Double glazing does not fail overnight. The process is gradual, and by the time most homeowners notice something is wrong, the unit has already been losing efficiency for months. Knowing what to look for means you can act early before small problems become expensive ones.
Here are the six most common signs that your double glazing has failed.
1. Condensation Between the Panes
This is the clearest sign of a failed sealed unit, and it is not something you can fix with a cloth.
Double-glazed windows work by trapping an insulating gas, usually argon, between two panes of glass. A hermetic seal keeps that gas in and moisture out. When the seal breaks down, the gas escapes, and humid air takes its place. As temperatures change, that moisture condenses on the inner surfaces of the glass, creating a milky, cloudy, or streaky appearance that sits permanently between the panes.
Unlike condensation on the inside surface of a window (which is usually a ventilation issue), condensation between the panes cannot be wiped away. The sealed unit itself needs replacing.
2. Draughts Around the Window
If you can feel cold air coming in around a window or door, the seal has broken down somewhere. The source matters:
- Draught between the frame and the brickwork, the external sealant or masticant has failed. This is relatively straightforward to address, but needs prompt attention to prevent water ingress.
- Draught between the glass and the frame, the internal rubber weather seal has deteriorated or lost its elasticity. Replacement seals are sometimes available, but on older windows, it is often more cost-effective to replace the unit entirely.
- Draught through the glass itself usually points to a cracked pane, which requires immediate replacement.
Draughts not only impact comfort and quickly increase heating costs, but combined with air moisture, rapidly increases the effects of damp.
3. Visible Damage to the Frame or Glass
Damage to the windowpane is self-explanatory. A broken windowpane causes a security issue, and results in a sealed unit that inevitably becomes condensated, leaks, and becomes draughty.
Damage to the frame causes a different issue and may depend on frame material.
- uPVC frames can warp, crack, or discolour over time, particularly on south-facing elevations with heavy sun exposure. Warped frames no longer close correctly, leaving gaps that allow air and water in.
- Timber frames are susceptible to rot if the paint or preservative coating has not been maintained. Soft, spongy wood around the frame is a serious sign; left untreated, it can lead to structural issues and provide an entry point for damp.
- Aluminium frames are generally more durable, but corroded or bent frames will affect how well the window closes and seals.
Surface scuffs and minor scratches can often be left alone, but anything that goes deeper than the surface layer warrants a professional assessment.
4. Water Leaking Through the Frame
Water coming in around the frame is a sign that either the external sealant has failed or the window’s internal drainage system is blocked.
Most double-glazed windows and doors have small drainage channels built into the frame that carry water away before it can penetrate inside. When these channels become blocked with debris, water backs up and eventually finds its way through. Cleaning the channels often resolves the problem.
If the drainage channels are clear but water is still leaking through, the sealant fails and must be replaced. If not, there will be continual water penetration that will cause dampness and mold as well as damage the wall and nearby plaster over time.
On older windows, repeated water leaks can also be a sign that the frame itself has reached the end of its serviceable life.
5. Windows or Doors That Are Stiff or Hard to Operate
Some stiffness can be solved by simply applying lubricant on the hinges and locking mechanism. If stiffness has developed gradually or the window no longer closes flush with the frame, there is likely something more significant that is causing the problem.
Old windows warp over time and no longer sit properly in the frame. The result is a window that sticks, grinds, or requires significant force to shut completely.
In some cases, difficulty operating a window or door is a warning sign of something more serious: subsidence or structural movement in the building. If the stiffness has appeared suddenly, especially alongside diagonal cracks in the surrounding walls or above the window reveal, it is worth having a surveyor look at the property rather than simply replacing the glazing.
6. Noticeably Higher Energy Bills
The primary purpose of double glazing is to improve energy efficiency. When double glazing is sealed well and all units are intact, heating through windows is minimized. If the units are failing and frames become insufficiently insulated, heating bills will increase.
If your energy bills have risen without a clear change in usage or tariff, it is worth checking your windows. A single blown unit may not make a dramatic difference, but multiple failed units across a property add up quickly, particularly in older homes where the original glazing is approaching or past its typical lifespan of 20 to 25 years.
Energy-efficient replacement windows are now significantly better than those installed even ten to fifteen years ago, so replacing failed units can result in a noticeable reduction in heating costs.
What to Do If You Notice These Signs?
Some problems, such as blocked drainage channels in the window frame or faulty external sealant, are rather easy to fix and do not require complete window replacement. Problems such as blown sealed units and cracked glass do require complete replacement.
If you are buying a property and notice any of these signs during a viewing, it is worth factoring potential replacement costs into your offer. At Trade UPVC, we will assess all accessible windows and doors, rate any defects by severity, and advise on the remedial work needed. Where failure is widespread, the findings can support a renegotiation of the purchase price.
If you already own a property, you should fix failed glazing as soon as possible to minimize the impact of window failure on your energy bills and the risk of damp and mould.
