# Optimizing the Performance of Window Frames: A Comprehensive Review of Materials in China
## 3 Improvement Methods and Sustainability Assessment
### Sustainability Assessment  
Sustainability is commonly defined as a development or practice that does not diminish the long-term productivity of natural resource assets on which a country's income and development depend; sustainable development encompasses several characteristics, such as energy efficiency, environmental friendliness, and recyclability [133]. Since the late 1990s, numerous studies have been conducted, primarily in the form of life cycle assessments (LCAs), to evaluate the environmental impacts of windows [134; 135; 136; 137]. Figure 7 illustrates the life cycle of different window frame materials. * Life cycle assessment  
Due to well-managed forestry systems, wood is now considered a renewable material, and the processing and production of wood have minimal environmental impact; at the end of their service life, they can be reused for another product, land-filled, used as fuel, or degraded through recycling [138,139]. The quantity of existing aluminum resources is not a significant concern because aluminum is a fully recyclable material; aluminum recycling avoids processes that consume a substantial amount of energy and the use of other chemical materials, contributing to a reduction in greenhouse gas emissions [140]. However, aluminum production requires substantial energy, including bauxite mining, Bayer refining, aluminum smelting, refining, and metal processing [141, 142, 143]. Polymers are energy-intensive during synthesis, and their entire life cycle has environmental impacts. For example, PVC releases many toxic elements during its production, and they are usually disposed of through land-filling or incineration that may leach additives like phthalates and heavy metals, potentially contaminating groundwater. PVC is a major source of chlorine in incinators, contributing to the formation of the highly toxic synthetic chemical dioxin [144]. The recycling of polymer products involves categorizing waste as general material, and the quality of recycled material depends heavily on the impurity content, including other polymers or additives, making recycling technically and economically challenging [145]. Additionally, some cradle-to-gate analyses found that aluminum-clad wood windows and wood windows have lower environmental impacts compared to aluminum and PVC window frames. Wood has a low impact on the environment, with values for embodied energy, GWP, and acidification potential (AP) being the lowest, even when excluding carbon sequestration potential. Aluminum-clad wood windows, aside from increasing the environmental impact mainly due to the production of the aluminum cladding, share similar environmental characteristics with wood windows and can decompose into their basic components, aluminum and wood, at the end of their service life [146,147]. * Thermal performance  
The significant variation in the thermal performance of windows is largely attributed to their frames having a higher thermal transmittance; this is especially pronounced in buildings with a high window-to-wall ratio, where windows exhibit higher thermal transmittance than the opaque parts of the wall [148]. A study by [PERSON] shows that the global warming potential (GWP) of wood, PVC, and aluminum window frames is 1:11:26, respectively [149]. Furthermore, [PERSON] et al. [150] conducted a market review of commonly used high-performance window frames in Europe, including wood, aluminum, PVC, and aluminum-clad wood materials. They presented 56 different window frames  
Figure 7: Life cycle diagram of different window frame materials. with various geometric shapes, materials, and styles. The study highlighted the incorporation of common high-performance insulation materials, such as rigid polyurethane (PUR), polyethylene (PE), and extruded polystyrene (XPS), into the frame cavities. The U-value (or U-factor), a measure of thermal transmittance, ranged from 0.63 W/m\({}^{2}\) K to 1.14 W/m\({}^{2}\) K. Improving the thermal performance of window frames will increase their ecological impact from cradle to gate.