed — it suggests a sealing issue. This could be:
A failed or compressed weather seal around the frame perimeter (common in older windows, and typically a maintenance fix rather than a reason to replace the whole unit). A frame that’s warped or has moved, preventing a full compression seal when closed. In some cases, failed window seals can be associated with broader frame deterioration.
Confidence level: Moderate. Drafts indicate a sealing problem but don’t specifically confirm IGU failure. The cause could be weather seal degradation rather than the glass unit itself. Both warrant attention, but the solutions differ.
A sealed double-glazed unit provides meaningfully better acoustic insulation than a single pane, primarily because the gas cavity absorbs and disrupts sound transmission. When the seal fails and the cavity is no longer hermetically sealed, acoustic performance is reduced.
If you’ve noticed that a previously quiet room has become noisier — more traffic noise, more neighbourhood sound audible through a specific window — and the increase is gradual rather than sudden, it can be a soft indicator of seal degradation. This is a supporting sign rather than a definitive one, because noise changes can have other causes: wear in the frame’s acoustic sealing, changes in the noise environment outside, or simply reduced attention to noise that was always there.
Confidence level: Low to Medium on its own. Useful as supporting context alongside other signs.
A functioning double-glazed unit significantly reduces the temperature differential between the glass surface and the room air. If you stand near a window in winter and feel cold radiating from it — not just draught, but cold emanating from the glass surface itself — it suggests the unit’s insulating performance has degraded. Failed double glazing can have an effective thermal performance approaching that of single glazing once the gas fill has escaped and the cavity is moisture-laden.
On a cold day, run your hand close to (but not touching) the interior glass surface. A significant sensation of cold radiating from the glass at close range indicates poor thermal performance. Compare it to windows you believe are functioning correctly if you have a reference point.
Confidence level: Low to Medium. Useful supporting indicator, but requires a comparative reference to be meaningful.
Understanding why seals fail helps set realistic expectations and informs decisions about replacement.
Thermal cycling is the primary cause. The glass, spacer, and sealant expand and contract at slightly different rates with temperature changes. Over thousands of cycles across years and decades, this mechanical stress accumulates and eventually compromises the edge seal. West and north-facing windows in Australian conditions — which experience the most extreme daily temperature swings — typically see faster seal degradation than south-facing windows.
UV degradation of the sealant compounds contributes, particularly in Australia’s high-UV environment. Polysulphide and silicone edge sealants degrade under sustained UV exposure, and windows with significant direct sun exposure age their seals faster.
Poor installation can initiate failure earlier. Units that were improperly bedded, installed in frames with inadequate drainage, or handled roughly during installation may have compromised seals from early in their life.
Product quality matters. Better-quality IGUs use more sealant, better desiccant, and warmer-edge spacers that reduce thermal stress at the edge. Budget units cut corners in ways that reduce seal longevity.
The short answer is no — a failed IGU cannot be repaired. Once the seal has broken down and moisture has entered the cavity, the unit needs to be replaced. There’s no effective way to re-seal from the outside and restore the unit’s performance.
What can usually be done, however, is replacing the IGU without replacing the whole window frame. If the frame is in good condition — structurally sound, with serviceable hardware and weather seals — a glazier can remove the failed unit and install a new one into the existing frame. This is typically considerably less expensive than full window replacement and is the appropriate response in most cases.
Full window replacement becomes the right answer when the frame itself is deteriorated, when the window is very old and replacement components are hard to source, or when you’re undertaking a performance or design upgrade at the same time.
“All condensation means failure.” Surface condensation on the room-facing glass is a humidity issue, not a product failure. Exterior condensation is actually a sign the window is working well. Only condensation or fogging between the panes confirms seal failure.
“You have to replace the whole window.” In most cases, the glass unit can be replaced within the existing frame if the frame is sound. Always get this assessed before assuming full replacement is necessary.
“Failed double glazing is dangerous.” It’s a performance issue, not a safety hazard in most circumstances. The glass remains intact; what’s lost is the thermal and acoustic benefit of the sealed cavity.
While all double glazing will eventually reach the end of its lifespan, a few simple habits can help extend its performance and delay seal failure.
Regularly clean window frames and inspect the sealant around the perimeter, especially after periods of extreme weather. Look for early signs of deterioration such as cracking, shrinking, or separation from the frame. Addressing minor issues early can prevent them from developing into full seal failure.
Keep window channels and drainage paths clear of debris. Blocked drainage allows water to pool around the frame, which accelerates wear on seals and increases the risk of moisture-related damage.
In coastal areas, it’s worth rinsing windows and frames with fresh water after exposure to salt-laden air. Salt buildup can gradually degrade sealant materials and shorten the lifespan of the unit.
Finally, when replacement does become necessary, product quality and installation matter. Units that fail prematurely are often the result of compromised materials or poor installation practices. Choosing well-specified products and experienced installers helps ensure the new system performs for as long as it should.
The diagnostic process for double glazing failure is straightforward once you know what you’re actually checking. Fogging, condensation, or mineral deposits between the panes — which don’t wipe off the glass surface — is the definitive confirmation of seal failure. Drafts, thermal discomfort, and increased noise are supporting indicators that warrant investigation but aren’t conclusive on their own.
Once you’ve confirmed failure, the typical path is IGU replacement within the existing frame rather than full window replacement — assuming the frame is in good condition. Getting an assessment from a qualified glazier will tell you whether that’s the right approach for your specific situation.
The clearest sign is fogging, condensation, or white mineral deposits visible between the panes — in the cavity, not on the glass surface. This confirms the hermetic seal has failed and moisture has entered the unit. If wiping the glass makes the fogging disappear, it’s surface condensation and not a unit failure.
No. Surface condensation on the room-facing glass is caused by high indoor humidity and wipes away easily — it’s a ventilation issue, not a product failure. Exterior condensation is actually normal and suggests the window is performing well. Only condensation inside the sealed cavity between the panes indicates genuine seal failure.
Once the IGU seal has failed and moisture has entered, the unit cannot be effectively repaired or resealed. The glass unit needs to be replaced. In most cases, this can be done within the existing frame without replacing the whole window.
The primary cause is thermal cycling — the repeated expansion and contraction of the glass and sealant with temperature changes, which over years eventually compromises the edge seal. UV degradation of the sealant, poor initial installation, and product quality all affect how quickly seal failure occurs.
Quality double glazing installed correctly typically lasts twenty to thirty years or more. The seal is usually the first element to fail, with windows in high-UV exposure, coastal environments, or west-facing positions typically experiencing earlier failure than those in more sheltered or cooler conditions.
In most cases, yes. If the frame is structurally sound with serviceable hardware and weather seals, a glazier can replace only the IGU within the existing frame. This is significantly less expensive than full window replacement and is usually the appropriate response to seal failure in a window with a good frame.
Yes. A failed IGU loses much of its thermal resistance as the gas fill escapes and the cavity becomes moisture-laden. The effective performance can approach that of single glazing, meaning more heat loss in winter and more heat gain in summer — both of which increase heating and cooling costs.