Energy loss through Windows and doors accounts for about 50% of the building, of which energy loss through glass accounts for about 75%. Under certain conditions, the heat radiation and conduction of glass is the dominant factor leading to the indoor energy loss.

The door and window is the weakest link of heat insulation, heat preservation, sound insulation and safety in the whole building envelope. There are roughly three ways for heat loss from doors and Windows:

1. Heat conduction between door and window frame fan and glass;

2. Air penetration heat exchange between door and window frame fan, between fan and glass, and between frame and wall;

3. Thermal radiation of window glass.

According to relevant data, the energy loss through Windows and doors accounts for about 50% of the building, among which the energy loss through glass accounts for about 75% of the doors and Windows. Under certain conditions, the heat radiation and conduction of glass is the dominant factor leading to the indoor energy loss.

As the heat transfer performance of plastic is 1/1250 of aluminum, the heat transfer coefficient of single glass window is 4.3~4.6%W/M2K, which is equivalent to the heat transfer effect of double glass window, indicating that the role of window frame fan material in window heat transfer cannot be underestimated, but the proportion of frame fan material in the heat transfer area of the whole window is small after all. The heat transfer coefficient of single glass window only reaches the performance index of energy-saving doors and Windows in some areas of south China. There is a larger gap between the 2.8W/M2K index of heat transfer coefficient of Windows and doors and the 4.0-2.0% W/M2K index of different heating areas, which is far from realizing the 65% energy saving policy of Beijing. Therefore, with the continuous development of national building energy conservation work, the use of energy-saving glass, effectively improve the thermal performance of Windows and doors will become the main target of energy-saving doors and Windows.

In recent years, the variety of glass available for selection has increased day by day, has developed from the past traditional daylighting, wind, rain, enclosure functions to modern heat insulation, heat preservation, safety, noise prevention, decoration and other composite functions. The main glass used for energy saving are heat-absorbing glass, coated glass, double (or three) glass, insulating glass, vacuum glass, etc. Coated glass is divided into anti-radiation glass, low-radiation glass and multifunctional coated glass.

Glass is a non-metallic material, although its heat transfer coefficient is only 0.8~1.0W/M·K, because the general thickness of glass is 3~8mm, its own thermal resistance is very small, for about 65~75% of the window area, heat transfer is still very considerable. Glass energy saving is mainly reflected in heat preservation and heat insulation: heat preservation is to control the loss of indoor heat energy to the outdoor by reducing the characteristics of glass heat conduction and convection. Such as using insulating glass, vacuum glass, etc.; Heat insulation is to improve the glass to the solar thermal radiation shielding characteristics, control the outdoor heat conduction to the indoor. Such as the use of endothermic, thermal reflection, low radiation glass. The above glass properties are briefly introduced:

1. Double (or triple) glass

Insulating glass is developed from two (or three) layers of glass

3. Heat-absorbing glass

4. Heat-reflecting glass

5. Low radiation glass and multifunctional coated glass

Low radiation glass (ILE) and multi-function coated glass (IMF), also known as thermal insulation glass, have the maximum daylight transmittance and the minimum reflection coefficient. 80% visible light can enter the chamber and be absorbed by the object, and more than 90% long wave radiated by the indoor object can be kept in the chamber, greatly improving the energy utilization rate. At the same time, it can selectively transfer solar energy and transfer most of the heat radiation into the room, which can play the role of heat preservation and energy saving in the heating building. Compared with ILE glass, IMF glass has the same principle of heat conduction control, but is superior to ILE glass in reducing heat input. Additional low radiation glass and muti-function glass pass through the sunlight of different spectrum to have selectivity, can take out ultraviolet ray, avoid indoor furniture, picture, artwork because of ultraviolet ray illuminate and fade, still can absorb part visible light, rise to prevent dazzle light effect.