How thermal cracks occur
When sunlight falls directly on an area of glass, the solar hear warms up the exposed glass area and expands. On the other hand, the shaded area covered by the sash or any shading devices does not warm up and remains cool. This cool part will resist the expansion of the warmer part and this leads to tensile stress around the edges of the glass. the proportion of thermal stress corresponds to the temperature differences between the incident light area and the shaded part. And if this tensile stress is greater than the strength of glass edge, thermal cracks will occur.
Characteristics of Thermal Cracks
Thermal cracks usually develop when severe states of sash structure, shading pattern and methods of use occur at the same time.
Generally, thermal cracks starts with a straight perpendicular line from the edge, and as it moves towards the center of the glass, the line develops into an undulating snake-like pattern.
Sealant
Use an elastic silicone sealant because putty is poor at channeling heat and poor at resisting moisture.
Fixing and structure of Sash and curtain walls
Allow over 4mm of clearance between glass and sash.
Concrete possesses a high heat capacity and even when exposed to the sun there is very little temperature change. If the sash is in direct contact with the concrete, the temperature of the sash remains relatively low. The rising temperature at the center of the glass will lead to thermal cracks.
Precautions on use
- Avoid placing thick curtains or blinds very near the windows, as these obstruct heat dissipation absorbed by the glass.
- Prevent cool air of air-condition or strong light of lamp from directly striking the glass surface as this will lead to a drastic temperature difference between the center and the glass edges.
- Avoid sticking decorative paper or painting the glass surface as it will drastically increase the absorption rate and raise the temperature of that specific area of the glass.
- Avoid installing a reflective film. It will reflect the transmitted rays back into the glass and will raise the glass temperature.
Preventive measures to avoid Thermal Cracks
Ensure that there is no great temperature difference occurring between the central area of the glass and its edge.
When cutting and installing the glass, care should be taken to ensure that there is no weal spot along the edges.
Types of Glass
Heat Absorbing Glass
Because of its immense solar radiation absorption capabilities, heat absorbing glass becomes very hot in the center and can incur large hear stresses. Therefore, the glass must be thermally insulated from the metal sash.
Wired Glass
The allowable stress of the wired glass edges is about half of float glass because it is difficult to achieve a clean cut for wired glass. Also there is a significant difference between the coefficient of thermal expansion of the wire mesh and that of the glass. Lastly, it should be rain-watered or dew seeped into the sash, rust will develop easily and this leads to further weakening, thus watertight and anti-rust measures should be adopted in the sash. If hear absorbing wired glass is used, the severest conditions in thermal cracks will occur, thus every caution must be exercised during application.
