Thermal Breakages

It is important to remember that a sealed unit is just that ! Sealed. Therefore, at the point of sealing the unit, the volume of air trapped inside the unit is fixed. It will expand and contract under the influence of temperature and pressure. When ambient conditions change due to weather, the volume of air will changecausing the sealed unit to exhibit inwards and outwards movement of the glass or spacer or both.

Deflection is a phenomenon common to ALL sealed units. It can manifest itself either as deviation of the glass, resulting in a distorted reflection, as an inward or outward deviation of the spacer bar or in extreme cases the sealed unit may crack - particularly on fanlight size units where decorative products have been applied to one or more of the panes such as bevels, lead or film.

The most common type of deflection reported is inward deflection of the glass or spacer bar. This will be most pronounced on a cold winters day with high barometric pressure on a unit manufactured in a warmer environment (such as a warm factory or during the summer months) with a warm edge seal and low ambient pressure. These conditions give rise to the greatest difference in pressure between the internal volume of air within the unit versus the external pressure on the unit. The greater the pressure on the outside of the unit causes the glass to flex inwards.

In these cases, especially when the daytime sun temperatures are high and the night time temperatures are low, inwards deflection can result in brekage. These tend to occur on decorative fanlights more due to the influence on the surface of the pane of glass with the decorative additions. This is because the pane with the decorative additions cannot flex in the same way as the clear pane and as pressure alters between the inside and outside of the sealed unit, the glass that has reached the full extent of its flexing capability breaks with a characteristic arc.