The appearance of misted double glazing is the product of a failed double glazing unit, also referred to as a blown unit.

A common cause of misted double glazing from a failed unit is holes in the seal around the double glazed unit. Such holes need only be small to draw moisture from the atmosphere and get inside between the two panes, causing what may look like light condensation inside the unit. Once inside the unit’s cavity, changes in temperature can cause the moisture to condense. With nowhere to go and moisture being continually drawn in units can end up holding considerable amounts of water.

Causes of misted double glazing

Poor Manufacture

If a double glazed unit has not been sealed properly or all the way round then it is highly likely to fail within the first 12 months from installation.

Poor installation

Units should be sat on bridged glazing blocks to keep them elevated from draining water and provide water with a route to run away. The build up of trapped water could in time penetrate the unit, thus halving it’s usable life expectancy. Additionally, poorly fitted exterior seals can allow water to collect and become trapped. Even the seal on a well manufactured unit is prone to failure when immersed in water over time, particularly if the water contains washing-up liquid, solvents or oil-based products.

Poor Specification

The frames of a uPVC Conservatory should be fully reinforced to ensure that the structure is rigid, even during times of high winds. Failure to reinforce could allow excessive flexing of the frame and result in the twisting or sheering of a unit that could cause it to fail.

Fixing misted double glazing

There are various tales of techniques to resolve the problem of misted double glazing, including:

Spurious Method 1

Remove the sealed unit and drill a small hole through the seal between the two panes of glass. Position the unit over a heat source, such as a radiator until all the moisture has dried out, or use a hair dryer. Once the moisture has gone, seal the hole with a blob of silicon sealant.

Spurious Method 2

Drill small hole in top and bottom of the external pane and insert small valves or vents to allow the moisture to evaporate away.

Spurious Method 3

Pour silica gel crystals into the cavity through a small drill hole in the seal and reseal with silicon.

We do not recommend any of these approaches, as we believe they can result in air circulating between the panes of glass – no longer operating as sealed energy efficient windows Additionally these approaches may not remove any staining that has taken place. In our opinion, the only true, reliable solution is to replace the failed double glazed unit.