Collectively, buildings have significant environmental impacts throughout their life cycles, from material production and initial construction through use and eventual demolition and disposal/recycling. In the USA, residential and commercial buildings comprise 70 % of total energy consumption (D&R 2012) and emit 40 % of total CO2 emissions (EIA 2011).
As architects and engineers continue to strive to make buildings less energy intensive to operate, increasing attention is being paid to a broader set of environmental considerations (e.g., resource consumption, presence of chemicals of concern, land use change) and quantitative methods, such as life cycle assessment, capable of capturing the impacts over a building’s full life cycle (Bayer et al. 2010; Crawford 2011; Simonen 2014). Increasingly, life cycle assessment (LCA) is being adopted by, or mandated to, architects and engineers during the design process in order to give consideration to environmental impact information during the selection of materials, components, and assemblies (Bayer et al. 2010; Al-Ghamdi and Bilec 2015). Additionally, in recent years, several green building certification systems have integrated LCA into broader sustainability assessments, supporting the increased awareness and use of the methodology among design practitioners working to evaluate built projects (Ortiz et al. 2009; Optis and Wild 2010; Passer et al. 2012; Al-Ghamdi and Bilec 2015).
LCA in the building sector has primarily focused on two key areas: energy performance of buildings (Optis and Wilde 2010) and building materials (Gustavsson and Sathre 2006). To date, the majority of building material and construction-related LCAs have focused either on simplified models of whole buildings (with approximate values for an estimated bill of materials) or on the assessment of isolated building materials (e.g., steel, concrete, flooring, insulation). Complex assemblies composed of numerous materials, such as curtain walling, windows and doors, roofing assemblies, and structural systems, are just beginning to be better understood and modeled through the application of nuanced comparative LCAs.
While comparative LCAs and EPDs have become more readily available for a host of interior products, it remains difficult for architects and designers to evaluate complex, long-life assemblies. Particularly in the early stages of design, when a team is most able to make material selection decisions that will most affect the total environmental impacts of a project, quantitative assessments and comparative studies that connect system/material typologies with detailing and design considerations are particularly lacking. While system performance and durability play a significant role in both life cycle costing and energy modeling, these factors are difficult to study using Environmental Product Declarations (EPDs) and other cradle-to-gate life cycle assessments. Additionally, for many architectural conditions and systems, a design may consider a range of materials that serve similar functions but differ in durability, expected service life, maintenance concerns, and end-of-life options such as material recovery and recycling.
This study uses window frame assemblies as an object of study to explore the role of use phase and end-of-life variables on environmental impacts across a full building life.
Durability, maintenance, and expected lifetime
Generally, questions of maintenance practices and material replacement have been insufficiently addressed in LCA work due to the difficulties associated with quantifying the benefits of physical properties such as durability for building materials (Aktas and Bilec 2012; Liu and Müller 2012; Miller et al. 2015). For windows, questions of durability and material replacement are particularly significant (Andreson et al. 2001). Frame assemblies are composed of a number of materials serving different purposes and subjected to varying stresses and wear. Therefore, the act of window refurbishment and replacement is a meaningful part of the building life cycle—and a growing topic of interest for high-performance building design and retrofit.
In a published literature, the majority of LCAs conducted for window frames have focused on the manufacturing and end-of-life impacts as stand-alone products with no specified service life (Sinha and Kutnar 2012; Salazar and Sowlati 2008; Asif et al. 2002, 2007; Citherlet et al. 2000). Or, they have shortened model time frames to 40 or 50 years, so that they do not include significant maintenance or material replacement for some or all of the materials studied (Mösle et al. 2015).
Additionally, most EPDs are cradle-to-gate assessments, which leave out the question of use phase impacts entirely. This trend can, in part, be explained by the difficulty of approximating an accurate service life for window frames—a long-life product whose replacement can hinge on a number of factors, from esthetics to performance to user needs. Although a product manufacturer may wish to provide a set of default scenarios in an EPD from which the building assessor may choose, the number of potential influencing factors on the use phase makes this impractical, due to the large number of combinations of factors related to climate, wear, and maintenance practices. However, when examining material use on buildings that must achieve high-performance standards for 80–100 years, the assumed service life of a component such as window frames does matter, as a single full replacement of an assembly will effectively double the product’s life cycle impacts.
While it is indeed difficult to overcome the uncertainty of selecting a single assumed service life for an assembly without fully understanding the building context and maintenance regime (Aktas and Bilec 2012; Minne and Crittenden 2015), comparative LCA facilitates the exploration of this topic through the testing of multiple use scenarios. This approach recognizes that maintenance regimes are project specific, influenced by a range of factors (e.g., building type, location, budget, use), and that LCA results will be influenced by assumptions made during the modeling process.
When comparing architectural assemblies composed of different materials, it can be a challenge to assess material recycling along the product life cycle, as discussed in the many papers that approach the issue of allocation in less complex material assemblies through system expansion (Klöpffer 1996; Ekvall and Tillman 1997; Ekvall 2000; Ekvall and Weidema 2004), a practice that adds complexity to both data collection and interpretation (Werner and Richter 2000). In response to increasingly complex products and comparisons, LCA methodology has developed an array of allocation methods (Ekvall and Tillman 1997; Ekvall and Finnveden 2001; Nicholson et al. 2009; EC 2010).
In the building and construction sector, variability in approaches to allocation of recycling credits and end-of-life impacts has posed challenges for cross-material and cross-industry comparisons (SIS 2012; Leroy et al. 2012). This is particularly true for building products that contain recycled content or those that involve material collection and recycling at end of life. Presently, two methods predominate: the recycled content method and the end-of-life recycling method. While the selection of an allocation method has a large impact on model results, there is a general lack of consensus as to which method should be used (Frischknecht 2010; Hammond and Jones 2010; Wardenaar et al. 2012; Huang et al. 2013).
The lack of consensus among LCA practitioners leads to difficulty of interpretation of data results across studies as calculation methods vary. The recycled content approach is particularly difficult to apply in the case of metals such as aluminum, as it requires the precise knowledge of recycled content by mass in an assembly, which is difficult to determine when availability of scrap is variable and scrap is incorporated in production melts with no change in performance properties (Schlesinger 2013; Puga et al. 2009; EAA 2013). For this reason, the end-of-life recycling method is preferred within the metal industry (Atherton 2007; Liu and Müller 2012; EAA 2013; PE International 2014) and increasingly used in both building material and whole building life cycle assessment reporting, such as EPDs (SIS 2012; ISO 14040:2006; ISO 14044:2006; ISO 21930:2006). On the other hand, using this method for building materials may be inaccurate, as application of the end-of-life recycling method to buildings requires prediction of reclamation rates and impacts of recycling at the end of life for long product lifetimes (Hammond and Jones 2010).