If your windows have started to look cloudy, fogged or permanently misted, there's a good chance your double glazed window has blown. This page explains what that means, why it happens, and how the problem can be quickly resolved – often without replacing the entire window.
What is blown window repair?
Blown double glazed window repair is the process of fixing failed double glazing, usually by replacing the sealed glass unit while keeping the existing uPVC frame.
A double glazed unit is made from two panes of glass separated by a sealed gap. This gap is filled with dry air or an insulating gas such as argon, which helps slow down heat transfer and improve the window's overall performance.
The unit relies on its edge seal to keep moisture out and keep the insulating gas inside. Over time, this seal can break down through age, weather exposure, movement in the frame or general wear and tear. Once the seal fails and the window is "blown", moisture can work its way in and any inert gas inside the unit will escape.
Common signs you need misted window repair:
- Cloudy or fogged glass
- Misting or condensation between the panes
- Water marks, streaks or staining inside the unit
- Draughts or cold spots near the glass
- Cracks or visible damage
A failed double glazed unit can cause a number of problems:
Reduced thermal insulation
A panel with a failed seal is no longer working as effectively as it should. Moisture can enter the gap between the panes and any insulating gas inside the unit will escape, reducing the window's ability to slow heat transfer through the glass.
Lower energy efficiency
When a glass unit performs poorly, the room may lose heat more easily. This can make the space harder to keep warm, especially in colder months, and means your heating has to work harder than it would with a properly sealed unit.
Poor noise-cancelling performance
Double glazing helps reduce outside noise, but a failed unit will not perform as well as intended. If the sealed gap has been compromised, traffic, weather and general outdoor noise may become more noticeable inside the property.
A tired, neglected appearance
Misted or cloudy glass will make otherwise good windows look older than they are. This is especially noticeable on front-facing windows, where failed units are likely to affect the overall appearance of the property from the outside.
In most cases, the frame itself does not need to be replaced. At Crystal, fixing a blown window involves simply removing the failed unit and replacing it with a new double glazed unit, restoring clarity, insulation and appearance without the cost of fitting a completely new window.
We are sometimes asked whether a blown window seal repair can be carried out without replacing the failed unit itself. Unfortunately, this is not possible. Once the seal has failed, moisture can enter the gap between the panes and insulating gas can escape from the unit. The original sealed environment cannot be reliably restored in place, and it is also not possible to reach the inside faces of the glass panes to remove staining.
Why not replace the whole window?
Full replacement is often suggested, but is rarely necessary when dealing with uPVC. These frames are designed to be durable and long-lasting, and in many cases will remain structurally sound long after the glazing unit has failed. Replacing a full window when only the glass needs attention involves more work, more cost, and more disruption than required.
Keeping the existing frame means:
- No disturbance to surrounding plaster, trims or external finishes
- No need to match new frames to others in the property
- Faster, cleaner installation
- Less material waste
At Crystal we adopt a straightforward approach – replace what has failed and retain what still works.
Our misted window repair process
Replacing a double glazed unit is a controlled job when done properly. Everything is measured and prepared so the replacement fits exactly as it should.
- The existing unit is assessed and measured to ensure accuracy
- A new sealed unit is manufactured to the correct size and specification
- The failed glass is carefully removed from the uPVC frame and recycled
- The replacement unit is installed using the correct packers and supports
- Seals are checked to ensure a secure, weather-tight finish
Once the new unit is in place, the window should look and perform as it did when it was first installed. In many cases, it may perform even better.
Glass coatings have improved significantly over the years, which means older double glazed units can sometimes have a slight tint, tone or reflective finish that differs from modern replacement glass. When carrying out blown window repair, care will be taken to match the new glass to existing glass units in the property, but if it is not possible to achieve an exact match we will advise you in advance of the install.
Cloudy window repair improves performance
A replacement unit does more than just clear the mist. Modern double glazing can offer better insulation than older units, thanks to improved coatings and inert gas fills. That can mean less heat loss, fewer cold spots, and a more comfortable room overall.
In practical terms, when you fix a blown window you will notice a warmer feel near the window, reduced condensation on the room-facing pane, and improved clarity. The overall appearance is restored without changing the frame.
Misted window repair also offers you the opportunity to upgrade your windows – with acoustic glass, integrated blinds or Super Spacer® non-metal warm edge spacer bars.
Misted window repair cost
The cost to replace a blown double glazed window is usually much lower than replacing the full window, as only the failed double glazed unit needs to be changed rather than the frame, sill and surrounding fittings. Whether you need misted, blown or cloudy window repair, the price will depend on the size of the glass unit, the glass specification and how many units need replacing.
Use our simple instant quote tool to estimate your blown window repair cost.