Why does double glazing mist or steam up?

Summer and winter: failed sealed units

Misted double glazed failed units are double or triple glazed sealed window units that have excess moisture trapped in the cavity between the panes of glass. This happens when there is a breach in the hermetic (airtight) seal around the edge of the unit, allowing moist air to enter the cavity over days, weeks or months.

When the temperature rises, the moist air trapped within the unit condenses and, over time, gives a white cloudy effect that is irritating and unsightly to look at.

Why south-facing windows mist more often

Over 90% of failed double glazing sealed units are south-east to south-west facing. As the sun rises in the east and sets in the west, these elevations see the greatest daily temperature change. That expansion and contraction creates the most movement in the window or door frame and puts extra stress on any weak points in the sealed unit.

Failings in double glazed windows

If there were any weaknesses or faults in the original manufacturing process of the sealed unit, continual expansion and contraction of the frame will eventually cause the unit to fail at the weakest point.

In our experience, the only long term solution is to fully replace the failed unit with a new one. We can repair timber, uPVC and aluminium frames of almost any age and typically offer a five-year warranty on replacement units.

Winter months: internal condensation

In winter, when the outside temperature drops significantly, you may see condensation on the internal surface of the glass. The glass surface can be much colder than the room temperature, so warm moist air inside the room condenses on the colder pane.

A simple test is to place the palm of your hand on the frame, count to ten, then place your hand on the glass. You'll usually feel that the glass is much colder.

To reduce this type of condensation, the glass temperature needs to be raised. This can often be improved by fitting more energy efficient units using warm edge spacer bar technology, low-emission glass and argon gas between the panes.

Other double glazing repairs we can help with

• Repairs to double glazed windows and window glass
• Repairs to double glazed window hinges and handles
• Repairs to double glazed door locks and handles
• Repairs to double glazed doors and frames

If you're seeing misted units or hardware issues, you can request a free repair quote and we'll match your enquiry to a local technician.