The widespread popularity and acceptance of the green building rating system is testament to the increased energy awareness and environmental consciousness on the part of the stakeholders. The energy consumption of buildings accounts for 40% of total energy consumed in the developed world [1
]. The green building rating systems (e.g., Leadership in Energy and Environmental Design—LEED) and green codes around the world, such as International Green Construction Code (IGCC) [2
], Energy Performance of Building Directive (EPBD) [3
., introduces stringent requirements for reduction of energy use in buildings. Windows are typically responsible for a large fraction of heat loss in a building. This is because the combination of glass and frame in windows generally has a higher degree of heat transmission, i.e.
, higher U value than the other components of a building.
Development of technology has led to a considerable reduction in heat loss through windows. Some examples are glazing of the glass [4
], which helps in reducing the heat loss and also contributes to solar heating of the building. Other technologies, such as lazer glazing [5
], low emissivity coatings, electrochromic materials, and thermochromic materials [6
] used in windows, have demonstrated technological evolution and reduced the loss of heat. Most technological developments, however, have focused on window panes, while overlooking the frames. A major component of a window is the window frame, which can cover 20%–30% of the area of a window and has a negative impact on energy performance [7
]. The most common window frames used presently are either materials with high conductivity such as aluminum for office buildings, or materials with low conductivity like wood and polyvinylchloride. Some argue that frames made of low conductivity material usually have low strength requiring wide frame profiles that reduces the total transmittance of the window [8
]. While these claims have not been validated, it remains true that window frame impacts the energy performance of the buildings significantly.
With sustainable design being a necessity, it is important to not only consider the energy performance of a window frame, but also consider other performance metrics to gain a holistic appreciation. These performance metrics are embodied energy over the product lifecycle, thermal performance, and structural performance. Sustainability forces us to consider holistic approaches, which have previously not been addressed comprehensively [9
]. A window frame that performs better than another from an energy standpoint, might have significantly higher embodied energy over its lifecycle—raw material extraction, processing, manufacturing, transportation, and installation. This would therefore make it a bad choice from an environmental standpoint. One of the most important choices that faces anyone installing or replacing windows will be the materials used in the frames. While the shape, size and operation of a window is aesthetically significant, the material from which a frame is constructed is crucial when considering cost and energy efficiency. While the panes themselves are typically constructed of glass, there are three most common types of window frame materials, wood, aluminum, and un-plasticized Polyvinyl Chloride (uPVC). All these materials have their advantages and relative shortcomings. Different homeowners make their decisions based on features and factors that are particular to their lifestyle, tastes, and preferences. The material from which a window frame is constructed can greatly affect overall installation cost and energy efficiency.
With sustainability being the driving force in the creation of a building, environmental impact of selected materials should be included in planning, considering the life cycle and embodied energy of the materials used. Therefore, the Life Cycle Assessment (LCA) methodology should be used to reveal the environmental and energy performances of the used materials, as well as the developed products through the whole life cycle. Since the 1980s, when LCA analysis was developed, until today, numerous methodologies to classify, characterize, and normalize environmental effects were developed. The most common, for example CML 2 (2000), IPCC Greenhouse gas emissions, Ecopoints 97 and Eco-indicator 99 [10
], focus on the following indicators: acidification, eutrophication, thinning the ozone layer, various types of ecotoxicity, air contaminations, usage of resources and greenhouse gas emissions. At first, LCA analysis was mostly focused on environmental effects like acidification and eutrophication, while in the past years mostly on greenhouse gas emissions, which are also called carbon footprint. The carbon footprint is expressed in terms of the amount of emitted carbon dioxide or its equivalent of other greenhouse gases. In Europe, carbon footprint is gaining immense importance and expected to be mandated to accompany products and services. As solutions are sought to reduce the impacts of buildings, LCA is seen as an objective measure for comparing building designs. Very few studies have analyzed window frames form a sustainability standpoint using LCA. Lawson [11
] and Asif et al
] performed LCA on various window frames and observed that aluminum frames had the highest environmental impacts.
In sustainable design, “durability” is also increasingly being included on priority lists under the assumption that designing for longevity is an environmental imperative. However, this is unsupported in the absence of LCA and accurate lifespan predictions. In the worst case, designing for longevity can lead to design choices that are well-intentioned but, in fact, yield poor environmental results. Rather than attempt to predict the future and design permanent structures with an infinite lifespan, design for easy adaptation and material recovery should be acknowledged.
This study aims to provide a holistic performance metrics for window frames by comparing three widely available window frame materials, wooden, PVC, and aluminum. First a window was designed having the same volume of material, spacer, and glazing system. Subsequently, their U values and thermal performances were calculated and compared. Furthermore, carbon footprint of the three window frames was calculated, focusing only on initial embodied energy, non-renewable energy consumed in the process from the acquisition of raw materials to the construction of the building. Finally, carbon footprints and performances were compared to identify the best holistically performing window frame material for a given U value.
Thermal performance and carbon footprints of three common window frames, wooden, PVC, and aluminum, were evaluated to provide a holistic performance metric for a window frame. From the conceptual design and numerical analysis, it was found that the wood window frame performed better than uPVC and aluminum frames, thermally as well as environmentally. The carbon footprint of aluminum window frame is almost four times higher than that of the wooden window frame. Also the PVC window frame is double that of the wooden window frame. Furthermore, the thermal performance of wooden windows was superior. An overall better performance of wooden window frame makes it the preferred material of choice for window frame holistically. The study proved that wooden window frames should be chosen in sustainable design, where energy performance as well as from the point of view of other performance metrics, such as embodied energy over the product lifecycle, thermal performance, and structural performance. A cradle to gate analysis was performed in this study. Altering the system boundaries would yield different results; for example, if the impact during building operation had been taken into consideration, the results would have been different. Similarly, results would have been modified if the carbon footprint calculation accounted for carbon sequestration of wood, the use of recycled aluminum and other similar issues pertinent to LCA.