Thermal safety of glass should be assessed considering the amount of radiation incident on the surface and the thermal capabilities of the glass. For example, the solar radiation intensity on the glass surface should be determined along with the air temperature range applicable to the location of the building. These measurements, together with the energy transfer coefficients and the glass absorption allow determination of the appropriate basic temperature difference between the central area of the glass and its edge. The difference is related to the thermal stress and then modified for the type of glazing system, taking account of extraneous effects resulting from curtains,
| Sl No. | Clear glass Thickness and Type mm | Cavity Width mm | U-Value W/m2K | |
| Air (4) | Argon (5) | |||
| (1) | (2) | (3) | ||
| i) | 4 + 4 | 6 | 3.3 | 3.1 |
| 12 | 2.9 | 2.7 | ||
| 16 | 2.7 | 2.6 | ||
| 20 | 2.8 | 2.6 | ||
| ii) | 4 + 4 Low E ( ) | 6 | 2.7 | 2.3 |
| 12 | 1.9 | 1.6 | ||
| 16 | 1.7 | 1.5 | ||
| 20 | 1.7 | 1.5 | ||
| iii) | 4 + 4 Low E ( ) | 6 | 2.5 | 2.0 |
| 12 | 1.6 | 1.3 | ||
| 16 | 1.4 | 1.2 | ||
| 20 | 1.4 | 1.2 | ||
| iv) | 4 + 4 + 4 | 6 | 2.4 | 2.1 |
| 12 | 1.9 | 1.8 | ||
| 16 | 1.8 | 1.7 | ||
| 20 | 1.8 | 1.7 | ||
| v) | 4 Low E ( ) + 4 + 4 Low E ( ) | 6 | 1.7 | 1.4 |
| 12 | 1.7 | 1.0 | ||
| 16 | 1.0 | 0.8 | ||
| 20 | 0.9 | 0.8 | ||
| vi) | 4 Low E ( ) + 4 + 4 Low E ( ) | 6 | 1.6 | 1.2 |
| 12 | 1.0 | 0.7 | ||
| 16 | 0.8 | 0.6 | ||
| 20 | 0.7 | 0.6 | ||
| NOTES | ||||
| 1 U-values for argon gas-filled cavity based on 90 percent argon/10 percent air. | ||||
| 2 is the declared (normal) emissivity. | ||||
| Sl No. | Clear Glass Thickness and Type mm (1) | Separation mm (3) | U-Value W/m2K (4) |
| i) | 4 + 4 | 25 | 2.8 |
| 75 | 2.8 | ||
| ii) | 4 + 4 Low E | 25 | 1.7 |
| 75 | 1.9 | ||
| NOTE — is the declared (normal) emissivity. | |||
blinds, back-up walls, proximity to heaters, etc, to derive a stress of actual service conditions.
1 High air temperatures, low rates of air movement, and the insulation provided by curtains, blinds, back-up walls and multiple glazing tend to reduce the loss of energy and uphold the centre temperature. Low temperatures at the edges are maintained by conduction from the glass through the frame to a cold building structure with a large thermal capacity.
2 Advice may be sought from the manufacturer of glass as to the methods for assessing the thermal safety of the glass.
The resultant service stress should then be compared with the design stress for the glass. If on comparison, the service stress is less than or equal to the design stress, the glass and glazing system may be accepted as thermally safe provided, that the edges of the glass are of adequate quality.
NOTE — Where the application of a solar control film is being considered to existing glazing, advice should be sought from the manufacturer on the effect of any additional thermal stress likely to be induced in the glass.
The normal mode of thermal breakage of glass is by the action of tensile stress located in and parallel to an edge, and so the breaking stress of the glass is mainly dependent on the extent and position of flaws in the edges. The condition of the glass edge is therefore extremely important.
Solar control glasses should not be nipped to size and any panes with shelled or vented edges should not be accepted for glazing in orientations subject to direct sunlight. Although a wheel-cut edge is the most satisfactory, laminated glasses with worked edges may be used.
Where clean-cut edges are not permitted, arrises should be created by a wet process, working parallel to the edge and not across the thickness, and the design implications of such an action should be examined.
Where solar control glasses are to be used in sliding doors and windows there is always the possibility that, when opened during sunny periods, the overlapping will function as double glazing with little ventilation in the air space, and it is this condition that should be assumed in assessing the thermal safety of glass.
Thermal safety assessment is based on the behaviour of glass in good condition and properly glazed. Even if the glass is shown to be thermally safe, this depends on close adherence to the recommend glazing procedures. All necessary precautions should be taken to see that only glass with edges of an acceptable condition is used. The glass should be stored and handled so that no contact with hard bodies can damage the edges and each pane or insulating glass unit should be carefully examined immediately before glazing.
5.1.6 Energy Conservation
Good window design can, by reducing reliance on artificial lighting, can be one of the largest single means of saving energy and should be carefully considered.
Account should be taken of shape of the room, window orientation, occupancy patterns and task, together with the relationship of windows to surrounding buildings and other obstruction. As with task lighting, possible
solar overheating might result and similar precautions should be taken.
5.1.7 Light Transmission Properties of Window
Light transmission is defined as the fraction of visible light at normal incidence transmitted through the glazing. For properties of plastic glazing sheet materials, manufacturers may be consulted.
Dirt on glazing reduces the light transmission, often by an appreciable extent before becoming noticeable. To ensure day lighting levels are adequate, an allowance for the reduced light transmission should be made in day lighting calculation by introducing a ‘dirt factor’ between 0.7 and 1.0. Regular cleaning of glass and plastic glazing sheet materials therefore call their importance.