Glass:
Exploding Glass

Double-glazed sealed unit exploding, why?
I have gone into the subject quite deeply of why double-glazed sealed units crack and explode, because not only is it very puzzling to the general public but it is also not very widely understood in our industry either.

And here's why double-glazed sealed units implode:
This sometimes quite shocking occurrence most often happens in the UK between around late October and early February, but mostly in late December – mid- January.

It may be an inner pane in a sealed unit that suffers breakage. This happens when the inner glass is warmed from the heating in the home and the outer pane of a double-glazed sealed unit is cold from a drop in outside temperatures. It may be an outer pane that suffers breakage. This happens when a cold outer pane is warmed quickly by the sunlight bearing on it. The pane of glass subjected to the thermal stress cannot cope beyond a certain amount of deflection, and it will suddenly crack along the whole length of the unit, always along the long length, or it can suddenly shatter altogether.

Now a load of boring stuff, for those of you who are really that interested in double-glazed sealed units.

Double-glazed sealed unit technical considerations

Aspect ratio:
Double-glazed sealed units that are a lot wider than they are high are most at risk. Also, it seems that units facing in certain directions suffer more than others. On squarish sealed units, the glass is able to flex more, in both directions, before it will break. I also have a theory that too much desiccant for the unit’s area may contribute; some manufacturers have a standard practice of filling the two longest spacers with desiccant irrespective of aspect ratio.

Desiccant used in the spacer bar of double-glazed sealed units: 
The desiccant used in the spacer bar of double-glazed sealed units (think of silica gel), i.e. in the bar that holds the two panes of glass apart, is one of several things that may play a part in causing a unit to implode. This is because some desiccants desorb nitrogen when subjected to high temperatures, as would be the case in the 'hot melt' method of manufacturing units. With this method of manufacturing I believe low-deflection desiccant is often not used out of ignorance or penny pinching. The high temperature at which the melted Butyl edge seal is applied will force air out of the unit cavity and, once the edge seal is complete, the desiccant will reabsorb the nitrogen on cooling. Some desiccants absorb more nitrogen than others, adding to the undesirable effect of a negative pressure, often referred to as a 'vacuum' within a sealed unit. Note: The 'twopart' polysulphide edge seal methods do also sometimes suffer the problem of implosion, but I believe not so commonly. With hot-melt manufacture of doubleglazing sealed units it is important to use a 3A desiccant and not a 3A/10A, as 3A does not absorb nitrogen.

Atmospheric pressure and hot weather effects on double glazing:
By this I mean at the actual time of manufacture, in the factory. If, when the double-glazing sealed unit is actually sealed, it is during a period of low atmospheric pressure and high ambient temperature, then, during a cold snap when the external atmospheric pressure is high, the relative pressure inside the sealed unit will fall, producing a greater negative pressure within the sealed unit and more inward deflection. This is why sealed units tend to suffer from the problem of implosion during midwinter, when the differential is at its greatest.

Humidity:
Again, I mean at the actual time of manufacture of the double-glazing sealed unit, in the factory.

The more humid the air at the time of manufacture, the more moisture is held in the air that is sealed up within the cavity of the sealed unit. When the edge seal is complete and the desiccant does its job and absorbs the moisture, this will be yet another factor that could lower the internal pressure within the cavity of the sealed unit.

'Hot melt' - hermetically sealed double-glazing units:
In this method of manufacture, melted sealant is applied to seal the perimeter. When this is done the air adjacent is heated up and therefore expanded. As the perimeter of the seal is completed a certain amount of air will have been expelled from within the cavity. Then, as the sealed unit equalises its temperature, a negative pressure within the cavity of the sealed unit will have been created.

Physical strength of the glass:
If a sealed unit was made up of one pane of 4mm glass and one of 6mm (thicker than normal) glass, then the one pane of 4mm glass would be more flexible and therefore suffer greater deflection and be more at risk of implosion than if both panes were 4mm (normal).

The actual breakage of a double-glazed sealed unit often occurs because of thermal stress: Both panes of a double-glazed sealed unit experience thermally induced stresses, and when either an inner or an outer pane breaks this is caused by the same forces.

In the outer pane thermal stress is caused by solar radiation (heat from the Sun) falling on the double-glazed sealed unit when the outside air is cool. Doubleglazed sealed units located on the south elevation experience their maximum thermal stress in winter. Coloured sealed units (particularly those containing red) are more prone, because of the solar radiation warming up a localised area, more than it would with clear glass. In cool weather and in the absence of the Sun, the outer pane of a double-glazed sealed unit, both edge and centre, is relatively cool. When this cool pane is suddenly bathed in sunlight, the temperature of the centre portion of the pane is raised but the edge, being shaded from direct sunlight, experiences a much smaller temperature rise, producing thermal stress. A light-coloured indoor curtain or blind, particularly if very close to the glass, thus restricting airflow, will reflect the sunlight and produce more heat absorption in the outer pane.

The primary cause of breakage of an inner pane of a double-glazed sealed unit is the same as of an outer pane breakage, only the other way round, because of the difference between the indoor and the outdoor air temperature acting on the sealed unit, causing more stress on the pane than its tensile strength can take. Thermal cracking of the inner pane of a double-glazed sealed unit is usually associated with a very low outdoor temperature and localised heating on the inside pane. Indoor curtains or blinds should not be fitted on the indoor side of an under-window heating radiator, but rather between the radiator and the window, and with enough clearance at the top and bottom to allow a free air exchange between the room and the space behind the curtain or blind. Maintaining a relatively even room temperature, rather than timed heating cycles where the room (and the inner pane of glass) cools right down, could reduce the risk of breakage due to thermal stress on inner panes.

Also see:
Double glazed sealed units - do they have a vacuum