​Double-glazing sealed units misting up or showing condensation in-between the 2 panes.



Once you have misty double-glazed sealed units:

In the trade we say that the double-glazed sealed unit has ‘broken down`. This means that the perimeter edge seal has lost its integrity and that it is letting in moisture somewhere. I would like to point out at this stage that the perimeter edge seal is not the black rubber seals that you can visibly see at both sides of the Double-Glazed Unit. The unit has its own seal which you cannot see once it has been installed. The moisture is hard to spot at first, sometimes becoming visible only as the Sun or another heat source draws the moisture up.Typically,the moisture will be drawn in as the atmospheric pressure changes, and it will build up, eventually condensing into water inside the sealed unit.

What you can do about misted-up (Condensed) double-glazed sealed units:

The only answer is to replace the double-glazed sealed units with new ones. That means no alteration or disruption to the framework as the glass sealed unit is designed to be removable and replaceable. Trying to separate the glass panes to clean them is not practical as the sealed unit is factory made with the panes bonded together with the aluminium spacer bar. In timber windows it will be more time consuming to replace the unit as the butyl compound (looks like putty) can become very hard over the years and the existing glass can be more difficult to remove. Richard Singleton will fix new sealed units into timber frames on glazing tape and not butyl compound as this will significantly increase the lifespan of the new unit.

The job of replacing the new panes is quick, with no mess and in 99% of jobs; the curtains do not even need removing. Were the panes have leaded strips stuck on the glass or the Georgian bar in-between the glass we can replicate them exactly.

Here we replaced some failed leaded units with clear ones as the lead obstructed such a nice view.(photo before and after)


Before and After: Toughened Double-Glazed Unit Fixed Before & After: Replaced Double Glazed Unit, No Condensig
Before After

Here is a before and after of a Toughened Double-Glazed Unit we replaced in a Aluminium Patio Door

Double Glazed Seal Unit Broken Down Double Glazed Seal Unit Fixed
Before After


Here is a before and after of a failed Leaded Double Glazing Unit in a PVCu Frame

Before After

Why did the double-glazed sealed unit break down?

There are a number of different glazing techniques for fitting double-glazing sealed units into different frame materials such as wood ( as mentioned above) as opposed to either PVCu or aluminium frames, and there are some important Do’s and Don’ts to ensure a reasonable life expectancy for the sealed unit.

Although there are many reasons why a double-glazed unit may mist up inside when it has ‘broken down’ because of the premature failure of the perimeter seal, the essential cause will lie in how the sealed unit was made, how the framework it is fitted into was made or how the double-glazed unit was fitted into the framework. We have removed units where the frame has not been drained for water escape and the unit has been sat in a ‘puddle’. We will drain every window where necessary before installing the new unit, on a minimum of a 5mm plastic glazing packer. Most units replaced by us are south or west facing where the sun is at its strongest during the day. This alone is the major contributor to premature sealed unit failure combined with the winter freeze and thaw, which all puts extra pressure on that seal between the panes.