How Do I Know If My Double Glazing Needs Replacing?

If something about your windows does not look or feel right, there is a good chance your double glazing could be starting to fail. The most common signs include misting between the panes, noticeable draughts, difficulty opening or closing the window, and your energy bills increasing without another explanation. Recognising these signs early can save you money and prevent further damage to your property.

Many homeowners notice a change in their windows but are unsure whether they need new double glazing or if a simpler repair will do. Perhaps there is a faint haze between the glass that was not there before, or a room feels colder than it used to, despite the heating being on. These issues can develop gradually, making them easy to dismiss.

To help, this guide covers the most common double glazing replacement signs, explains what each one typically means, and sets out what to do if you spot one or more of them in your home.

Misting or Condensation Between the Panes

Condensation forming between the two panes of a double glazed unit is one of the clearest signs that your double glazing needs replacing. A double glazed unit consists of two sheets of glass separated by a spacer bar, with the gap between them filled with an insulating gas, usually argon. The unit is sealed at the edges to keep that gas in and moisture out.

When the seal breaks down, the insulating gas escapes and is replaced by moist air. As temperatures change, this moisture condenses on the inner surfaces of the glass, producing a foggy or misty appearance that you cannot wipe away because it sits between the panes.

A misted unit is no longer providing effective insulation. The thermal performance drops significantly once the gas fill is lost, which means the window is letting more heat escape from your home. This is not something that can be repaired by resealing the existing unit. The sealed unit itself needs to be replaced.

If you have noticed this in one or more of your windows, our misted double glazing replacement service can resolve the issue, often during a single visit. Montrose Glass provides a 10 year guarantee against misting on all replacement double glazing units.

Are Draughts or Cold Spots a Sign of Double Glazing Failure?

If you can feel cold air near a window even when it is fully closed and locked, the seal around the glazing unit or the frame itself may have deteriorated. Over time, the rubber gaskets and seals that hold the glass in place can harden, shrink, or crack. UPVC frames can warp slightly with prolonged heat exposure, and timber frames can swell or contract with moisture changes. Any of these issues can create gaps that allow cold air to enter your home.

A simple way to check is to hold your hand close to the edges of the window on a cold day. If you feel a distinct chill or a flow of air, the window is no longer sealing properly. This means your heating system has to work harder to maintain a comfortable temperature, and the window should be assessed by a professional glazier to determine whether the seal, the unit, or the entire frame needs replacing.

Can Failing Double Glazing Cause Higher Energy Bills?

Windows account for a significant proportion of heat loss in a typical UK home. The Energy Saving Trust estimates that replacing single glazing with double glazing can save around £100 to £120 per year on energy bills for a semi-detached property. When double glazing units fail, their thermal performance drops closer to that of single glazing, meaning the savings you once gained are gradually lost.

If your energy bills have risen over recent months or years and you have ruled out other causes, such as a tariff increase, a change in usage, or a boiler issue, your windows may be the culprit. This is particularly likely if your double glazing is over 20 years old and you have noticed any of the other signs described in this article. Replacing failing units with new, properly sealed double glazing restores the insulation barrier and can make a measurable difference to your heating costs.

Difficulty Opening, Closing, or Locking the Window

Windows that stick, jam, or refuse to close flush against the frame are more than an inconvenience. If a window does not close fully, it cannot provide a proper seal against draughts, rain, or noise. If the locking mechanism no longer engages, the window is also a security vulnerability.

These problems are often caused by frame warping, hinge wear, or the settling of the building over time. In some cases, the hardware can be replaced independently of the glass. In others, the frame distortion is severe enough that a full double glazing replacement is the most practical and cost-effective solution. A professional on-site assessment will clarify which option applies to your situation.

Visible Cracking, Chips, or Damage to the Glass

Any visible damage to the glass pane should be taken seriously. A small chip on the outer surface may appear cosmetic, but it can weaken the structural integrity of the pane and, if it extends to the sealed edge, allow moisture to enter the unit. A crack that runs across the pane compromises both the insulation and the security of the window.

Cracked or chipped double glazing should be assessed by a qualified glazier rather than left in place. Even if the window still appears to function, a damaged pane is more susceptible to further breakage from thermal stress, impact, or high winds. Replacement is typically recommended for any crack longer than a few centimetres or any damage that is close to the edge seal.

How Long Does Double Glazing Typically Last?

Double glazing generally lasts between 20 and 30 years, depending on the quality of the glass, the frame material, the standard of the original installation, and how well the windows have been maintained. Units installed in the 1990s and early 2000s were manufactured to different standards than those available today, and many are now reaching or exceeding their expected lifespan.

If your double glazing was installed more than 20 years ago and you are noticing any of the signs described above, replacement is likely the most cost-effective long-term option. Modern sealed units offer improved thermal performance, better gas retention, and longer-lasting seals. Rather than repeatedly addressing individual symptoms, replacing ageing units provides a lasting solution. For a fuller discussion of whether to repair or replace, read our guide on signs it’s time to replace vs. repair your double glazing.

What Should You Do If You Spot These Signs?

If one or more of these signs of your double glazing is failing are present in your home, the next step is to arrange a professional on-site assessment. A qualified glazier can confirm whether the sealed unit, the frame, or both need replacing and provide you with a clear quote before any work begins.

Montrose Glass is a family-run business established in 2012 with over 20 years of experience in the glazing industry. We are ISO 9001 and ISO 45001 accredited, members of the Guild of Master Craftsmen, and approved under the Safe Contractor Scheme. We charge no call-out fee under any circumstances, and our glaziers typically respond within 30 to 90 minutes. We operate 24 hours a day, 365 days a year, with nationwide UK coverage including Scotland, Wales, and Northern Ireland. Our vans are fully stocked with glass that can be cut on site, and a free no-obligation quote is provided before any work starts.

FAQs On When To Replace Double Glazing

How can I tell if my double glazing seal has failed?

The most obvious sign of a failed seal is misting or condensation between the two panes of glass that cannot be wiped away from either side. This happens because the airtight seal around the unit has broken down, allowing moist air to enter the gap where insulating gas once sat. If you can see this haze or fogging, the sealed unit needs replacing.

Do I need to replace the whole window or just the glass?

In many cases, only the sealed glass unit needs replacing rather than the entire window frame. If the frame is structurally sound and not warped, a glazier can remove the failed unit and fit a new one into the existing frame. A professional assessment will confirm whether the frame can be retained or whether a full replacement is more appropriate.

Is condensation on the inside of my window a sign of double glazing failure?

Condensation on the room-facing surface of the glass is usually caused by high indoor humidity and poor ventilation rather than a fault with the glazing unit itself. It is condensation between the two panes that indicates seal failure. If you are seeing moisture trapped inside the unit that you cannot reach or clean, the double glazing has failed, and the unit should be replaced.

How much does it cost to replace a double glazing unit?

The cost varies depending on the size of the unit, the type of glass required, and the complexity of the installation. Montrose Glass provides a free no-obligation quote on site before any work begins, and no call-out fee is charged. Call 01702 346304 for an accurate assessment tailored to your specific windows.

Can I replace double glazing myself?

Replacing a sealed double glazing unit involves handling heavy glass, ensuring a correct fit, and achieving a proper seal. Incorrect installation can result in further seal failure, draughts, or water ingress. Professional installation by a qualified glazier ensures the unit performs as intended and is covered by a workmanship guarantee.

Call Montrose Glass on 01702 346304 to arrange a free, no-obligation assessment of your double glazing. There is no call-out fee, and we are available 24 hours a day, 7 days a week, 365 days a year.