The main function of building doors and windows is lighting and warmth, with the improvement of people's living standards, residential users are more inclined to use large-area glass doors and windows, so that the indoor can get full natural lighting and outdoor open vision during the day. However, the insulation performance of doors and windows is the worst compared to the four major envelopes of walls, roofs and grounds, and the energy consumption of doors and windows accounts for about 40%~50% of the total energy consumption of the building envelope. In order to maintain a comfortable temperature indoors, it is only possible to use a lot of energy, such as air conditioning in summer and heating in winter.

In order to solve the problem of glass energy loss, the industry has developed Low Emissivity Glass, which has become one of the main products in the glass industry today. If the double silver Low-E glass with emissivity of e = 0.04 is selected in the insulating glass, its heat transfer coefficient can reach 1.3~1.1 after being filled with inert gas, which is equivalent to the thermal insulation performance of a 370mm thick brick wall, which can not only increase the lighting area, but also reduce energy consumption. We call this kind of glass with excellent thermal insulation, heat preservation and lighting performance, and effectively reduce energy consumption called energy-saving glass, so what indicators are the key energy-saving indicators of energy-saving glass?

(1) Visible light transmittance TV

For the most basic function of glass windows, "lighting", that is, the ability of glass to transmit visible light, it is measured by the transmission ratio of visible light to TV. The industry standard JGJ151-2008 defines the visible light transmittance as the ratio of the visible light flux of the standard light source to the visible light flux projected onto the glass, doors, windows or glass curtain wall by weighting the human visual function. The greater the visible light transmittance, the better the indoor lighting effect.

(2) Heat transfer coefficient K value (or U value)

For the second major function of glass windows, "heat preservation", that is, the ability to block heat transfer from temperature differences, measured by the heat transfer coefficient K value. The industry standard JGJ151-2008 defines the heat transfer coefficient as the heat passing through doors, windows or glass curtain walls per unit area per unit time when the ambient temperature difference between the two sides is 1K (°C). The smaller the heat transfer coefficient, the better the thermal insulation performance.

(3) The total transmittance of solar infrared heat energy gIR

The third function of glass windows is "heat insulation", that is, the ability of glass to block solar radiant heat, which is measured by the total transmission ratio of solar infrared heat energy gIR. In the latest monogram HB002-2014, gIR is defined as the total solar transmittance in the wavelength range of 780nm~2500nm. The smaller the gIR value, the stronger the ability of the glass to block solar radiant heat.

So what is the total solar transmittance (g-value for short, also known as the solar heat gain coefficient SHGC)? In the industry standard JGJ151-2008, it is called "total sunlight transmittance", which is defined as: the ratio of the solar radiation part that becomes the heat gain of the room through glass doors, windows or glass curtain walls to the solar radiation illuminance projected on glass, doors and windows or glass curtain wall components. The part of solar radiation that becomes indoor heat gain includes: the heat gain transmitted by solar radiation through radiation; The solar radiation is absorbed by the components and then transferred into the room. The g-value characterizes the heat-gaining capacity of the glass, and the higher the g-value, the stronger the heat-gaining.

來源：城市幕墻、新玻網

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