# Optimizing the Performance of Window Frames: A Comprehensive Review of Materials in China
## 1 Introduction  
The escalating global environmental crisis is increasingly pronounced, with the construction industry consuming 50% of the world's electricity and contributing to 38% of global carbon emissions [1, 2]. Figure 1 illustrates the environmental impact of windows, one of the building elements, and their frame materials throughout their life cycle. Typically, windows only occupy 1/8 to 1/6 of the building envelope area, but their heat dissipation can reach approximately 40-50% of the total building envelope [3]. Simultaneously, the energy content of the materials involved and their environmental impact accentuate the significance of windows in the ecological context of construction. The improvement of energy-efficient and eco-friendly windows can directly reduce environmental impacts. Therefore, minimizing the ecological burden throughout their entire life cycle is crucial for achieving a general level of sustainable development, particularly within the realm of buildings.  
One of the most crucial elements of a window is its frame, which not only ensures the structural integrity of the window system but also plays a significant role in energy efficiency and environmental conservation. Over the past several decades, the design of window frames has undergone significant innovations, ranging from the introduction of new materials to novel design concepts. Wooden frames are commonly used in residentialconstruction. It is well-known for its high strength and relatively good thermal performance; however, wooden frames are prone to cracking, warping, and moisture damage, leading modern frame designs to explore alternative materials.  
Due to its low material cost, light weight, and high strength, aluminum frames have become prevalent. Aluminum window frames dominate the market in high-rise buildings, as these structures require more robust frames. However, aluminum has high thermal conductivity, making it the least thermally efficient among commonly used frame materials. Another option is polymer frames, typically made from polyvinyl chloride (PVC), unplasticized polyvinyl chloride (UPVC), or glass fiber-reinforced polyurethane (GFRP). However, they have limitations when it comes to size and weight since they are not inherently sturdy. GFRP, with a strength eight times that of PVC, offers advantages in terms of moisture resistance, corrosion resistance, and resistance to UV degradation. Nevertheless, GFRP thermal conductivity is nearly twice that of wood and PVC [4]. From this, it is evident that there is no perfect window frame material. High-performance frames often explore composite material options, such as vinyl-clad aluminum or aluminum-clad wood.  
The work of this review addresses issues arising from the use of modern window frame materials. It analyzes improvement methods employed by researchers from the past to the present and conducts a sustainable re-evaluation of materials. The aim is to provide a reference for architectural designers. Following a brief introduction in Section 1, Section 2 delves into the material performance of wooden window frames, polymer window frames, and aluminum alloy window frames, respectively. Section 3 addresses the issues encountered during the use of window frames; this review surveys improvement methods and provides a summary. The environmental impact of different window frame materials is also discussed in the context of their entire life cycle, accompanied by a sustainability assessment. Finally, Section 4 serves as a conclusion to the overall survey, offering recommendations for future work.