Building glass skylight.

Originally, building glass skylights are for the purpose of daylighting. Through large-area natural daylighting structures, the use of artificial light sources is reduced, making the building more transparent and bright and bringing a pleasant psychological experience to users. In addition, buildings with glass skylights can improve the quality of the building and give the building noble and luxurious attributes, which also makes glass skylights more and more popular in many high-end buildings.

Glass structure and size of skylight.

The glass of the skylight is a special part of the building envelope. In terms of receiving solar radiation, the horizontal roof receives twice the daily solar radiation of the west-facing side [1]. Intense sunlight passing through a large-area glass skylight will lead to an obvious "greenhouse effect". Therefore, the design of skylight glass not only needs to consider daylighting and safety, but also needs to consider performances such as thermal insulation and sound insulation.

The industry standard JG/T 231-2018 "Building Glass Skylight" stipulates that laminated glass, insulating glass containing laminated glass or vacuum glass containing laminated glass should be used for glass skylights, and the laminated glass should be located on the lower side. The main purpose is to prevent the glass from falling off when it is damaged and ensure the safety of indoor personnel. The glass configurations that meet the standard requirements are shown in Table 1. The function of the skylight is to let in light but not heat. Therefore, shading double-silver or triple-silver Low-E glass is selected for low-emissivity glass to reduce the amount of solar radiation heat entering the room.

Note:

 1. The data in the table is calculated by Window7 software. The boundary conditions are selected according to JGJ 151-2008 standard, and the K value is taken under winter boundary conditions.

2. Meaning of letter codes: T is tempered glass, TL is tempered Low-E glass, A - insulating layer, P: laminated layer, V is vacuum layer.

As mentioned earlier, the daily solar radiation received by a horizontal roof is twice that of the west-facing side. Therefore, skylights have higher requirements for the thermal insulation of glass. As building energy conservation requirements become higher and higher, ordinary laminated glass no longer meets the standard requirements. Instead, laminated insulating glass or laminated vacuum glass with better thermal insulation performance is selected. As can be seen from the data in Table 1, compared with the insulating structure, laminated vacuum glass has better thermal insulation performance.

The size of skylight glass should not be too large. According to JG/T 231-2018 "Building Glass Skylight", the area of skylight glass panels should not be greater than 2.5m², and the length of the long side should not be greater than 2m.

Change of glass K value at different installation angles.

Compared with vertical installation, the heat transfer coefficient K value of insulating glass will increase when installed horizontally or inclinedly, reducing the thermal insulation performance of the entire skylight and increasing building energy consumption. The main reason is that when insulating glass is installed horizontally, the heat convection path becomes shorter and convection accelerates, increasing heat transfer and increasing the heat transfer coefficient. The heat convection paths of insulating glass installed vertically and horizontally are shown in Figures 1 and 2. There is no gas inside vacuum glass, and heat transfer is only by radiation and conduction. The K value remains unchanged at any installation angle.

 Table 2 shows the heat transfer coefficient K values of insulating glass and vacuum glass at different installation angles calculated by using Window7 calculation software.

For vacuum glass with an insulating layer, since the gas convective heat transfer of insulating glass has little influence on the overall heat transfer of composite vacuum glass, the heat transfer coefficient K value remains unchanged at different installation angles.

Note:

1. The data in the table is calculated by Window7 software. The boundary conditions are selected according to JGJ 151-2008 standard, and the K value is taken under winter boundary conditions.

2. Meaning of letter codes: T is tempered glass, TL is tempered Low-E glass, A - insulating layer, P: laminated layer, V is vacuum layer.

3. 0° represents horizontal installation and 90° represents vertical installation.

Conclusion:

1. From the perspective of glass structure, laminated insulating glass is currently widely used. However, the "insulating + vacuum + laminated" structure has a lower heat transfer coefficient K value. Therefore, for both energy-efficient buildings and ultra-low energy consumption buildings, vacuum glass is a more ideal kind of skylight glass.

2. The K value of vacuum glass does not change with the tilt angle and is more suitable for the skylight structure.

References:
[1] Yan Huasheng, Zhang Zhuhui. Analysis of heat transfer characteristics and energy-saving optimization of glass skylights, Silicon Valley [J] 2012(06).