Collaborative and life cycle-based project delivery for environmentally sustainable building construction: views of Finnish project professionals and building operation and maintenance experts

Purpose – The energy performance gap (EPG) in building construction has been one of the major barriers to the realization of environmental and economic sustainability in the built environment. Although there have beenafewstudiesaddressingthisissue,studyingthistopicwithaspecialfocusontheprojectdeliveryprocesshasbeenalmostoverlooked.Hence,thisstudyaimstoaddresstheEPGinbuildingconstructionthroughthelensofcollaborativeandlifecycle-basedprojectdelivery. Design/methodology/approach – In order to realize the objective of this study, the development of a theoretical framework based on the literature review was followed by a qualitative study in which 21 semi-structured interviews were conducted with Finnish project professionals representing clients, design/planning experts,constructorsandbuildingoperation/maintenanceexpertstoexploretheirviewsonthetopicunderstudy. Findings – Thefindingsrevealtheprojectdelivery-relatedcausesofEPGinbuildingconstruction.Moreover,the obtainedresultspresentacollaborativeandlifecycle-baseddeliverymodelthatintegratesprojectandproduct(i.e. building)lifecycles,anditiscompatiblewithalltypesofcontractualframeworksinbuildingconstructionprojects. Researchlimitations/implications – Althoughthefindingsofthisstudysignificantlycontributetotheory and practice in the field of collaborative and sustainable construction project delivery, it is acknowledged that these findings are based on Finnish professionals ’ input, and expanding this research to other regions is a potential area for further studies. Moreover, the developed model, although validated in Finland, needs to be tested in a broader context as well to gain wider generalizability. Originality/value – Theobtainedresultsrevealthesignificanceandimpactofcollaborativeandlifecycle-based project development and delivery on the realization of environmentally sustainable building construction.


Introduction
Buildings, in general, consume a striking amount of energy, accounting almost to 40% of the whole energy consumption in the world (Laconte and Gossop, 2016).This huge consumption profiles buildings as one of the main areas under focus for further research and development.Consequently, sustainable development goals (SDGs), outlined by United Nations, to a high extend apply to construction industry and, in particular, building construction projects.This high-level recognition has resulted in extensive research on the energy efficiency of building construction and renovation.Regarding building construction, there have been significant advancements (e.g.building information modeling, geothermal energy system), and subsequently the enhanced design expertise and capability in the past decade has aimed for high efficiency or even net-zero energy buildings, in which the amount of consumed and produced energy (i.e.electricity) are even.Although there have been some successes in the construction of highly energy efficient or net-zero-energy buildings in some of the developed countries (e.g.USA) (Kibert, 2016), many of newly constructed buildings have been still struggling to achieve the energy efficiency targets, developed in the design phase.This phenomenon is called energy performance gap (EPG) (Laconte and Gossop, 2016).
Energy performance gap has been one of the major barriers for the realization of environmental and economic sustainability in the built environment.Looking at the definition of EPG, it basically refers to one or more factors in the project life cycle and probably in the commissioning phase of the constructed building, which hinders the efficient performance of the building in terms of energy consumption.In this regard, studies addressing the barriers and enables of the EPG in building construction (e.g.H€ akkinen and Belloni, 2011; Li and Yao, 2012;Moradi et al., 2023;Qian et al., 2015) have found that factors such as collaboration between parties, early involvement of key participants, designer's competence and integrating project delivery contribute toward solving the performance gap issue.These findings imply project delivery model's prominent role in filling the EPG because it accounts for the successful accomplishment of building construction projects.In this regard, there have been very few, if any, studies, employing collaborative project delivery as a theoretical lens for looking into the EPG issue.An important point to note is that the project delivery model's impact is not limited to the project life cycle; it also considerably affects the completed building's operational life cycle and the realization of energy efficiency goals.Thus, construction project delivery model needs to be collaborative and inclusive in terms of covering both project and product (i.e.building) life cycle.
However, the existing construction project delivery models mostly address project life cycle and almost avoid completed building's operation period.This is not a surprise as the terminology highlights the focus of the delivery model on the project only and not the product (i.e.constructed building).Consequently, the project parties are not usually held accountable in terms of their responsibility for the performance of the constructed building.This is particularly important for three reasons.First, the research shows that a completed building's operating costs in its operational life cycle can be even higher than its construction costs (Mike et al., 2015).Second, realizing sustainable built environment is highly dependent on the actual performance of the buildings in terms of energy efficiency, not the design intentions.And third, actual performance of the building can be seen only in the operation phase.Hence, it seems that project and product life cycle and management are interconnected and need to be integrated in the context of building construction.Thus, further developments and improvements are needed.
In this regard, it is necessary to acknowledge that construction project delivery models have evolved significantly over the past 30 years.In the big picture, the mainstream typology of project delivery models divides them into three categories of traditional, collaborative, hybrid (Moradi et al., 2022).Traditional delivery models in construction projects refer to design-bid-build, design-build and different types of construction management (e.g.

SASBE
Construction Management (CM) and CM at Risk) (Forbes and Ahmed, 2010).In other words, the terminology associated with traditional delivery models comes from the name of the contract type used in those delivery models.The same logic somewhat applies to the collaborative delivery models which include alliance, partnering, lean project delivery (LPD) and integrated project delivery (IPD) (Engebø et al., 2020;L€ ahdenpera, 2012;Mesa et al., 2019).The hybrid category refers to those project delivery models which employ traditional contract but also take advantage of collaborative working practices like co-location of the project participants (Darrington, 2011;Moradi et al., 2021a).Traditional delivery models are usually characterized by adversarial relationships, mistrust, unfair share of risk-reward, working in silos and dominance of low prince criteria for contractor selection.Conversely, collaborative delivery models feature early involvement of key participants; joint design, planning, control and decision making; open book cost management; aligned interests of stakeholders, continuous learning, fair share of risk-reward; open communication; and trustbased relationships (Moradi et al., 2021b).
The emergence of collaborative delivery models has had a significant impact on the performance results of construction projects (e.g.Hanna, 2016;Ibrahim et al., 2020).However, there are still two problematic issues left.The first one is the fact that traditional project delivery models are still dominant in many developing and developed countries and these countries are struggling to realize productive and sustainable building construction.The second issue is that even in collaborative project delivery models the shared risk-reward mechanism applies to the project life cycle and therefore the completed building's operational life cycle is taken into account in a limited manner.Thus, it is imperative to discover a solution for overcoming the mentioned challenges.Such solution could be developing a delivery model which is compatible with various contractual frameworks and tendering process in building construction projects and covers constructed building's life cycle.This study aims to realize this solution in order to the fill the mentioned knowledge gap and enable the realization of productive building construction and sustainable built environment in practice.Accordingly, this study's objective is to answer the following research questions: RQ1.What are the project delivery-related barriers and solutions which affect the realization of energy efficiency targets in the operation phase of the constructed buildings?
RQ2.What kind of model can enable collaborative and life cycle-based delivery of building construction projects for filling the EPG?
The resultant article is structured in six sections, including the introduction, theoretical background, methodology, results, discussion and conclusions.

Theoretical background
Energy performance gap When the measured (or actual) energy consumption of buildings differs from the expected energy consumption, the building is said to have an EPG (Zou et al., 2018).This can mean the difference between simulated and measured energy performance or the difference between targets set by specifications or standards vs the measured performance.The EPG may exist may be observed in existing building as well as in retrofitting and new construction projects (Mahdavi et al., 2021).In Europe, building energy efficiency is typically measured through the energy efficiency classification from A to G.While the EPG is typically mentioned in the context of higher-than-expected energy consumption, the gap may exist in either direction.For example, in the Swiss residential building stock buildings of low energy classification generally consume significantly less energy than assumed, while buildings of higher energy efficiency class tend to consume slightly more energy than expected (Cozza et al., 2020).

Collaborative project delivery
However, Laconte and Gossop (2016) refer to cases where buildings are consuming as much as two or three times the designed energy.The EPG is also related to other types of building performance gaps, like issues with operations and indoor conditions (Rasmussen and Jensen, 2020).Frei et al. (2017) noted that the EPG can arise in three life cycle phases of the building: (1) design and planning (poor early design decisions, uncertainty in energy modeling, oversizing of systems), (2) construction and commissioning (economy over design, poor commissioning) and (3) operation (equipment issues, user interaction and change of building purpose).Boge et al. (2018) especially highlighted the role of early-phase planning.Saving money by not investing enough in the early stage may result in costly remedies in the operational stage and sometimes even permanent problems that cannot be fixed.
In the operational phase of the building, facility managers have a significant role.Borgstein et al. (2018) found that energy performance issues relate to poor management and improper operation of systems.Insufficient energy performance guidelines and poor documentation can result in a lack of proper setpoints or high night-time loads.Floor plans with too large control zones for equipment also prevent correct operation of building automation systems.Facility managers from the USA report that the main reasons for the EPG are (1) higher than expected use of energy by the occupants, (2) there being more than the designed number of occupants and (3) technology failures (Liang et al., 2019).Facility managers are in principle expected to continually improve energy efficiency in buildings, but are not actually required or incentivized to do so.In fact, some facility managers actively avoid trying to fix issues so as not be held responsible for possible worsening of the gap, referring to unavoidable differences between theory and practice (Willan et al., 2020).Fears of causing disturbances in building operations and unfamiliarity with data-driven tools prevent the use of data-based recommendations (Markus et al., 2022).However, it can be argued that continual energy performance improvement should be a key role for facility managers.This role should be started early on, while planning and construction is still taking place (Boge et al., 2018).While the complexity of modern building services technology can be a cause of the EPG, new technologies may also offer a solution.For example, machine learning can be used to predict EPG based on risk data.This allows the project participants to react to potential energy performance issues early on, before final decisions are made (Yılmaz et al., 2023).

Definition of project delivery model
Building construction projects go through different phases which include definition, design, planning, construction, closure and handover.This process is usually called project delivery model which is also known as project delivery method or project delivery system.In this article, the term project delivery model is utilized.Project delivery model, according to Mesa et al. (2019), has three defining elements which are project organization, operational system and contractual relationships.Although the mentioned elements by Mesa et al. (2019) are inclusive, they seem to be missing an important piece which is the delivery process, referring to the steps and activities encompassing project and/or building life cycle and the involved people in each phase.If the delivery process is added to this collection, a new framework can be developed for defining project delivery model.This framework is shown in Figure 1.This theoretical foundation is of prime importance as the authors' have often observed in the literature and practice that a certain contract type or operational system or project organization is called as project delivery model whereas all the defining elements shown in Figure 1 need to be in place to have a construction project delivery model.

Collaborative project delivery
According to Moradi et al. (2022), "Collaborative delivery model is one of the umbrella terms which has been utilized by different scholars in reference to alliance, partnering, integrated SASBE project delivery, and lean project delivery."In terms of typology, according to Engebø et al. (2020), Mesa et al. (2019) and L€ ahdenpera (2012), it can be argued that partnering, alliance, IPD and LPD are the pure collaborative project delivery models.However, this study aims to provide an in-depth conceptualization of collaborative project delivery model in construction.To do so, if the framework shown in Figure 1 combined with the features of collaborative delivery models (mentioned earlier in the introduction), the result would be something like Figure 2, which provides a new framework for defining/distinguishing collaborative project delivery model in construction.The framework, shown in Figure 2, is consisted of two main elements.The first element is the defining factors of construction project delivery which include project organization, operational system, contractual framework and delivery process.And the second element is the relevant features of collaborative project delivery to the mentioned defining factors.For instance, the collaborative features related to project organization are trust-based relationships and join decision making.

Collaborative project delivery
Previous research on collaborative project delivery models Collaborative project delivery models have been extensively discussed in the recent review studies (e.g.Engebø et al., 2020;Moradi et al., 2022), and this study neither has aim to repeat those discussions in different words, nor it fits to the scope of this article.Instead, an abstract level analysis of the major studied themes is presented in Figure 3.
As can be seen, success factors and barriers is the only common theme among the conducted research on alliance, partnering, LPD and IPD.The study conducted by Moradi and K€ ahk€ onen (2022) has identified commonalities between success factors of collaborative delivery models.Among the research themes shown in Figure 3, success factors, trust and working relationship and team integration are the most relevant topics to the scope of this article.Hence, the findings of the studies representing those themes have been summarized and are shown in Table 1.

Research gap and theoretical framework
Collaborative project delivery models emerged, mainly, as a response and reaction to the five common challenges in traditional construction projects.These challenges include accidentfree construction, reliability of planning, constructability of design, adversarial working relationships and dominance of low price for selecting the contractor (Forbes and Ahmed, 2010;Oakland and Marosszeky, 2017).The research shows that collaborative delivery models have had promising results in overcoming those challenges (e.g.Ibrahim et al., 2020).
However, while the building code sets requirements for building energy consumption and developers set their own energy performance targets, a pitfall in both traditional and collaborative delivery models has been lackluster enforcement of these targets over the building's operational life cycle.Malfunctioning or inadequately calibrated systems due to lacking construction processes can often result in higher-than-expected energy consumption an EPG.This gap is of prime importance for realizing sustainability goals, in particular environmental sustainability (energy efficiency and emission), as buildings account for almost 40% of global energy consumption (Laconte and Gossop, 2016).

Collaborative project delivery
This becomes even more important if it is noted that there is usually a considerable EPG in building construction projects in terms of the discrepancy between design intentions and actual energy consumption of the building (Laconte and Gossop, 2016).In this regard, the project delivery model seems to have a big role in this EPG.Thus, studying sustainable and collaborative project delivery for building construction with a life cycle perspective is a major research gap which needs to be addressed.Hence, this study aims to do so through developing a conceptual framework (see Figure 4), identifying the causes of EPG associated with the project delivery and validating the developed framework to be resulted in the development of a state-of-the-art delivery model which can enable the realization of productive building construction and sustainable built environment in practice.

Research design
This study aims to address the EPG in building construction through the lens of project delivery model.To do so, the research process started with formulating the following research questions: (1) What are the challenges/barriers of achieving energy efficiency in building construction projects which are related to project delivery process?
(2) What kind of project delivery model could contribute toward filling the EPG in building construction projects?
Due to the adequacy of literature on the addressed topic in this study, the deductive approach was adopted (Saunders et al., 2019).Accordingly, literature study and semi-structured Collaborative project delivery interviews were selected as the data collection methods and thematic as well as content analysis as the data analysis methods.These choices were justified with regard to the exploratory purpose of the research (Saunders et al., 2019).The next step in the research design was to determine the context of study and make a choice about the sampling method.To do so, building construction and renovation projects was selected as the focus of the study.In terms of the building type (construction category), residential buildings, institutional buildings (i.e.school and hospital) and commercial buildings (i.e.shopping mall and office building) were included in the scope of the study.
Concerning sampling method, a combination of quota sampling and purposive sampling method (Saunders et al., 2019) was utilized in this study through which four groups of interviewees were specified by the research team.These interviewee groups included (1) client project manager, (2) contractor project manager, (3) design manager and (4) property management (i.e.building operation and maintenance) experts.The research team targeted at least five interviewees in each group with a provision to conduct more interviews in each group if data saturation was not achieved (Saunders et al., 2019).Then, the research team filled each quota by intentionally choosing relevant individuals (i.e.interviewees) in the possession of relevant knowledge and experiences related to the quota and the research topic.The defined interviewee groups in this study provided a basis for life cycle-based and inclusive study of performance gap through the lens of project delivery process based on the input from key project participants in different phases of project life cycle.The lifeperspective in data collection was imperative due to the diversity of disciplines involved in the design, construction and operation of a building.

Data collection
Data collection started with formulating the protocol and questions of the semi-structured interviews.The developed questions aimed to the explore the project delivery-related causes behind the EPG in building construction projects based on the viewpoints of key project participants involved in different phases of project life cycle.The developed interview protocol and questions was piloted in the first four interviews (one interview in each interviewee group) to seek feedback from the interviewees.Since there was neither negative feedback nor any changes in the interview protocol and questions, the first four interviews, which had been conducted with piloting purpose, were also considered valid to be analyzed in the data analysis stage.
In the next step, the research team conducted 21 semi-structured interviews in Finland with project professionals representing client, design/planning experts, contractors and building operation/maintenance experts.Since data saturation was achieved in each interviewee group, there was no need for conducting additional interviews (Saunders et al., 2019).The conducted interviews were audio recorded based on the obtained consent from the interviewees.Then they were transcribed and translated to English language by the native Finnish speaking member of the research team.Table 2 shows interviewees' discipline, role and their latest project's type, budget and duration.In addition, Figure 5 shows the demographic information of the interviewees.

Data analysis and validation
The analysis process started with thematic analysis which was performed by inductively coding the extracted research data as a result of analyzing the interview transcripts.The labels of the codes were data derived by the researcher (Saunders et al., 2019).Validating the generated codes was accomplished through reviewing them three times (each time by one member of the research team) and making the required corrections.The validated codes representing project delivery were formed a theme titled "project delivery."

SASBE
The establishment of the themes was done based on the sameness or similarity of the codes in terms of the meaning and/or title.Then, a content analysis was performed through which the challenges/barriers and solutions/enablers in the established themes were listed and synthesized based on the similarity or sameness of the title and/or meaning.Finally, the cross validation was carried out through showing the results of thematic and content analysis to the interviewees to ensure the interpretations made in the analysis process were valid.All the interviewees approved the results of thematic and content analysis.

Model development
Following the cross validation, the identified barriers and enablers together provided a basis for modifying the developed conceptual framework (Figure 4) in the literature study and developing a collaborative and life cycle-based delivery model for sustainable building construction.The developed model was validated in two steps.The first step of the validation included two case studies in which the modified model was shown to the project managers of one successful and one unsuccessful building construction project (in terms of energy efficiency and on time and on budget completion) to seek their feedback.The obtained feedback from the case projects was then applied, and the developed delivery model was validated.

Project delivery related challenges and solutions of energy performance gap in building construction
Analyzing the conducted interviews resulted in the identification of several barriers and solutions for achieving energy efficiency in building construction projects (see Appendix).Among them, some were frequently mentioned by the interviewees, which are shown in Table 3.As the barriers and enablers implies, the existing delivery models (both collaborative and traditional) ignore the building performance in its operational life cycle and lack Challenge/barrier Lack of/inadequate involvement of HVAC contractors and operation/maintenance people in the project definition and design phase Fragmented (i.e.divided/isolated) procurement, project delivery, and maintenance process (multiple contracts) of energy efficient systems Dominance of low-price criteria in the tendering process for selecting contractors which usually have low capacity to deliver their promises Solution/enabler Life cycle contract Collaborative project delivery model Involving building services people in the project definition and design phase Involvement of client and its representative (consultant) from project definition phase until the end of the project Early definition of the use of the building and its spaces Source(s): Authors' own work  3. Project delivery-related challenges and solutions of realizing energy efficiency SASBE sufficient strength for involving building services and maintenance experts in building design and construction phases.Moreover, limiting the contractor's responsibility to the project life cycle causes fragmentation in the maintenance and optimization of building operation.Dominance of low-price criteria for tendering is another chronic problem which results in the selection of low-capacity contractors who fail to deliver the project efficiently and are incapable of taking responsibility for the building performance in its operational life cycle.Thus, collaborative and life cycle-based delivery model seems to be a viable solution for filling the EPG in building construction projects.

Collaborative and life cycle-based project delivery model (CLCPDM) for sustainable building construction
The literature study and obtained data provided a basis for the development of a collaborative and life cycle-based project delivery model (CLCPDM) for sustainable building construction.This model has two versions: (1) The abstract version, as can be seen in Figure 6, shows the main steps in the delivery of the project and operation of the building and the main output in each step, and (2) the detailed version also includes descriptions of what happens in each step (see Figure 7).
This model has two key differences with the existing delivery models in the literature.First, CLCPDM is inclusive and covers both project life cycle and operational life cycle of the constructed building.Second, it has a combined feature of both traditional and collaborative construction projects, thereby increasing its compatibility with both contexts.The second feature also combines the strengths of both collaborative and traditional delivery models and covers their weaknesses.In other words, it is new a generation of construction project delivery model with capability to realize productivity and sustainability in both project and product life cycle.
In short, the developed model: (1) Fully realizes the significance of proper project definition, feasibility study and competent as well as price-based contractor selection, (2) Involves the design team and contractor when they have the highest impact, (3) Features life cycle-based and collaborative project definition and design, (4) Treats essential design and planning as an iterative cycle to realize the required improvements, (5) Employs collaborative tools and working practices in design and construction phases and (6) prioritizes systematic and continuous documentation of project and building performance data.

Discussion
Project delivery has been a mechanism for the successful completion of construction projects.
The traditional model of this mechanism has not yielded satisfactory results most of the time, particularly in the complex projects, resulting in the over budget, waste, low quality, accident full and delayed delivery of building construction projects (e.g.Forbes and Ahmed, 2010;Moradi and Sormunen, 2023).Collaborative project delivery emerged to be an effective replacement, and it has had promising performance results (e.g.Hanna, 2016;Ibrahim et al., 2020).In spite of this advancement, building construction projects, to a high extent, are still struggling to meet the environmental sustainability goals; their actual energy consumption is Collaborative project delivery considerably higher than expectations (e.g.Laconte and Gossop, 2016).Of course, there are several factors behind this EPG phenomenon one of which is project development and delivery process (Moradi and Sormunen, 2022).In fact, this factor happens to be a major cause of the EPG.In particular, the findings showed that inadequate and/or late involvement of key project participants (including building services people) together with fragmented project delivery and maintenance process and dominance of low-price criteria for contractor selection are the key barriers of achieving energy efficiency in building construction (Moradi et al., 2023).The identified enablers in this study were relevant to the barriers, which can be seen as an indication of the reliability of the obtained results.
The involvement issue can be explained as the missing impact which contractor as well as maintenance experts can have in the project definition and design stages.In other words, these people are a dynamic database of building performance data which can help the client and design team to first reasonably define the goals and then provide input for ensuring constructability of the design in the construction phase and functionality of building in its operation phase.The fragmented project delivery and maintenance exactly reflects on the discovered research gap in this study and its purpose, addressing the fact that project and product life cycle need to be integrated and the key people involved in project life cycle need to be involved and accountable in the product life cycle as well.Finally, the third issue, dominance of low-price criteria, has been a problem for a long time which results in the selection of lowcapacity contractors which do not have the required resource and competence.Although collaborative delivery models (e.g.alliance, IPD) has removed this dominance and mostly consider the competency as the selection criteria, they are also missing an important point which is the reasonable price offered by the contractor.Thus, it seems that a mature contractor selection mechanism needs to take into account both tendering (based on a reasonable price range specified in the project definition) and contractor's capacity (i.e.experience, knowledge, adequate financial resources, sufficient and competent workforce), as the competency criteria.The same selection logic must be also applied for employing the design team.The mentioned solutions in Table 3 concisely characterize the project delivery model, developed in this study, which can overcome the related barriers for achieving energy efficiency in building construction.
The obtained results in this study contribute to the existing body of knowledge in two aspects.First, the findings fill the knowledge gap on the role of project development and delivery in the EPG in building construction.To the best of the knowledge of the authors, this is the first study looking into the EPG in building construction through the lens of project development and delivery process.The second contribution is the development of a novel delivery model which features collaboration and life cycle perspective as its building blocks, and it is yet compatible with the traditional contracts and tendering processes.In other words, CLCPDM is the new generation of construction project delivery model which contributes toward productivity and sustainability achievement in both project and product (i.e.constructed building) life cycle.
From practical perspective, the discussed challenges and solutions together with the developed delivery model informs project professionals and clients on the project delivery related causes of EPG and then provide a practical solution for collaborative and life cyclebased project development and delivery.In particular, the model provides a practical guidance for clients on how to develop their project with a life cycle perspective over the benefits and loss resulting from different decisions.It also reveals the best time for involving the design team and contractor to benefit from their impact.
Although the main focus of the developed model (Figures 6 and 7) is on the delivery process, it still includes the application of relevant tools for measurement, simulation, monitoring and optimization purposes, but does not prescribe/recommend any specific tool.Such optimization could be performed with the help of a digital twin that allows the real-time comparison of actual energy performance to that predicted by simulations (Spudys et al., 2023).A digital twin might be created from building information modeling (BIM) data that are SASBE used in the design phase of the building.The combination of BIM and digital twins could also be used to expand the life cycle optimization to cover not only environmental, but also social and economic impacts (Boje et al., 2023).As more and more data from buildings becomes available, increasingly accurate prediction of building energy consumption can be made using machine learning methods (Miller et al., 2020).Artificial intelligence (AI)-based systems may be used to optimize various aspects of buildings, such as energy consumption, thermal comfort and lighting conditions, both in the design and operational phases of the building life cycle (Mousavi et al., 2023).Accordingly, there is a great potential for the continuous improvement of construction project delivery through the integration of dynamic digital tools like BIM and digital twins.

Conclusions
This study aimed at discovering the project delivery related barriers and solutions of realizing energy efficiency in building construction projects and to develop a collaborative and life cycle-based delivery model for sustainable building construction.This was accomplished through a literature review combined with a qualitative study involving semistructured interviews.The opinion of project professionals representing client, design, contractor and property management (i.e.building operation and maintenance experts) were obtained and analyzed.Accordingly, it is concluded that: (1) The project delivery model considerably accounts for the success or failure of the realization of energy efficiency in building construction projects.
(2) Involvement of building services experts and maintenance people in the project definition and design seem to enhance the constructability of the building services design and functionality of the building's Heating, Ventilation, and Air conditioning (HVAC) system in the operation phase.
(3) Project delivery contract should expand the responsibilities (including risk and reward) of project parties into the constructed building's operational life cycle.
(4) Collaborative and life cycle-based delivery model combines strengths of both traditional and collaborative delivery models' and covers their weaknesses.The developed model in this study fulfills this purpose.
The obtained results in this study considerably contribute toward existing body of knowledge in two areas of EPG in building construction and collaborative project delivery.However, it is acknowledged that the findings are based on Finnish professionals' input and expanding this research to other regions is a potential area for further research.Moreover, the developed model, although validated in Finland, needs to be tested in a broader context as well to increase its generalizability.Furthermore, it is also acknowledged that in this study the interviews were conducted with certain groups of professionals involved in project delivery process and building operation as well as maintenance, and including building users as the fifth groups of interviewees could have been value adding.Hence, obtaining building users' input is suggested to be considered in the future relevant studies.
Figure 1.Framework for defining project delivery model

Figure 3 .
Figure 3. Major themes in the previous studies addressing collaborative project delivery models Figure 5. Demographic information of the interviewees Figure 6.Abstract version of collaborative and life cycle-based project delivery model (CLCPDM) for sustainable building construction