The value of thermal stress depends on the temperature difference between the cold and warm part of the glass pane and also on temperature distribution in the glass pane. Normally a glass pane is glazed so that the edges of a glass pane are covered with the window profile. The uncovered part of the glass pane remains under the impact of thermal radiation and absorbs heat, the temperature of the glass pane rises and spreads all over the glass pane. The edges of the glass pane that are protected against thermal radiation remain cooler. Distribution of different temperatures in the glass pane creates thermal stress in the edges of the glass pane and if this exceeds the breaking force, thermal breakage will take place. The risk of thermal breakage is the highest in body tinted glasses.
Factors increasing thermal stress:
Intensive solar radiation
Intensity of solar radiation on the insulating glass unit is determined based on the geographical location of the building, season, cloudiness, air pollution and reflection from the ground or neighbouring buildings.
High absorption coefficient and thermal radiation caused by solar energy
The most important property of a glass pane responsible for generating stress is absorption. The higher is the thermal radiation absorption capacity of a glass pane (it is the biggest in body tinted glass panes), the higher is the temperature of the glass pane and as a result is will increase thermal stress.
Environment or materials behind the glass unit
Regardless of the way thermal radiation reflects from a room back on the glass pane, it is still partially absorbed in the glass pane. It is especially noticeable when there is some insulation, a pent or piping of the heating system close to the glass pane and where air circulation is not sufficient either.
Condition of the edges of a glass pane
The condition of the edges of the glass pane is very important, since the tensile strength of a glass pane depends on the cracks on the edges of the glass pane and expansion of these. The fewer micro-cracks there are on the edges of the glass pane, the more durable it is. Grinding of the edges of a glass pane will increase the durability.
Major and long-term shades on the glass may cause big temperature fluctuations on the glass. Thermal stress may occur in the glass pane when a glass pane is partially covered from the sun by other buildings or some other way (houses, sun-blinds, balconies, window jambs, etc.).
Some window blinds and shadows inside the room may have an impact on air moves over the glass pane. If the glazed surface of the window is partially covered, the temperature of the covered and uncovered glazed surface may be different. This can lead to self-breaking of the glass pane.
Internal heating sources
The sun is the primary energy source increasing thermal stress. Secondary sources like convection or heat emitting elements may increase thermal stress, especially if radiation is directed straight on the glass pane.
Films controlling solar energy
Attaching films on the glass panes increases absorption of solar energy in the glass panes and thus also thermal stress. Before covering the glass pane with a film we recommend toughening the glass, since it helps to reduce the risk of breaking the glass pane.
To reduce thermal stress insulating glass units, especially tinted, reflective and wired glass panes may not be stored in the direct sunlight before glazing. Place soft plugs between the glass panes, so that air can circulate between the glass panes. Avoid damaging the edges of the glass panes. Do not cover glass panes partially at the construction site or cover with any cover material since different temperatures generated on the glass surface may lead to self-breaking of the glass pane. Glass panes must be entirely and very carefully covered. Do not glue anything on the surface of un-toughened glass, since this may increase thermal radiation and stresses caused by expansion of the insulating glass unit due to temperature fluctuations. This may lead to self-breaking of the glass pane.
In rare cases the internal glass pane of the insulating glass unit may self-break in cold climate. The reason of this is that at night the temperature of the window profile gets very low, which in its turn cools the edges of the glass pane, whereas the central part of the glass pane is influenced by room temperature and remains warm.
When manufacturing the insulating glass units also the physical parameters of different glass panes should be observed. This is to avoid thermal stresses in the glass panes. If required, glass panes should be toughened. For instance if an insulating glass unit consists of a selective glass pane installed inside and Pilkington Optifloat body tinted glass pane installed outside, the body tinted glass must be toughened, since thermal radiation coming from outside is partially absorbed in the body tinted glass. But the surface coating of the selective glass pane reflects thermal radiation and reflects back the thermal radiation coming through the body tinted glass pane. As a result the surface of the glass pane is imposed to different temperatures and the result is thermal breakage of the glass pane.