Signs your double-glazed unit has failed
Last updated: 28 September 2026
Persistent misting or condensation between the panes is one of the clearest signs that the sealed unit has lost its seal. Moisture inside the cavity cannot be wiped from either room-side glass surface.
What is the clearest sign of a failed double-glazed unit?
A sealed unit is designed to keep the cavity between the panes isolated. When the edge seal fails, moisture can enter and the glass may appear cloudy, misted or streaked internally. The amount can change with temperature and weather, so the unit may look clearer at some times and worse at others. That changing appearance does not mean the seal has repaired itself.
How can you tell internal misting from normal room condensation?
Room condensation forms on the surface you can touch and can usually be wiped away. Condensation caused by a failed sealed unit is trapped between the panes and remains inaccessible from either side.
Morning condensation on the room side can also be influenced by humidity, ventilation and surface temperature, so it should not automatically be treated as failed glazing. The useful test is simply where the moisture is. If it is genuinely inside the sealed cavity, the glass unit itself is the likely issue.
Can a failed unit still look intact?
Yes. A sealed unit can fail without cracking. The frame may still open, close and lock normally while the glass becomes misted because the edge seal has deteriorated.
That distinction matters because it may allow the sealed unit to be replaced while retaining the existing frame. A visual inspection should still consider the frame, beads, gaskets and surrounding condition. If the window is distorted, rotten, insecure or leaking around the installation, the problem may extend beyond the glass.
What other signs should you look for?
Visible cracks, moisture staining near the edge, internal haze that returns after temperature changes and obvious damage to one pane can all justify a glazing assessment. A cracked unit should be treated more urgently if the damage creates a safety risk.
Photographs should show the full window as well as the affected area. A close-up of condensation alone does not reveal whether the frame is serviceable. Note any difficulty opening or closing the sash, failed locks or movement in the frame because those symptoms can point to a wider window problem rather than a sealed-unit issue only.
Can a misted unit be dried and resealed instead of replaced?
The normal glazing solution for a genuinely failed sealed unit is replacement of the unit rather than attempting to restore the original factory seal in place. The replacement must be measured and manufactured to the correct specification.
Short-term methods that remove visible moisture do not recreate the original sealed construction or guarantee equivalent performance. If the frame is sound, unit replacement can be a targeted repair. If the frame itself is failing, complete window replacement may be a separate construction decision.
What happens when a unit is replaced?
The opening is measured, the required thickness and glass specification are confirmed, the replacement sealed unit is manufactured and the old unit is removed from the frame before the new one is fitted and secured.
The measurement should account for the actual glazing system, not just the visible glass area. Safety glass, privacy glass or other specifications may be required depending on the location. Once installed, the frame and beads should be checked so the new unit sits correctly.
How much does sealed-unit replacement cost?
Price depends on dimensions, thickness, glass specification, access and whether there are additional frame or hardware issues. There is no responsible universal price per window that covers every unit.
M25 Glazing will add real ranges only after they are confirmed. Pricing is confirmed from the actual opening and work required. Until then, the website states that prices depend on size, glass type and access. Use the double glazing service page to start a measured replacement enquiry.
Related services and local pages
If you can see moisture between the panes, the double glazing service page is the most relevant next step. For a cracked or unsafe unit, use the emergency glass replacement page.