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 it is very puzzling to the general public, but also because it is not very widely understood in our industry either.
And here's why double glazed sealed units implode:
This sometimes quite shocking occurrence of double glazed sealed units exploding most often happens in the U.K. between around late October, and early February, but mostly in late December - middle January.
When an inner pane in a sealed unit is the one to suffer 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. When an outer pane is the one to suffer 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.
Other contributing factors to double glazed sealed units breaking in this way can be: (1) Very high atmospheric pressure at the time and (2) An 'as manufactured' original high inbuilt negative pressure within the sealed unit. (3) One sealed unit laboratory recreated the effect of the glass surface tension being damaged by being scratched when the lead/coloured film/bevels were being cut around with a blade. If the glass is scratched in this way during manufacture it can be a weak point when the glass is under stress and deflection.
Now a load of boring stuff, for if you are really that interested in double glazed sealed units.
Double glazed sealed unit technical considerations:
Double glazed sealed units that are relatively a lot wider in size than their height are most at risk. Also it seems that units facing certain directions suffer more than others. On sealed units that a lot squarer, the glass is able to flex more, and in both directions, before it would break. I also have the theory that too much desiccant for the units area may contribute when a manufacturer has the 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) which goes within the spacer bar which holds the two panes of glass apart, can be just one of the several things that may play it's own destructive part in a unit which suffers implosion. This is because some desiccants desorb nitrogen when subjected to high temperatures, as would be the case during manufacture of the 'hot melt' method of making units. With this method of manufacturing double glazed sealed units 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 effectively force air out of the unit cavity and, once the edge seal is complete, the desiccant will re-absorb 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 'two-part' polysulphide edge seal methods do also sometimes suffer the problem of implosion, but I believe not so commonly. With hot-melt manufacture of double glazing 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:
For 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 lower, 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 mid Winter, when the differential is at it's greatest.
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 of the double glazing sealed unit, 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 it's job and absorbs the moisture, this will be yet another factor that would add to the decrease in internal pressure with the cavity of the sealed unit.
'Hot Melt' - Hermetically Double Glazing Sealed Units:
As the melted sealant is applied to seal the perimeter, 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 it's temperature, a negative pressure within the cavity of the sealed unit will have been created as a natural by-product of this method of manufacture.
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 due to thermal stress:
Both panes of a double glazed sealed unit experience thermally induced stresses, and when either an inner or outer pane breaks this is caused by the same conditions.
In the outer pane thermal stress is caused by solar radiation (heat from the Sun) falling on the double glazed sealed unit when the outdoor air is cool. Double glazed sealed units located on the south elevation experience their maximum thermal stress in Winter. Coloured sealed units (particularly with red in) are more prone, because of the solar radiation warming up a localised area, and 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, and even more so if very close to the glass restricting airflow, will reflect the sunlight and produce more heat absorption in the outer pane.
The primary cause of an inner pane of a double glazed sealed unit breakage is the same as an outer pane breakage, only the other way round, because of the difference between the indoor-outdoor air temperature acting on the sealed unit, causing more stress on the pane than it's 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 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.
It is not usually one, but rather it is a combination of the above factors that cause a sealed unit to implode. If you have a recurring problem with sealed units imploding then I would suggest (1) Enquire if the sealed unit manufacturer understands what low deflection desiccant is, and do they use it, or feel that they should be using it, (2) Consider having the replacement unit made of thicker glass, i.e. both panes of 6mm glass instead of the normal 4mm, (3) Think about your room heating cycles, and (4) How close the main curtains are to the window, and if air can circulate behind them.