When a double glazed sealed unit fails…

It is often said that a sealed unit “is blown” or “has gone misty”, etc. This means that a fault, and it may only be a pinhole to start with, like a small puncture, has developed somewhere in its perimeter, and moisture is getting inside and between the two panes of glass. At different times of the year there will be different amounts of moisture in the atmosphere, and even in the hottest of balmy summer days the atmosphere that we breathe has a moisture content (humidity). With changes in sonic and atmospheric pressure being put upon the ‘sealed’ unit, moisture will be drawn in to mix with the otherwise arid interior of the unit through this breach. As temperatures change the moisture will condense into a liquid, which will continue to build up and up, as the liquid cannot escape anywhere as easily as the moisture that is being drawn in. I have seen sealed units that have had several inches of water laying at the bottom of them because the unit is acting like a tank! When sealed units are manufactured they are not designed to be taken apart again in the future, and therefore in practice they cannot be economically cleaned and put back together. When a sealed unit has failed it will need replacing, and the old glass is usually just scrapped.

The most common causes of premature failure in this type of frame are:

Not sealed correctly during manufacture:
If a sealed unit is not fully sealed all the way round, or not sealed properly, the unit is likely to fail within a relatively short time scale, and this will be normally within a year or so.

Not seated correctly on glazing blocks:
Difficult to be exact about to what extent this will contribute to premature failure, as each case will vary. Worst case would be if drainage was blocked altogether with even a small amount water getting in, which could reduce the life of a ‘good’ unit by around 50% in terms of its otherwise fair life expectancy, see below.

Exterior seals not fitting correctly, letting water in:
Again, similar to above, but if some of the water getting in contains washing up liquid or similar, then this will attack and degrade the perimeter seal, usually along the bottom edge. Anything stuck together will come apart quicker if immersed in water, and even quicker if that water contains a solvent or any oil based contaminants.

Flexing of the framework:
Very difficult to quantify, but any pressures put upon the sealed unit will not do it any good at all. Installations most at risk are those of the frames which go to make up the structure of a Conservatory, which should always be fully reinforced if PVC-U to help reduce flexing caused by wind loads.


1.     One of the BIGGEST mistakes a window supplier or the fitter on site can make is in the use of solid ‘flat’ packers to sit the glass unit on. All sealed units NEED to be sitting on ‘bridge’ packers, that is so that any water ingress past the outside gasket will have ready access to egress out through the drainage system of the frame. The use of solid ‘flat’ packers, and often  blocking drainage may well be the biggest single cause of premature sealed unit failure that we see today. Leaded designs let even more water past the outer gasket because of the bumps the lead causes on the outer pane of glass.

2.     Any company that (wrongly) does not use bridge packers should at least make certain that their fitters are trained in the correct placement of their flat packers (I.E. inside of the drainage slots, and certainly not over them), AND should also engineer the frames so as to have a drainage slot in between the packers in the middle, as well as at each end.

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Warmseal Windows

2b Mansfield Road,
Sutton in Ashfield,
NG17 4GR.

Call: 0800 052 0060
Email: info@warmsealwindows.co.uk