If a pane in your external bifold doors looks cloudy from the inside of the glass, check whether you can wipe the moisture away. Misting trapped between the panes is different from condensation on the room-side surface: it can indicate that the sealed double-glazed unit has failed.
The change often becomes more noticeable as autumn turns to winter. The good news is that a misted panel does not automatically mean you need a whole new doorset. In many cases, the practical solution is to assess and replace the affected sealed glass unit while retaining the existing frame and door.
What does a blown bifold door glass unit look like?
A double-glazed panel: also called an insulated glass unit, or IGU, has two panes separated by a sealed cavity. If the edge seal deteriorates or becomes damaged, moisture can enter that cavity. The unit is commonly described as “blown”.
Look for:
- Haze, droplets or streaks between the panes
- A cloudy area that remains after cleaning the accessible glass surfaces
- Misting that clears and returns as temperatures change
- A milky or marked appearance that reduces the view through the panel
A failed seal can also affect the unit’s insulating performance. The cavity is no longer working as designed, so the affected area may feel colder and contribute to heat loss. The effect on energy use depends on the door, the rest of the home and how much glazing is affected; it cannot be judged from the mist alone.
Why does the fog become more obvious in winter?
A sound sealed unit should keep moisture out of the space between its panes. Cold weather does not, by itself, cause a healthy unit to mist internally. But winter can make an existing seal failure much easier to see.
When outdoor temperatures fall, the glass cools. Moisture in a failed unit’s cavity can then condense against the colder glass, appearing as droplets, haze or streaks. Changing temperatures between day and night can make the fog come and go, which may give the impression that the panel has cleared permanently. It has not necessarily done so.
Autumn and winter also mean darker days and more time indoors, making misted glass more noticeable. If the affected panel is large, a colder patch by the doors may add to the feeling of a cold spot, though a cold area or draught can also have other causes, such as worn seals or a gap around the frame.
Is it between the panes, or ordinary condensation?
Before arranging work, try this simple check:
- Room-side condensation: Wipe the inside surface of the glass with a clean cloth. If the moisture comes away, it is on the room-side face. Indoor humidity, cooking, bathing and drying clothes can contribute, particularly when the glass is cold.
- Condensation inside the unit: Clean the room-side surface, then check the outside face if it is safely accessible. If the haze or droplets remain between the two panes, the sealed unit may have failed.
- Exterior condensation: Dew on the outside can form in cold conditions, especially overnight. It is not, on its own, evidence that the sealed unit has blown.
If you are unsure, take a photograph in daylight and note whether the mist shifts or disappears as conditions change. Avoid trying to drill the glass or use DIY “defogging” treatments: these do not restore a failed sealed cavity.
For more on diagnosing a complete doorset, see how the Pro-Lock assessment works.
What can a failed sealed unit mean for the door?
The first effects are usually visual: the glass looks dirty even after cleaning, and the view through the panel becomes less clear. A failed unit may also insulate less effectively, potentially contributing to heat loss and higher heating costs over time. The scale of any difference varies, so a misted panel should not be treated as a precise measure of energy loss.
It is also worth checking how the affected panel operates. Fogging alone does not prove that the panel has become heavier or that the hardware is failing. But if water is reaching the glazing rebate, the panel is wet, or the door has started dragging or catching, have the whole doorset checked. Extra moisture or a change in panel support can add strain to hinges, rollers and alignment.
A repair assessment should consider the glass unit alongside its glazing support, gaskets, panel position and operation. That helps distinguish a glass fault from a separate issue with the frame, seals or hardware.
The usual solution: replace the glass unit, not the whole door
Where the frame and panel are suitable to retain, the repair may be to remove the affected sealed unit and fit a correctly specified replacement. Replacing the glass unit is different from replacing the complete bifold doorset.
The replacement needs to match the opening and the door’s requirements, including its dimensions, thickness, glazing specification and safety-glass type. Correct fitting and glazing support matter too: the unit sits within a working panel, and its position can affect how that panel sits and moves.
A professional assessment can establish:
- ✅ Whether the mist is inside the sealed unit
- ✅ Whether the glass, panel and surrounding frame are suitable to retain
- ✅ What specification of replacement unit is needed
- ✅ Whether the gaskets, glazing support or drainage also need attention
- ✅ Whether suitable parts can be sourced and a repair is economically sensible
The Pro-Lock team provides repair-first assessments and written recommendations for external bifold doors. Our bifold door services cover external uPVC, aluminium, timber and hybrid doorsets. We do not sell new doors, so our advice is focused on the condition of your existing doorset and suitable repair options.
Book an assessment before the deep cold
If you can see persistent mist between the panes, or the affected panel has become noticeably colder, arrange an assessment rather than waiting for the fog to clear on its own. A clear diagnosis helps you understand whether the issue is confined to the glass or whether another part of the doorset needs attention.
The £79 assessment fee is credited against the repair cost if you proceed. It is refunded if suitable parts cannot be sourced or the doorset is beyond economical repair. Any recommended work is set out in a written quotation before you decide how to proceed. Completed repair work carries a 12-month warranty.
External bifold door repair across our service area
Pro-Lock covers seventeen counties: Norfolk, Suffolk, Cambridgeshire, Bedfordshire, Essex, Hertfordshire, Surrey, Kent, East Sussex, West Sussex, Lincolnshire, Northamptonshire, Leicestershire, Berkshire, Hampshire, Oxfordshire and Buckinghamshire. London is excluded. We work on external bifold doors only; internal wardrobe, cupboard and room-divider bifolds are not covered.
For local information, see bifold door repair in Berkshire and bifold door repair in Hampshire, or our Oxfordshire coverage. We also provide bifold door repairs in Reading, bifold door repairs in Southampton and bifold door repairs in Oxford. In West Sussex, our coverage includes Worthing and the coastal areas around Chichester Harbour.
Talk to the Pro-Lock team
Don’t assume a misted panel means replacing the entire door. Contact the Pro-Lock team to explain what you can see and arrange an assessment.
- Call 0330 311 0975 ✅
- Email enquiries@bifold-door-repair.co.uk ✅
- Message Pro-Lock on WhatsApp ✅
A clear assessment can help identify the right repair, restore the panel’s appearance and give you peace of mind before winter sets in.