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In recent decades, there has been increased attention to reduce the operational energy performance of buildings. Stringent legislation on building energy performance has stimulated facade design to evolve to serve numerous functions and meet complex technical requirements. This has in part been achieved by an increase in the use of materials, processing methods and construction techniques, which paradoxically may reduce the ability to recover material that is high-value in terms of embodied carbon after its first use. Existing environmental assessment methodologies assign accreditation for the use of low embodied carbon materials associated with re-use and re-cycled products in the input and production stage (module A), however, the ability to recover the materials and associated environmental benefits as a function of design, also known as reclamation potential, is not usually considered. This study aims to develop a robust disassembly assessment framework to evaluate the reclamation potential of materials from facade systems and forms part of a larger ongoing research program to address the end-of-life challenges in facade re-use. The disassembly methodology will allow the reclamation potential, in terms of environmental impact, of different facade designs to be assessed as a function of time in terms of component and system service lives, with reference to different recovery scenarios. The proposed methodology has been applied to a reference system; the ubiquitous insulated glazing unit (IGU), with a reference service life of 25-years, to highlight some of the potential future applications of the assessment framework. Preliminary findings show that service life and early-stage design constraints can hold great influence on determining the recovery strategy that yields the greatest reclamation potential for systems at end-of-life.
Rising carbon emissions have produced a call for sustainability on a global scale with a need to revisit all areas in the design of engineered products and services. The building sector is responsible for the consumption of around 50% of resources on a global level, approximately 40% of energy consumption and around 35% of carbon emissions. , [2, p. 38] Carbon emissions arise from all stages of the building life-cycle, from production (embodied carbon), through to in-use (operational carbon), and end-of-life processing and recovery. A modern-day façade can typically constitute between 13 and 30% of the initial embodied carbon of buildings, exceeded only by the foundations and the structural frame. [3, pp. 21–22], – However, in contrast to the foundation and structural frame, the components within façade systems typically have a shorter lifespan calling for more maintenance and repair, resulting in a relatively larger amount of recurring embodied carbon throughout the building lifespan. , 
In the past few decades, the focus has been to improve the operational performance of buildings through higher performance facades that meet the increasing performance targets set in building legislation. ,  However, operational carbon constitutes only part of the problem in addressing the whole-life carbon emissions of buildings. Indeed, in the instance of reduced operational carbon, the relative significance of embodied carbon increases. , , ,  Further, the introduction of more materials into the building mix in the form of additional materials such as automated shading and energy control coatings, typically with shorter lifespans has increased the absolute value of initial and recurring embodied carbon. ,  Without effective recovery strategies in place to consider how these systems of high environmental value will be recovered at the end-of-life stage, the question arises as to whether the carbon balance is simply being comprehensively addressed or whether it is simply being shifted to another stage in the façade life cycle.
At present, there is no evidence of a design focus on the ease of disassembly of façade systems at the end-of-life in the interest of recovering material for re-use and re-cycling. A quantitative evaluation specific to the end-of-life reclamation potential of systems based on early-stage design decisions does not currently exist. Hildebrand made a call for the potential for re-integration of materials through product recovery for further use on component (reuse) or substantial (recycling) level to be enhanced to support the façade design options with the best whole-life environmental performance for architects and manufacturers. [13, pp. 121–142] The current study will introduce a methodology for assessing reclamation potential at the end-of-life, in terms of embodied carbon, of façade designs based on their material selection, processing and joining methods. The reclamation potential will be assessed in the instance of different possible recovery scenarios constructed by the author. Service life uncertainty will be taken into account, by developing an approach proposed by Fawcett when assessing whole-life costing for buildings, to assess the reclamation potential as a function of time. 
Shifting Design Focus for Façade Systems
At present, metals are commonly recovered for re-cycling from buildings, whilst glass and concrete products are often grouped with mixed inert waste streams and sent to landfill, or at best down-cycled into road aggregate. The lack of a well-developed recovery supply-chain and increasing technological constraints from design does not favour the re-use or optimal re-cycling for façade components. This, in turn, has created self-sustaining linear material life cycle. The concept of the circular economy, built upon the early theories of industrial ecology, has been the subject of recent study within the built environment. , – Circular design focuses on moving towards more regenerative systems in which resource input and waste, emission, and energy leakage are minimised by slowing, closing, and narrowing material and energy loops. This can be achieved through a holistic and integrated approach of design, maintenance and effective product recovery.
Hierarchy of Product Recovery
Product recovery seeks to obtain materials and parts from old or outdated products through recycling and remanufacturing in order to minimise the amount of waste sent to landfills. When considering reclamation potential in terms of environmental impact, it is important consider the relevant recovery route (Fig.1).
Pomponi and Moncaster proposed a framework highlighting the six pillars to achieving regenerative design within the built environment including governmental; economic/supply-chain; technological; environmental; societal and; behavioral.  From the supply-chain side, some existing efforts to move towards a more circular business model have been made to create new business models for facades, for example leasing concepts, in order to align incentives of demand side (investors and users) and supply side (industry and designers) within the environmental context. 
The EU Landfill Directive set up in 1999 has had a positive impact in diverting a large percentage of waste away from landfill sites but policy and/or legislation to promote the best-case product recovery is yet to be enforced on industry.  Design for disassembly (DfD) principles have been found to be a key enabler in product recovery and realising the circular economy in construction products such as concrete, steel and timber. – Little focus has been made specific to façade systems with consideration of the complexity of the materials and construction methods employed. As such, typical existing façade technologies such as permanent and durable connections diverge from the principles of DfD and place technological constraints on the recovery and secondary use of components from existing systems at the end-of-life.  For example, in the present technological landscape, laminated glass, commonly used in curtain-wall facades, can at best be crushed up and used as glass fibres or road aggregate which will have a lower reclamation value than reconditioning or re-use.
Existing Environmental Assessment Tools and their Shortfalls
Life-cycle assessment (LCA) has become a well-utilised method for quantifying the environmental impact of building products.  LCA on the material level has the ability to calculate the energy and emissions related to the depletion of resources, the generation of energy, and the steps of production. LCA-based databases (EcoInvent, Inventory of Carbon and Energy, Oekobaudat, Wecobis) contain inventory data and accompanying software (Gabi, SimaPro, Umberto, OpenLCA) to calculate the environmental impact of products and services in terms of modules A-D (Fig. 2), and have been the subject of recent review. [13, pp. 17–36, 143–158],  It has been found that LCA is heavily subject to data reliability and study assumptions.
Embodied carbon, EC (kgCO2equiv), provides a reflection of the ecological impact of a product or system in terms of carbon emissions; and is closely related to design in terms of the materials and construction type deployed. It can be decreased by: the use of re-used and re-cycled materials, the inclusion of deconstruction in scenarios by the type of connections, the application of durable materials and/or, the utilisation of renewable materials.  Moncaster and Pomponireviewed the existing academic knowledge of LCA in buildings and found that impacts attributed to the end-of-life stage are often over-looked.  Existing façade-related LCAs have largely focused on modules A-B, sometimes C, to compare the building envelope as an element within the building mix and/or; to allow comparisons between different design alternatives, mostly in terms of the effect of changing design on embodied and/or operational carbon. – When considering the after-life phase, module D, most existing assessments assume a typical disposal scenario and do not perform any comparison between different recovery strategies. Further, few studies go so far to quantitatively assess the link between design choices and module D, end-of-life reclamation potential, which could help to highlight any technological barriers that undermine material recovery in its highest value. An effective methodology for linking the unique features of façade design with reclamation potential is required to allow recovery opportunities to be realised; reflect the true value of material after its first usage phase; credit whole-life environmental impact and; open-up the availability of supply for second-use. Increased awareness of the environmental benefits of more circular designs and recovery methods could stimulate new legislation and policy, specific to product recovery, a key catalyst in advancing the circular economy within the built environment. 
A Novel Approach for Evaluating Reclamation Potential under Uncertainty
The assessment developed in this study will be bound to modules A and C-D (see Fig. 2). Energy in operation, module B, will not be considered in the proposed methodology as the framework is for assessing systems independently of each other, in which case, the operational performance will remain unchanged.
End-of-Life Reclamation Potential
Environmental assessments for building products are typically deterministic when considering end-of-life, i.e. consider one end-of-life disassembly method alone either demolition and landfill or some form of recycling. ,  This approach can lead to biased recommendations to designers in terms of oversimplified outcomes or unjustified confidence in outcomes such as a focus on reducing the environmental impact of one life-cycle stage at the sacrifice of benefits from end-of-life recovery.  The proposed methodology provides a framework for assessing façade systems in terms of their reclamation potential within different end-of-life scenarios. The end-of-life scenarios were constructed based on conceivable present and future scenarios. These scenarios take the form of system re-use, component re-use, re-cycling, incineration and landfill. This approach will allow an informed environmental impact ranking, in terms of embodied carbon, of alternative recovery strategies.
Component Interconnectivities and Disassembly Method
The re-cycling potential of homogenous components i.e. individual materials, has been assessed in the literature.  In contrast, a typical facade system could be described as a complex component, one which consists of a set of irreversibly connected homogenous components. The separation of components or disassembly and therefore product recovery, is closely related to the removal of connections. There are a considerable variety of connection types that are used in complex products for example: screw, snap-fit, press-fit, glue and seal, weld, solder connection; all of which are associated with different disassembly tasks.  At present, the effects of material selection; together with processing and joining methods on disassembly has not been clearly quantified.
Lambert & Gupta developed a theory of disassembly via the use of connection diagrams.  A connection diagram is an undirected graph where the nodes represent the parts and the edges represent the connections (Fig. 3).
The diagram in figure 3(ii) indicates all of the connections between parts and their connection type which can allow the series of disassembly operations required for product recovery and highlight any technological challenges in terms of permanently-connected (unrecoverable) homogenous components. 
Component Interconnectivities and Service Life Uncertainty
The service life of a component in a building, is the period for which it remains in productive use before being replaced or abandoned. The end of service life may be reached for a variety of environmental influences and functional influences.  Other reasons for the end of a building or component’s service life depend on wider processes of economic, social and technical change, as well as fashion. Some of the reasons for service life depreciation are shown on figure 4(i). Service life and the method of demolition/disassembly is a source of uncertainty in the assessment of reclamation potential. When the service life of components is considered, system re-use may not necessarily be the most appropriate recovery strategy. As such, service life, specifically in the case of building components with typically long lifespans and; separation capability to enable re-conditioning and re-manufacturing must be considered when comparing different end-of-life scenarios.  (Fig. 4).
The multi-component nature of glass façade systems, means that the service life and therefore re-use applicability of one component, is dependent on the service life and/or deterioration of its nearest permanently connected neighbour component. Some information is available for façade component and system service life , including a twenty-five year study conducted by Lingnell for the insulated glazing unit (IGU) , however, service life data is generally sparsely populated or given as approximate figures making it difficult to create precisely defined probability distributions. Service life estimates for typical curtain walling components are shown in figure 4(ii).
Mathematical distributions can be calibrated using empirical data of the probability of system/unit failure over the unit lifetime. Fawcett considered service life predictions of building elements through lognormal distributions, which correspond to a relatively low probability of failure in the first few years of a component’s service life, and a small probability of a very long service life which is similar to the empirical distributions seen for the service life data that does exist for some building components.  BCIS completed a survey of the life expectancies of common building components; an example of failure frequency over the lifetime of a curtain-walling system (Fig. 5). 
Indeed, façade systems are complex systems that contain a mix of components with different lifetimes. A probabilistic approach to service life approximation, will be suggested in this framework that considers façade system interconnectivities.
Up until recent years, sustainable product recovery as a function of design has received little attention within the façade industry. This study develops a methodology to effectively assess design options in terms of disassembly and reclamation potential with consideration of the system interconnectivities, component/system service life and different end-of-life scenarios. The reclamation potential will be calculated in terms of net environmental impact by considering the A, C-D stages of the system life-cycle. A methodology to evaluate service life using a lognormal probability distribution about available service life data will be developed. The overall aim is to create a robust framework to perform a quantitative assessment that relates design with disassembly to be applied to several different façade systems in future research. From this, it will be possible to propose the most effective recovery strategy; highlight favourable designs and; suggest technological disassembly strategies that could be developed to enable more regenerative designs.
The framework proposed in this study will detail the necessary sourcing and processing of data from a specific system under evaluation to assess reclamation potential as a function of service life and system interconnectivities (Fig. 6).
Bill of Materials
The type and quantities of all materials and manufacturing processes used in the façade system will first need to be established by inspecting construction drawings and the associated bills of materials from façade manufacturers.
All materials and processing methods have an embodied carbon coefficient, ECCoeff (kgCO2equiv) associated with them which can be found from various available databases that contain the input-data in terms of ECCoeff for LCAs. Databases include EcoInvent, Inventory of Carbon and Energy (ICE) (UK), Oekubaudat, ELCD. EcoInvent and ICE contain generic life cycle data sets that provide suitable averages of ECCoeffand other environmental indicators for building materials for life-cycle modules (A1-A3, B1-B6, C1-C4 and D). Both datasets comply with ISO-14040 which allows for a comparison of all datasets to be made. This includes the ECCoefffor the processing step, which could be production, recycling, incineration, transportation and/or demolition/disassembly. The ECCoeff can then be used to calculate the total value of EC for the system using equations 1 and 2.
Connection Diagram - Sequence of Disassembly Operations
Based on the information taken from the input stage in terms of bill of materials and construction processes, the system unit under analysis can be evaluated to form a hierarchy of materials (Fig. 7).
With this information, the systems can be represented in the form of a connection diagram to highlight the sequence of necessary disassembly operations and limitations of achieving each, given the current technological landscape. For completeness, an additional table that outlines the given tasks for disassembly can be written in terms of descriptions, time taken to complete and their interdependencies ie. predecessors (Fig. 8).
Connection diagrams such as the one shown in figure 8 will be constructed for the façade system under study, to be able to easily infer the disassembly method in each recovery scenario. For example, a limiting point for recovering float glass in the system shown in figure 8 would be the adhesive fixing to PVB meaning that the system can not be effectively separated into its constituent components without a significant loss of value in the existing technological landscape. In the instance of technological barriers, potential solutions can be considered as discussed with industry.
The specific system unit under study will be made in reference to the same construction site. Depending on the recovery scenario, transportation data will vary, based on the existing available locations for reconditioning, re-use and recycling efforts.
Recovery Route Scenario Variations
Four hypothetical scenarios, explained in table 1, have been constructed to draw comparisons on the end-of-life route that performs best in terms of achieving maximum reclamation potential.
Table 1: Demolition and corresponding material recovery method for different end-of-life scenarios to be compared
Material Recovery Method
Mixed Rubble to Landfill and/or incineration
Metal components to recycling facility
Glass components to road aggregate
Effective sorting on site
Disassemble/ separate components
Metal components re-used in new system
Glass re-used in new system
Technological constraints; Supply-chain
Remove system for re-use
Direct re-use of façade unit in new building
At present, the most common end-of-life scenario within industry is either; façade system demolition and landfill (S1) or dismantle and component recycle (S2). Component reuse (S3) and system re-use (S4) may be found to be limited to various degrees by technological and supply-chain factors. For example, a non-verified reconditioning process, or design-limited separation capabilities. However, it is important to still consider these scenarios, alongside a full statement detailing the limitations to them being achieved. Depending on the degree of limitation to achieving a relatively high reclamation potential, this may provide a strong justification for further research into addressing the barriers in achieving component and system re-use from existing and future designs.
Material Routes within Scenarios
For each scenario, S, the route of recovery for each constituent material, M, within the system will be mapped to find the percentage recovered through each recovery pathway - re-use, re-cycle, down-cycle, incineration and/or landfill. For example, in the demolition scenario (S1), it is assumed that all materials are disposed of in landfill.
Table 2: Material recovery route mapping reference table
Output: Reclamation Potential
For each scenario the reclamation potential measured as a percentage of the initial embodied carbon value can be calculated. From the product recovery hierarchy (figure 1), it can be assumed that:
ECDowncycled Product < ECRe-cycled Product < ECNew Component/System
Therefore if, no consideration were made for deterioration and the production stage was considered alone:
ΣRPSystem Re-use = Initial Embodied Carbon of System
In other words, the system has fulfilled its maximum reclamation potential. However, the additional processes involved in preparing, separating and/or transporting for alternative recovery strategies must be included to ensure the total EC required to achieve the different recovery pathways is characterised, which may change the most favourable outcome.
RPRe-use constitutes savings that can be made in place of production of a new system from raw materials, processing and construction into a specific component/system with the same original function and can be calculated as follows:
ΣRPRe-use = ECDismantle + ECSeparate + ECRecondition + ECTransportation - ECNew Component/System
RPRe-cycle constitutes savings that can be made in place of new production and processing from raw materials into a specific component/system and can be calculated as follows:
|ΣRPRe-cycle = ECDemolish + ECSeparate + ECReprocess + ECTransportation - ECRe-cycled Product|
RPDown-cycle constitutes savings that can be made in place of new production and processing from raw materials into a lower-grade product and can be calculated as follows:
|ΣRPDown-cycle = ECDemolish + ECSeparate + ECReprocess + ECTransportation -ECDown-cycled Product|
Incineration refers to the process of heat recovery which can be converted into usable energy form e.g. electricity, heat or combined heat and power. RPIncineration is the energy recovered through incineration of materials calculated as follows:
|ΣRPDown-cycle = ECDemolish + ECSeparate + ECTransportation - ECIncineration|
In the case of material being sent to landfill, no saving is attributed and ΣRPLandfill, is positive indicating an EC deficit. It can be calculated as follows:
|ΣRPLandfill = ECDemolish + ECTransportation|
A negative sum for ΣRP, indicates an embodied carbon (EC) saving was incurred. From equation 2, and the relevant equations for the scenario route under analysis, the % reclamation potential, %RP, can be calculated as follows:
% Reclamation Potential, %RP = -ΣRPScenario(n) / Initial ΣECSystem
Uncertainty Analysis: Service Life Effects
In the instance of unknown reconditioning processes, product acceptance in terms of service life and deterioration must be considered. It is suggested by the author that the RP in the system and component re-use scenarios be compensated by a probability of survival, Ps, factor, which will depend on the reference recovery lifetime, t, to find a lifetime-compensated reclamation potential, LCRPt, that considers service life and probability of failure. In the case of re-cycle, down-cycle and landfill, the service life and consequential deterioration, has no effect on the RP, because the reprocessing step involves returning the product/components to full functionality.
Service Life Data Sources
The probability of survival (Ps) for a component within a facade system or the system itself, can be plotted as a lognormal distribution about the typical service life figures. Service life figures can be taken from discussions with façade manufacturers and existing published building component service life data. 
Lifetime-Compensated Reclamation Potential (LCRP)
For recovery scenario, S, the LCRP can be calculated at time, t, as follows:
|LCRP(t) = Ps x RPScenario|
This first approach assumes that when Ps = 0, LCRP = 0. However, this does not make a consideration for the potential for the system to continue to function to a lesser performance than originally specified.
A second approach is proposed by the author to consider a compromised performance that the system provides. In this instance, a specific function, F, will need to be defined. For example, if assessing the LCRP of an insulated glazing unit (IGU) the specific function taken may be thermal transmittance. Thermal transmittance, also known as U-value, is the rate of transfer of heat through a structure, divided by the difference in temperature across that structure. U-values are the reciprocal of R-values. The units of U-value measurements are W/m2K. When Ps = 0 for the IGU, the system is no longer operating to the performance of an IGU. The performance will be compromised, and the FU-value will have increased to that of a single glazing unit (SGU).
|P.R.F = FOriginal as New / FCompromised Function|
Therefore, the LCRP with compromised function can be calculated from:
|LCRP(t) = [ Ps x RPScenario ] + [ (1-Ps) x RPScenario x P.R.F ]|
The implicit assumption here is that, it is acceptable for the component/system to continue operating in its compromised functional state.
The percentage reclamation potential, %RP, calculated for each scenario will allow an informed ranking in terms of associated environmental impact of alternative recovery strategies. By modelling the re-use scenarios as a function of time, it is possible to provide results in the form of a range of values over time. As an example, if the proposed methodology was applied to a typical IGU, the function, F, may be taken as the U-value in which case for an IGU with a U-value of 2.7 W/m2K, and a compromised system taking the form of a single-glazed unit with a U-value of 5.9 W/m2K, the performance reduction factor, P.R.F, calculated using equation 12, would be equal to 0.46.
As previously stated, empirical information on service life data for façade elements is limited. A lognormal distribution can be plotted about a reference service life of 25-years of a typical IGU system, in which the logarithm of time (years) over a 50-year lifetime is normally distributed.
Figure 9(i) shows the probability density function as a lognormal distribution by taking the reference service life of 25 years, over a 50-year reference lifetime for an IGU component, with a reference mean of ln(x) equal to 2.21 and standard deviation of ln(x) equal to 0.92. Figure 9(ii) highlights the cumulative probability of the probability density shown in figure 9(i) which suggests that at 25-years, it would be expected that at least 86% of IGU units would fail to meet their initial functional performance requirements.
Figure 10 provides a visual representation of the comparison of different recovery strategies over time in terms of reclamation potential. RP using approach 1, has been calculated using equation 11, in which no compromised function has been considered. The RP using approach 2, calculated using equation 13 and a performance reduction factor of 0.46, considers the unit continuing operation as a compromised function as a single-glazing unit. Recovery scenarios involving recycling would offer reclamation potentials that are invariable over the lifetime. An estimate of 30% reclamation potential from recovery through re-cycling has been plotted in figure 10. It is shown that a system with a compromised function maintains the highest LCRP value, suggesting that if the compromised function of an IGU to perform as a single-glazing unit is acceptable to the next user, a system re-use strategy may be considered more advantageous than alternative recovery strategies from an embodied carbon perspective. However, in the instance that a compromised function is unacceptable, as shown by LCRP calculated using approach 2, it is evident that after a certain time period, re-cycling may be considered to be more advantageous than the system re-use scenarios. Alternative recovery strategies such as component re-use could prove more viable options over a long lifetime, however, this would be dependent on the technical ability to separate the constituent components from their original system.
Uncertainty in service life and consideration for different end-of-life scenarios are frequently missed from existing life-cycle assessments. [13, pp. 121–142],  The novel approach of this research is the development of an assessment framework that allows the 1) evaluation of the end-of-life reclamation potential as a function of time for different recovery scenarios; 2) approximation of service life uncertainty; 3) suggestions for the most valuable recovery route in terms of embodied carbon for existing designs. The proposed framework was applied to a typical IGU glazing unit with a reference service life of 25-years, to exemplify the potential applications in comparing end-of-life recovery strategies for specific systems. These findings are sensitive to: the approach used to calculate LCRP, the performance function selected for comparison and original design constraints. It must be noted that methods incorporating a probability distribution for service life are limited by the empirical service life information made available and therefore must be revised as appropriate.
The framework and methodology proposed in this work will be applied to several different façade typologies in later research conducted by the authors enabling suggestions for the most suitable recovery route and associated reclamation potential; and allow recognition of the most impactful technological barriers that limit the best end-of-life recovery routes from being achieved. Future assessments will look to extend the framework to include operational energy to evaluate the trade-off between operational energy and reclamation potential. To further the service life assessment, a review of the maintenance options; reasons for deterioration specific to functional and environmental influences and; associated embodied carbon would be appropriate.
Future regulation will call for more sustainable recovery methods. At present, the benefits of the low embodied carbon associated with re-use and re-cycled products are credited in module A (input and production stage). Figures for reclamation potential and the after-life stage are often not included or even considered. Without a stimulus to focus on designing to enable product recovery, what is built today, may not be available for product recovery in the future. It would be useful to consider a fair approach to credit reclamation potential through design for disassembly within the whole-life cycle assessment. Any approach to fairly distribute the benefits of re-use and re-cycling across the cradle-to-cradle life-cycle would have to ensure they are not double-counted. That said, a standardised method for holistically assessing the whole-life environmental impact of designs will allow well-informed, efficient and traceable decision-making across the supply-chain.
Ongoing research by the authors looks to address end-of-life challenges in façade design by; reviewing the behavioral and societal dimension to façade material recovery through interviews with industry and the exploration of separation methods that could alleviate the technological barriers to system and component re-use.
This work was supported by the UK Engineering and Physical Sciences Research Council (EPSRC) for the University of Cambridge Centre for Doctoral Training in the Future Infrastructure and Built Environment (EPSRC grant reference number EP/L016095/1).
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