Multiple choices and considerations for glass in glass curtain wall systems

Glass curtain wall, a beautiful and innovative way of decorating building walls, with its lightweight design and crystal clear transmission and reflection properties, not only brings a bright and comfortable indoor environment to the building, but also achieves a harmonious integration of the interior and exterior spaces. Therefore, it quickly became the preferred choice for various types of building facade designs, displaying a strong visual impact, deeply loved by owners and architects, and has become an indispensable iconic element of modern urban architecture. However, with the widespread application of glass curtain walls in China, their negative impacts have gradually emerged, such as light pollution, increased building energy consumption, and intensified greenhouse effect. The root causes of these problems are closely related to the choice of glass.

Many advantages of glass curtain walls

Firstly, glass curtain walls stand out due to their lightweight and aesthetically pleasing characteristics. It not only adds luster and beauty to the building, but also often uses aluminum alloy or other metals to make the skeleton, combined with glass and enclosed metal hollow rods, to create a lightweight enclosure wall. This type of wall has an extremely light weight, weighing only about 50 kilograms per square meter, greatly reducing the burden on the building.

Secondly, glass curtain walls also perform well in terms of energy efficiency. Modern high-rise buildings often use insulated glass that combines coated glass or Low-E coated glass with tempered glass. The partition is filled with dry air and has a transmittance of over 60% for solar radiation of 0.3-2.5um. This means that during the day, most of the outdoor radiation energy can pass through the glass curtain wall, providing ample natural light indoors. At night or on rainy days, more than 50% of the thermal radiation from indoor objects is reflected back into the room, and only a small portion is absorbed and dissipated through re radiation and convective exchange, effectively preventing the leakage of indoor heat to the outside. This design not only overcomes the shortcomings of traditional glass in terms of strong heat transfer and high thermal transmittance, but also achieves excellent shading and energy-saving effects.

The following text will delve into the key points for selecting glass curtain walls from four aspects: energy-saving performance, safety performance, light pollution performance, and appearance effect. Firstly, in terms of building energy efficiency, although glass curtain walls can reduce the extensive use of steel bars and concrete in traditional concrete exterior walls, thereby saving energy and resources, their insulation and thermal insulation performance still has a significant gap compared to traditional walls, with a heat loss rate as high as 5 to 6 times. Therefore, in the energy-saving design of public buildings, curtain wall energy-saving is particularly important, accounting for about 40% of the total building energy consumption.

From the perspective of controlling heat gain and transfer, the key to energy efficiency of glass curtain walls lies in the selection of glass. Energy saving glass needs to have two major characteristics: insulation and heat preservation, which are closely related to visible light transmittance and reflectance. At present, glass types that meet both "insulation" and "thermal insulation" requirements include low emissivity (Low-E) insulated glass, multi insulated glass, as well as Low-E vacuum glass and composite vacuum glass. The thermal energy of sunlight is mainly composed of visible light energy (short wave heat) and invisible light energy (long wave heat, i.e. infrared), of which visible light energy accounts for 46%, infrared energy accounts for 52%, and another 2% is ultraviolet light. The unique feature of Low-E coated glass is that it allows visible light to pass through while reflecting invisible light back, effectively blocking the transmission of thermal radiation. In summer, this type of glass allows visible light (sunlight) to enter while blocking strong invisible light energy (infrared radiation heat) emitted by asphalt roads, buildings, etc., helping homeowners save on air conditioning investment and indoor air conditioning costs. In winter, it allows visible light energy to enter while reflecting invisible light energy (infrared radiation heat) back into the room, reducing energy loss through glass, effectively maintaining indoor temperature, reducing winter heating costs, and achieving energy-saving effects. In addition, the thermal insulation performance of insulating glass benefits from its sealed air layer. Due to the excellent thermal resistance of air, the heat transfer coefficient K of insulating glass is closely related to the thickness of its air layer. When the thickness of the air layer is less than 13mm, the K value decreases with increasing thickness; When the thickness approaches 13mm, the minimum limit is reached; Afterwards, the K value increases with the increase of thickness, mainly due to the formation of closed-loop convection in the air layer, which leads to an increase in heat transfer. Therefore, the air layer thickness of insulating glass should generally be selected as 12mm. If further reduction of K value is needed, inert gases such as argon can be filled into the insulating glass to suppress the closed-loop convection of air; Alternatively, the method of increasing the number of air layers can be adopted, such as double-layer or multi-layer insulating glass, or by sandwiching a transparent film in the air layer and dividing it into several independent areas through decorative partitions, in order to reduce the impact of convective heat transfer on the insulation properties of the glass.

The selection of curtain wall glass should consider preventing light pollution

Light pollution refers to the adverse effects of excessive light radiation on human living and production environments, covering pollution sources such as visible light, infrared radiation, and ultraviolet radiation. The widespread use of glass curtain walls has endowed modern buildings with unique charm, but their reflected light has brought many troubles to surrounding residents. The glass curtain walls of luxury office buildings, large commercial buildings, and star rated hotels, which reflect and gather light over a large area, pose a potential threat to human health. In the scorching summer, these reflected lights cause a sharp rise in indoor temperature and even trigger fires. At the same time, the reflected light from the street facing glass curtain wall also interferes with vehicle and pedestrian traffic, blurs the road surface and traffic signals, and increases the risk of traffic accidents.

In order to curb the light pollution caused by reflected light from glass curtain walls to the surrounding environment, the country has set corresponding standards for the proportion of glass curtain wall area and visible light reflectivity. For example, Shanghai stipulates that the area of glass curtain walls should not exceed 40% of the total wall area, and the reflectivity of visible light should also be controlled within 15%; Hangzhou limits the area of glass curtain walls to no more than 60%, and the reflectivity must not exceed 16%. These standards aim to limit the impact of glass curtain walls on the surrounding environment due to light reflection.

The key to solving light pollution in glass curtain walls is to control the reflectivity of the glass. Because the higher the direct reflectance of sunlight, the more sunlight is obtained outdoors, which in turn affects and pollutes the outdoor light and thermal environment. Therefore, when selecting curtain wall glass, priority should be given to glass varieties with low reflectivity. In addition, for outdoor light and heat environments, a lower reflectance is more beneficial. According to the test results, the reflectance of various types of glass is ranked as follows: coated glass, white glass, coated glass, Low-E glass, and laminated glass. Among them, the visible light reflectance of laminated glass and coated glass is relatively high, reaching 48.68% and 34.87% respectively. However, the application of glass in architecture is relatively limited. Therefore, in practical selection, we tend to use Low-E glass and coated glass, or combine them with ordinary white glass to make insulated glass or vacuum glass. This choice can effectively reduce the light reflection pollution of glass curtain walls to the surrounding environment, thereby meeting relevant national standards.

Considering the selection of curtain wall glass from the perspectives of safety and fire prevention

High rise buildings often use glass curtain walls in pursuit of lightness and aesthetics, but this also brings many safety hazards, especially the damage and falling of curtain wall glass. For example, in 2007, a glass explosion occurred in a square in Beijing, where flying glass hit pedestrians and damaged the glass roof of a subway station.

In order to ensure the safe use of glass curtain walls, safety glass should be given priority in material selection. In the past, tempered glass was a commonly used choice, but it carries the risk of self explosion, especially in high-rise curtain walls where the danger is self-evident. The cause of self explosion is often related to the impurities of nickel sulfide in the glass, which come from mineral raw materials. In contrast, ultra white glass has significant advantages. It uses high-purity chemical raw materials such as aluminum hydroxide, calcium carbonate, magnesium carbonate, etc., thus completely avoiding the presence of nickel sulfide and reducing the possibility of self explosion. In addition, ultra white glass is also known as low iron glass, which further reduces the self explosion rate. Practice has proven that the self explosion rate of ultra white glass is only 0.03%, far lower than the 0.3% of tempered glass.

In addition, according to the building fire prevention regulations, glass curtain walls must meet certain fire prevention requirements. The fire resistance limit of the exterior wall should not be less than 1 hour. Therefore, it is recommended to use composite fire-resistant glass, which has excellent fire resistance and a fire resistance limit of 1.0 to 3.0 hours, making it very suitable for use in high-rise buildings.

Half tempered glass does not self destruct due to its lower surface stress, and after heat strengthening treatment, its strength reaches twice that of ordinary float glass. It has excellent temperature resistance and impact resistance, which can withstand sudden temperature changes and human impacts. Even if it breaks, it is not easy to fall off, ensuring safety.

Selection of Curtain Wall Glass and Architectural Appearance Effect

If the appearance of the building is crucial, it is recommended to use ultra white hollow laminated glass. This type of glass is composed of three pieces of glass, with a single piece of glass on the inside, an air layer in the middle, and double laminated glass on the outside. The overall effect is uniform and beautiful. The transmittance of ultra white glass is over 98%, with an iron content of less than 100ppm, presenting an ultra transparent effect. At the same time, it has excellent physical, mechanical, and optical properties, and can be processed in various ways.

Based on the current era, sustainable development has become a core global development concept, and building energy efficiency is an inevitable trend for the long-term development of the country. Green ecological buildings have therefore received widespread attention from all sectors of society. Although glass curtain walls were controversial in the early stages due to energy consumption issues, after technological innovation, glass curtain walls have gradually taken a place in the field of sustainable development, demonstrating a new trend of ecological intelligence and highly compatible with the concept of sustainable development.

In urban construction, energy-saving, environmentally friendly, safe, efficient, and aesthetically pleasing buildings are increasingly becoming a necessity, and the selection of glass materials is particularly crucial. According to personal needs and cost control, suitable building materials can be selected, but they must meet the energy-saving and safety standards required by regulations. Meanwhile, pursuing high quality, aesthetics, and safety are also optional goals. Against the backdrop of promoting green buildings and energy conservation and environmental protection in the country, these efforts are particularly important and in line with the trend of the times.