Building energy efficiency includes energy-saving measures for newly constructed buildings (including renovations and expansions) as well as energy upgrades for existing buildings. In China, the total area of existing buildings stands at 40 billion square meters, yet most of these structures are far from energy-efficient.
　　Nationwide, the total area of public buildings stands at approximately 4.5 billion square meters, of which 500 to 600 million square meters are equipped with central air conditioning—primarily in commercial spaces like shopping malls, office buildings, and hotels. Meanwhile, these centrally air-conditioned public buildings, though accounting for just 11% of the existing building stock, consume about 10 times more energy per unit area than typical residential structures, making them significant energy hogs.
　　In some public buildings, the area of doors and windows exceeds 20% of the total floor area, yet energy consumption through these openings accounts for approximately 50% of the building’s overall energy use. Of this, around 75% of the energy loss occurs via glass, making glass—the material comprising about 80% of window areas—the primary contributor to window-related energy losses.
　　There are only two options for energy-efficient glass renovations in existing buildings: either demolish the original glass and replace it with energy-saving glass, or apply a building-grade, heat-insulating, safety film to the existing glass.
        
　　Types and Functions of Energy-Saving Glass
　　1. Coated Glass: Coated glass features one or more layers of metal, alloy, or metal compound applied to the glass surface to enhance its performance. Depending on their specific properties, coated glass can be categorized into heat-reflective glass and low-emissivity glass.
　　Thermal reflective (sun-control) glass typically features one or more thin films—made of metals like chromium, titanium, or stainless steel, or their compounds—coated onto the glass surface. These coatings give the product rich, vibrant colors while maintaining appropriate visible light transmittance. At the same time, they reflect a high percentage of near-infrared radiation and block most ultraviolet rays, making this type of glass also known as sun-control glass. Compared to ordinary glass, thermal reflective glass reduces the shading coefficient, thereby enhancing its ability to block solar heat—but it has minimal impact on the overall heat transfer coefficient.
　　LOW-E (low-emissivity) glass features a multi-layered thin film made of metals like silver, copper, or tin, along with other compounds, coated directly onto the glass surface. This innovative product boasts high visible-light transmittance while reflecting a significant amount of infrared radiation, providing excellent thermal insulation. However, due to the relatively low strength of the coating, LOW-E glass is typically used as part of insulated double- or triple-glazed units rather than being installed on its own.  
　　2. Insulated Glass Units: Insulated glass units consist of two or more panes of glass separated by an aluminum hollow spacer frame and sealed together using adhesive or welding, creating a free space in between. This cavity can be filled with dry air or inert gases, significantly reducing the thermal transmittance coefficient (U-value) compared to single-layer glass, thus providing excellent insulation performance. However, the shading coefficient (SC) decreases only slightly, resulting in minimal improvement in solar radiation heat reflection.
　　3. Composite products of coated glass and insulated glass: These include heat-reflective coated insulated glass and low-E coated insulated glass. The former effectively reduces both the thermal transmittance and solar shading coefficient, while the latter offers superior light transmission. Types and functions of architectural glass films
　　Architectural glass film is made by bonding and pressing high-quality polyester (PET) film with a metallic coating layer using vacuum magnetron sputtering technology, providing excellent sun-control performance for various types of glass. Architectural glass films are broadly categorized into three main types:
　　(1) Architectural Insulated Glass Film
　　Building insulation films primarily aim for energy efficiency, while also offering UV protection and safety features like shatter resistance. These building insulation films are categorized into two main types: heat-reflective films and low-emissivity films.
        
　　Heat-reflective films (also known as solar control films) are applied to glass surfaces, allowing a certain amount of visible light to pass through while reflecting a high percentage of infrared radiation (IR) and minimizing the Solar Heat Gain Coefficient (SHGC). This helps keep indoor temperatures from rising excessively during hot summers, ultimately reducing energy costs associated with air conditioning.
　　Low-emissivity (LOW-E) film—also known as LOW-E coating—allows a certain amount of short-wave solar radiation to pass through, enabling solar heat (near-infrared rays) to enter the room. At the same time, it reflects more than 90% of the long-wave infrared radiation (far-infrared rays) emitted by indoor heat sources, such as heating appliances, back into the room. By effectively utilizing both the short-wave solar radiation from outdoors and the long-wave thermal energy radiated by indoor sources, LOW-E film significantly enhances insulation and energy efficiency in buildings designed for heating in cold climates.
　　(2) Architectural Safety Glass Film
　　Its primary functions are safety against shattering, theft protection, and bullet resistance. This film offers excellent impact resistance (explosion-proof strength), puncture resistance, and UV-blocking capabilities, while maintaining high or even complete transparency.
　　(3) Decorative Glass Film
　　This type of film primarily serves a decorative purpose, coming in a wide variety of styles—such as basic colors, semi-transparent, opaque options, and designs featuring diverse geometric patterns. Opaque films include one-way vision options, while semi-transparent films resemble frosted glass. Compared to traditional methods like manufacturing frosted or etched glass, these films offer lasting durability, quick installation, and lower costs. Plus, they allow you to easily update the decorative pattern anytime, anywhere, according to your personal preferences.
　　The Ministry of Construction and the National Development and Reform Commission have stipulated that safety glass must be used in 11 specific areas of buildings where glass is specified as a construction material.
　　Non-safety glass in existing buildings can be upgraded to safety glass after applying a film, and laminated glass offers superior safety compared to tempered glass, as it prevents glass fragments from scattering and causing injury in the event of breakage. A cost comparison between replacing windows with energy-efficient glass and applying heat-insulating films is also provided.
　　Case: A certain exhibition center's lobby features a stick-system curtain wall with 12mm tempered clear glass. If this is replaced with 12mm heat-reflective coated tempered glass, the material cost would be 210 yuan per square meter, while the removal and installation of the old glass would add another 70 yuan per square meter, bringing the total renovation cost to 280 yuan per square meter. Alternatively, replacing it with two panes of 10mm-thick 12mm LOW-E insulated glass units would increase the material cost to 375 yuan per square meter, plus the 70 yuan per square meter for removal and installation, resulting in a total cost of 445 yuan per square meter.
　　By applying a heat-reflective film, the material cost is 100–130 yuan per square meter, while the installation fee is 30 yuan per square meter, totaling 130–160 yuan per square meter. Notably, the cost of installing the heat-reflective film is approximately 50% of the cost of replacing it with heat-reflective glass.
　　Applying both a heat-reflective film and a low-emissivity (LOW-E) film to the original glass can achieve energy-saving performance nearly equivalent to that of LOW-E insulated glass. Here are six advantages of retrofitting glass with window films:
          
　　In the energy-efficient renovation of building doors, windows, and glass curtain walls, existing glass films offer the following six advantages:
　　(1) More cost-effective. Applying a heat-reflective film to existing tempered transparent or tinted glass saves 50% of the total cost compared to replacing it with new heat-reflective glass—and this principle applies similarly in other scenarios. For example, upgrading standard insulated glass units by adding a heat-reflective or low-E coating transforms them into high-performance insulated glass with heat-reflective or low-E coatings, respectively.
　　(2) Faster. Applying glass film takes less time than removing the old glass and installing new ones, making it a simpler and more efficient process.
　　(3) More environmentally friendly. Replacing glass generates significant amounts of construction waste in the form of broken glass, which increases transportation and landfill costs. In contrast, applying window film allows you to reuse existing glass, enhancing its insulation and safety features. It also helps prevent fading of indoor carpets, curtains, fabrics, and paint, effectively protecting furniture, office equipment like computers, and extending the lifespan of these items.
　　(4) More secure. Standard window films also offer a degree of safety enhancement, featuring the ability to hold shattered glass fragments together—similar to laminated glass. In fact, their safety performance surpasses that of tempered glass, with professional-grade security films providing even greater protection.
　　(5) Healthier. The adhesive used in glass window films contains UV (ultraviolet) absorbers that block 98%-99% of ultraviolet rays.
　　(6) Lighter weight. Replacing the glass with hollow or hollow LOW-E glass—where the total weight of the two panes plus the air space doubles or even increases further—can significantly raise the building's structural load.