An Environmental Quality Assessment of Office Buildings: The Impact of a Glass Façade on Users in Different Climate Contexts in Croatia

Abstract

Glass façades are being increasingly applied in the typology of office buildings. The environmental quality parameters of indoor spaces have an exceptional influence on the wellbeing and productivity of users. In the past, climate defined the architecture of a particular area. Façades of contemporary office buildings have similar characteristics regardless of their location. This study comprises four office buildings with different types of glass façades located in areas of continental and coastal climate in Croatia. Also, the research relates to a graduate study course, during which the students designed a survey to gain insight into the satisfaction of users of the analyzed buildings. The aim of this research is to explore the impact of the design of glass façades in a certain climate context on users. This paper will address the following research questions: (1) Is a fully glazed office building pleasant to work in? (2) Is a fully glazed façade an appropriate solution for office buildings in hot and sunny climates? The research results show that to achieve a comfortable indoor environment for buildings’ users, it is necessary to design the façade according to the orientation, especially in terms of glazing characteristics, a sun protection system, and window control.

1. Introduction

The climate context in which buildings are created is one of the key parameters that influence design decisions. Vernacular architecture offers an insight into the climate influence on the design of the building and its envelope [1]. The above is particularly important in areas with climate extremes [2], where examples of specifically shaped climate adaptive envelopes can be found, to ensure the comfort of staying in the interior space [3]. Recently, the number of office buildings with a fully glazed envelope has been increasing, which represents an indoor comfort challenge, especially in areas with warm and hot climates and intense solar radiation [3,4]. A key challenge in the process of designing a glass envelope and selecting its features is balancing the requirements of energy efficiency and users’ satisfaction in a specific climate context. Previous studies [5,6,7,8,9] indicate a research gap regarding the qualitative comparison of the impact of different types of glass façades on users’ comfort in various climate conditions. Therefore, this paper targets the assessment of indoor environmental quality within the frame of glazed office buildings in different climate areas in Croatia.

The energy efficiency of buildings has been the focus of the public since the oil crisis in the 1970s [10]. Energy use in buildings accounts for about a third of the world’s total CO₂ emissions. The European Union’s Energy Performance of Buildings Directive 2010/31/EU EPBD [11] defined strict requirements for energy efficiency in buildings to reduce energy consumption and, consequently, CO₂ emissions and climate impact. Almost half of the building’s energy losses occur through the outer envelope [12]. In Croatia, the energy efficiency criteria for the outer envelope are defined in accordance with the Technical Regulation on Rational Use of Energy and Heat Retention in Buildings [13] and other regulations arising from it. Research shows that the energy efficiency of the envelope can sometimes be inversely proportional to the thermal and visual comfort of the interior space [12]. In temperate climate areas, the balance of providing sufficient daylight, passive heating of space, and protection from excessive sun exposure should be achieved. Excessive solar gain may result in high cooling energy consumption and indoor discomfort [14].

The characteristics of the façade in relation to the climatic conditions significantly affect the comfort of staying in the interior space. Previous studies of public buildings’ glass facades located in different parts of Croatia show that orientation of the façade in relation to the glazing characteristics, the sun protection system, and the operation of windows significantly affect user comfort [15]. The comfort of staying indoors is the result of the interaction of thermal, visual, acoustic comfort, and indoor air quality [16]. Office spaces should have an acceptable level of thermal and visual comfort without impairing the quality of lighting, which would lead to a decrease in employee productivity [12]. The thermal comfort of the room depends upon the air temperature, radiant temperature, relative humidity, and airflow speed. Users are usually able to control the inside air temperature. Most standards use operative temperature (a function of the air temperature, the mean radiant temperature, and the relative air velocity) as a representative of the indoor thermal comfort [17,18]. Other environmental factors are physiologically associated with overheating, including a lack of air movement and high humidity. The thermal comfort of the room also depends on the level of clothing and the type of activity that will take place in the space. Visual comfort depends on direct sunlight and the natural lighting of the space. Research shows that the degree of thermal and visual comfort strongly affects the productivity of users [19,20]. Users usually prefer the possibility to control the comfort conditions of the environment in which they work [21,22,23,24]. In Croatia, the Technical Regulation on Rational Use of Energy and Heat Retention in Buildings [13] prescribes that the comfort of the interior space should be achieved by proper heating, cooling, and ventilation of the space and by meeting the requirements of thermal stability and interior surface temperature, humidity, air quality, lighting, and noise in the interior space.

Personal preferences can represent a significant problem in achieving the comfort of staying in an interior space. Optimal comfort criteria will differ from one space user to another and will also depend on the possibility of adaptation inherent to the human body [25,26,27]. During the design process, architects and engineers, therefore, rely on theoretical assumptions for users’ behavior and preferences or recommendations from laboratory-based human factor studies conducted under controlled conditions that may not be directly applicable to buildings in use. It is of great importance to provide architects and engineers with reliable data of the relationship between the outdoor inputs, existing performance indicators, i.e., the indoor environmental conditions (daylight autonomy, thermal comfort, window operation, and sun protection) compared with the users’ subjective assessment in real office work environments [28,29].

The quality of the internal environmental conditions of office spaces is particularly important since users usually spend most of their waking time inside office buildings [30]. Also, office buildings are the third largest group of buildings in terms of size of the closed space, following residential and commercial buildings [31]. Over the years, various models have been developed for evaluating the comfort of users in indoor spaces, initially through quantitative analysis of the measured parameters of the indoor environment (e.g., temperature and relative humidity) [32]. Since the 1980s, the internal comfort has been evaluated through a combination of a quantitative analysis and qualitative assessment approach [31]. The post-occupancy evaluation method was intensively used in cases of sick building syndrome and severe user complaints related to the workplace environment. Since 2005, the number of published publications researching indoor comfort in office buildings has been increasing [33,34,35]. Subjective methods such as occupants’ surveys [36,37] are used in the analysis of the individual preferences of the users according to the conditions of the internal environment. Dynamic simulations using software can be of great help in researching the topic of indoor environment quality [38,39]. The literature shows that there is a correspondence between the results of subjective surveys and those from dynamic simulations [40]. Wellbeing and occupant satisfaction have become key factors in designing the façade of office buildings [41]. Through the analysis of user experience, this research shows the impact that contemporary energy-efficient glazed façades of office buildings in two different climate zones in Croatia (Zagreb, with a continental climate, and Split, with a Mediterranean coastal climate) have on the thermal and visual comfort as well as indoor air quality of the interior space.

The field of office glass façade energy efficiency research [3,4,8,9,19,42,43,44,45,46,47] and indoor environmental quality [6,15,16,18,19,20,21,22,23,27,28,30,31,32,33,35,36,38,39,41,45,48,49,50,51,52,53,54] is extensive. Numerous studies focus on the combination of the research into the energy efficiency of a façade and its impact on user comfort in certain climate contexts [3,5,7,8,12,18,26,28,34,55,56]. Some researchers have stated concern regarding the rising use of glass facades in hot climates [7]. Westphal and Andreis [4] focused on the impact of office glass facades on air conditioning energy consumption in different climate contexts. Baumgärtner et al. [9] researched a prototype solar thermal glass façade in hot and cold climates but without the influence on users’ comfort. Kim et al. [28] researched indoor environmental quality in offices in a hot arid climate without analyzing façade features. Le et al. [46] analyzed adaptive facades in terms of the energy performance and users’ comfort in different climates.

Hafizi and Vural [57] performed a similar study to ours while focusing on the interaction of user and climate adaptive building shells. Aste et al. [58] also carried out similar research to ours analyzing virtual reference room façade energy performance and the impact on indoor comfort in various climates. Their research was conducted via computer simulation (without a users’ survey). Ayçam et al. [37] showed the possibility of combining the post-occupancy survey and analysis of the envelope features concerning the energy and atmosphere parameter in office buildings, without real-time monitoring of the indoor climate data in various climate contexts. The literature review indicates the need for analysis through the combination of office glass facade features examined via energy and atmosphere parameters and their impact on user comfort in various climate contexts.

This research is related to the graduate study of the Faculty of Architecture, University of Zagreb, within the course Architecture and Technology 1 [59]. In the group mentored by Iva Muraj and Stanka Ostojić in the academic year 2021/2022, the students discussed the impact of the building’s envelope on its users. Students Petra Dragošević, Ana Jerković, Karla Radić-Lima, and Bruna Sorić analyzed the impact of the glazed envelope on users, researching case studies of two pairs of office buildings with single- (Case Study 1) and double- (Case Study 2) glazed façades in Zagreb and Split. Through the analysis and comparison of studied buildings’ façade characteristics and a users’ questionnaire, these students investigated the influence of different types of façade systems and different climate zones on the thermal and visual comfort and indoor air quality of the interior space [60]. Research was carried out on office buildings in two different climate contexts in Croatia: in Zagreb, in the continental climate area, and Split, in the coastal climate area.

Different types of environmental evidence were used to identify climate-related decisions in the design of buildings. Buildings with different types of glass façades were analyzed to establish the effectiveness of various glass façade systems. The aim of this research is to explore the impact of the design of glass façades in a certain climate context on users. Therefore, this paper addresses the following research questions:

Question 1. Is a fully glazed office building pleasant to work in?

Question 2. Is a fully glazed façade an appropriate solution for office buildings in hot and sunny climates?

In Croatia, there has been no study researching user experience regarding the indoor comfort of glazed office buildings in different climate areas so far. The results could improve the approach to assessing the impact of the building envelope parameters on user comfort and energy performance in different climates. Also, by including students in the research process, this study tends to raise awareness of the importance of adopting technical solutions early in the design process to avoid indoor environmental problems regarding the buildings’ glass façade performance in various climate areas. The research findings show students the practical application of their theoretical knowledge.

2. Challenges of Applying Glass Façades in Buildings

The architectural envelope represents a barrier between the external and internal environment and protects the internal space from external influences and weather conditions. The material of the architectural envelope will have a decisive influence on the impact of the parameters of the external environment on the internal space, especially in terms of energy transmission [3,12]. Glass is characterized by high solar energy transmission from the external environment to the internal space. Therefore, the proportion of glass in the building envelope will define the amount of solar energy that enters the interior space from the outside. Glass façades are often used in office building design in major European and North American cities [3,4], which results in a significant influence on the external environmental conditions on the internal comfort of office spaces.

The large glazed surfaces of the office envelope allow a view of the outside space and provide information about the environment in which the building is located [27,30,38,41]. However, glazed envelopes must be carefully designed so that, in relation to the climatic characteristics of the location, no increased energy consumption for heating and cooling the building occurs. Research shows that in the Mediterranean climate area, the energy efficiency of office buildings with glazed façades largely depends on the cooling energy consumption [3].

In general, façades can be designed as single- or double- (multiple) façades, depending on the composition and construction [10]. Considering their construction, both single- and double-glazed façades can be divided into post-and-beam façades, unit system façades, and point-fixed façades [43]. Post-and-beam and unit system façades have been analyzed in this research. The advantages of unit system façades are the production of complete units in the workshop, significantly accelerated assembly, floor-level closure of the façade and the avoidance of scaffolding during the installation of the façade [43]. The single glazed façade consists of one glazing with double- or triple-insulated glass. Double-glazed façades consist of an external façade, an air gap, and an internal façade [44,45]. The external façade is usually glazed with single safety glass, which protects the air gap from external influences and weather conditions. The air gap can be ventilated (naturally or mechanically) or closed. During the summer, a ventilated air gap enables the extraction of heated air [8]. Closed in the winter, the air gap serves as a buffer space for accumulating solar radiation and the passive heating of the interior space. The internal façade is usually made of double-insulated glass, and it is the main heat loss reduction element of the building envelope. It can be opened due to the natural ventilation of the interior space. One of the advantages of the double façade is the possibility of placing a shading system inside the air gap. The shading system serves as an external protection from the sun because it is located outside the internal façade without compromising the design of the outer envelope of the building (Figure 1). The disadvantage of a double-glazed façade can be a higher initial investment compared to a single-glazed façade [44]. Single unit system façades were analyzed in the first pair of studied buildings. Double façades were analyzed in the second pair of studied buildings. It should be noted that the following research is based on locations in the northern hemisphere.

The sun’s radiation passes through the glass, providing natural lighting and passive heating of the space, while reducing the lighting and heating energy consumption. The natural lighting can positively affect the visual comfort in space, and the glass façade enables desirable views towards the outer space [27,30,38,41]. However, excessive exposure of the glass façade to solar radiation (especially in areas with warm climates) can significantly contribute to overheating of the interior space, unpleasant glare, and increased cooling energy consumption [7,14,30,38,41,46,61]. Office buildings with glazed façades in warm climates should have an appropriate shading system (ideally on the outside of the façade) to prevent overheating of the interior space [3,37,48]. If the building does not have a built-in shading system, the mechanical ventilation and air conditioning system will have to consume larger amounts of energy to achieve a comfortable stay in the interior space.

3. Data Collection

The Republic of Croatia is climatologically located in the northern temperate zone, but it is characterized by areas with different climate characteristics. In Croatia, the topography of mountains, plains, forests, and an elongated coastal belt affects the local climate conditions. Based on the mean values of air temperature and precipitation, there are three main climate areas in Croatia: continental, mountain, and maritime [62]. The northern, northwestern, and eastern parts of Croatia are characterized by a continental climate, while the elongated coastal belt is a part of the coastal climate area. Higher altitudes of the Dinaric mountains are characterized by a mountain climate that differs from the wider area primarily in terms of lower temperature and intensive snow regime [62]. According to the Ordinance on Energy Audits and Energy Certification of Buildings [63], the territory of Croatia is divided into two zones regarding the mean air temperature of the coldest month (measured at the location of the building): continental (≤3 °C) and coastal (>3 °C). This research includes four buildings located in cities whose climate characteristics are used for the energy audit and certification of buildings in Croatia: Zagreb (45°48′ N; 15°58′ E) for the continental climate and Split (43°30′ N; 16°26′ E) for the Mediterranean coastal climate (Figure 2). The continental climate is a moderately warm and humid climate zone with hot summers, while the Mediterranean coastal climate zone is characterized by hot dry summers and cool rainy winters [62].

A comparison of the climate characteristics of Zagreb (Maksimir meteorological station) and Split (Marjan meteorological station) reveals significant differences between these two cities. Zagreb is characterized by a lower mean annual temperature (difference of 7 °C) and a lower mean temperature of the hottest (difference of 6 °C) and coldest month (difference of 8 °C) of the year. Zagreb is also characterized by a three times smaller number of hot days per year compared to Split and a three times a larger number of colder days [64]. Split is characterized by a higher average annual irradiated solar energy and average annual insolation and a lower average annual cloud cover [65,66,67]. Split is characterized by 900 more hours of sunshine than Zagreb according to the data on average annual insolation.

The dimensions, form, and design of the façade of the studied complexes strongly affect the town area silhouette. The first analyzed pair consists of two business buildings: the Business Center Strojarska in Zagreb and the Westgate Towers in Split. Both analyzed buildings have a single-unit system glazed façade. The second case study also comprises two business buildings: the Eurotower in Zagreb and the administration building at the University Library in Split. Both analyzed buildings have a double-ventilated glazed façade.

3.1. Case Study 1

The Business Center Strojarska is a part of the commercial–residential complex located in central Zagreb, south of the railway line, north of Vukovarska street, east of the Main Railway Station (and mostly abandoned factory halls), and west of the Bus Station. The complex was built in 2014 according to a project by Proarh d.o.o. Studio and architects D. Mateković, T. Stojan, M. Malez, I. Ister, and V. Pavličević. It consists of six buildings: two business towers, two residential slats, and two lower business buildings [68]. The higher tower of the complex is also the tallest tower in Zagreb (besides the cathedral towers) with a height of 96.15 m. The complex was built on flat ground surrounded by low building volumes in relation to the dimensions of the towers. The slats are nine floors high, while the towers are fourteen and twenty-six floors high. The business towers, trapezoidal in plan, are located in the north part of the complex (Figure 3a).

The towers are glazed with a single unit system façade with triple-insulated glass [69]. The total thickness of the glass is 56 mm. The internal gaps between the glass layers of the façade are filled with argon. This façade is divided into fixed transparent parts, non-transparent operable elements (aligned to the transparent ones), and non-transparent parapets. The glazing color is anthracite, while the non-transparent opening panels are painted in yellow and blue tones. The façade sun protection is dark gray tinted exterior glass and interior roller blinds. Analysis of the insolation of the Business Center Strojarska, focused on the volume shadows’ movement in July (when the shadows are the longest), shows that the larger tower shadows the entire southern façade and a smaller part of the eastern façade of the lower tower in the afternoon hours. The smaller tower overshadows an almost negligible part of the western façade of the higher tower (Figure 3b).

The Westgate Towers complex is located in Domovinskog rata Street, south of the industrial zone of the city of Split (North port) and north of the expanded city center, marking the western entrance to Split. The complex is located in a mostly low-rise building zone, 800 m from the sea (Kaštelanski Bay). The complex consists of two towers with the same floor plan dimensions but with different uses and heights. The lower business tower was built in the first phase of the project in 2016. The higher tower houses a hotel, and it is the tallest building in Croatia with a height of 135 m, built in 2023. The complex was designed by architects O. Barić and I. Potočić [70] (Figure 4a).

The towers are glazed with a single-unit system façade with double-insulated glass. The total thickness of the glass is 36 mm. The internal gap between the glass layers of the façade is filled with argon. The façade is divided into vertical strips by protruding profiles. Each strip comprises operable and non-operable parts of the glass façade. The façade sun protection is dark gray tinted exterior glass and interior roller blinds. An analysis of the sunlight path shows that the higher tower shadows half of the smaller tower (northeast and northwest part of façade), while in the afternoon, the smaller tower shadows the lower floors of the higher tower. (Figure 4b).

The analyzed examples differ in the thickness and the number of glass layers of the façade, profile design, window (U_w) and glass (U_g) thermal coefficient values, treatment of the opening parts, and sun protection, as shown in Figure 5.

3.2. Case Study 2

The Eurotower is a business complex located in Vukovarska Street in Zagreb. The complex was designed by architect M. Hržić in 2002 and was completed in 2004 [71]. It consists of two volumes, connected underground and aboveground, the higher of which is one of the tallest towers in Zagreb (96 m above ground). The ground floor and the gallery comprise a cafe, a restaurant, a reading room, and an entrance hall with a reception, and the rest of the building includes offices. The higher volume is twenty-three, and the lower one eleven stories high. The engine room is located on the roof (Figure 6a).

The tower is glazed with a double-ventilated façade. The internal façade is made of double-insulated glass and the external façade of single glass. Sun protection includes a reflective coating on the tinted glazing and electrically powered aluminum blinds located in the air gap between the internal and external façade. The internal façade can be opened, and the external glazing is fixed. The air gap is continuous and naturally ventilated. In the interior, the parapets are low, the wider parts of the façade are fixed, and the narrower windows are operable. The duality of the façade is barely visible from the interior and exterior due to the same design of the fields of layers and the minimal air gap (13 cm). An insolation analysis in the month of July, when the shadows are the longest, shows that the higher tower does not overshadow the lower one (Figure 6b).

The University Library in Split is the central complex of the Split student campus, located between the main square and the University Park, next to the main pedestrian street of the campus. In addition to the library, the complex also houses the university’s administration offices [72]. The University Library complex was designed by D. Bašić-Ožić, J. Jelavić, D. Perišić, L. Petrović Sika, J. Šenović, and E. Širola and completed in 2008 [73]. The complex is composed of three volumes. The south (analyzed in this research), the university’s administration building, is a glazed volume with seven aboveground floors, raised on columns. The basement and two underground floors include reading rooms and libraries. The north building is a four-story volume with additional office spaces for rent and spaces for student group work (Figure 7a).

The university administration building is glazed with a double-ventilated façade. The glazing is transparent, the internal façade is made of double-insulated glass, and the external façade is made of single glass. The air gap is continuous and naturally ventilated. The inner glazing can be opened by sliding, and the outer glazing is fixed. The duality of the façade is visible due to the different design of the outer and inner envelope. The air gap is eighty centimeters wide, partly because of the sliding of the interior façade. An analysis of the July insolation shows that the south volume overshadows the northern part of the complex in the central part of the day, but the façade of the analyzed building remains unshaded (Figure 7b).

Both analyzed examples have the same thickness of glazing and the principles of functioning of the double façade, opening of the inner glazing and a fixed outer envelope. The sun protection is similar for both buildings, with electrically controlled aluminum blinds in the air gap (the Eurotower’s reflective tinted exterior glass is an additional sun protection feature of the façade). The façades of the Eurotower and university administration building are different in the dimensions of the air gap and the color of the glazing, as shown in Figure 8. The analyzed examples also differ in the window (U_w) thermal coefficient values and the treatment of opening parts.

4. Methods

Four office buildings were selected and paired into two case studies to analyze the impact of glass façades on users in different climate contexts. There were two categories of data collection methods: qualitative assessments of the façade’s resilience to environmental variables and subjective assessments by users. The qualitative assessment was used to compare the façade characteristics on real-world sites, and the survey was used as the main data collection tool in assessing users’ experiences of the indoor comfort of the workspaces. The research process used in this study is shown in Figure 9.

A thorough analysis of the empirical evidence from four office buildings was conducted to deduce the used type of glass façade system and to understand the mechanisms for indoor comfort in various climatic scenarios. Using the existing building’s envelope as a case study, the building’s façade was analyzed to understand the architect’s logic behind its design. This qualitative assessment aimed to obtain valuable insights regarding the characteristics and advantages of the façade systems used in office buildings.

The study focused on an indoor environment, but first, certain inputs from the outside, including the climate, building geometry, and building envelope, were assessed (Figure 10). Therefore, this paper aims to differentiate numerous factors that affect the micro-climatic conditions of the workspace, including its location, exposure to sunlight and wind, availability of sun shading, and proximity to water (sea). Also, the various façade system types were analyzed by their defining features, including the envelope’s surface-to-building volume (A/V) ratio and parameters such as air permeability, water tightness, glass thermal coefficient (U_g), and the overall window thermal coefficient (U_w). The study also accepted the depth and height of the workspace, the control systems like ventilation and heating, and the individual user control of opening the windows and naturally ventilating the interior space.

Secondly, a web-based survey was carried out to examine the user’s experience in the occupied open-plan office environments, where participants performed real work tasks. Employing user-experience research methods is crucial in gathering valuable data and insights into indoor environmental conditions (visual comfort, air quality, and thermal comfort). The personal preferences of users should be considered when defining the terms of indoor workspace comfort.

The survey consisted of 21 questions aimed at collecting information about the layout and the comfort of the workspace, the use of lighting devices and windows, sun protection systems, the users’ perception of solar heat and air quality, the temperature levels, and the overall experience of the workspace (Appendix A).

Survey Development and Design

A set of specific questions was created for users of selected office buildings. The real end users of an office building are many and varied: employees in an office building, employers, labor workers, personnel in a shop, etc. To collect employee feedback, an anonymous survey can be effective and can provide more reliable and honest data. The survey was developed by the authors and students of the Faculty of Architecture, University of Zagreb [60]. The buildings were assessed based on 13 aspects that were divided into the following categories: sun protection operation, the influence of micro-climatic conditions on productivity, control systems (cooling and heating), air quality, window opening frequency, sufficient daylight illuminance, heat transfer, summer overheating, the influence of the façade on productivity, workspace comfort, illumination of the space, turning on the lights while using a sun shading device, and turning on the lights in winter.

Questions were presented in the form of multiple choice and were easy to answer. The office buildings were assessed using a qualitative scale from 1 (poor) to 5 (excellent), a frequency scale from 1 (never) to 3 (often), a performance scale from 1 (does not meet) to 3 (exceeds), and closed questions (yes–no questions). In addition to closed questions, at the end of a questionnaire survey, an ‘open’ question was included, where respondents were invited to write comments or suggestions.

The survey answers were collected online using a survey questionnaire (Google Forms). The survey was carried out over one month from 20 November to 20 December 2021. Seventy valid responses were obtained.

5. Results

5.1. Results for Case Study 1

In total, 43 respondents provided feedback for Case Study 1. The survey was completed by 21 employees from the towers of the Business Center Strojarska complex in Zagreb and 22 employees from the Westgate Tower in Split (lower tower).

The façades of both analyzed examples were designed uniformly, regardless of their orientation. The workspaces of both observed open-plan buildings were evenly distributed along the north, south, east, and west façades. Accordingly, the perimeter office zone was divided into four differently oriented areas to collect and analyze the data. The number of survey respondents in relation to their workspace orientation for Case Study 1 is shown in Figure 11.

Both buildings had internal sun protection and, according to the survey responses, obtained similar and very good ratings for their effectiveness. The Business Center Strojarska had a rating of 3.80 and the Westgate Tower had a sun protection operation effectiveness rating of 3.90, both on a scale of 1 to 5. According to the survey results shown in Figure 12, 81% of the respondents working in the Business Center Strojarska in Zagreb claim that the micro-climatic conditions had an impact on their productivity, in contrast to Split, where just over half of the Westgate Towers’ respondents (55%) said that micro-climate conditions impacted their productivity. Users in both buildings frequently use heating and cooling devices, with only a minority using them occasionally. In both Zagreb (76%) and Split (68%), users reported frequent use of control system devices. Additionally, the air quality in both buildings was rated as satisfactory for the workspace. The air quality at the Business Center Strojarska averaged 3.76 on a scale of 1 to 5, whereas the Westgate Tower had an average value of 3.63.

In Zagreb, many users sometimes opened the windows in the north (72%), east (67%), and south (75%), but in the west-facing offices, 43% of respondents often opened the windows. In Split, many users in the north (72%) and east (80%) sometimes opened the windows, but in the south- (50%) and west-oriented offices (33%) some users often opened the windows.

When questioned about daylight levels and their impact on users’ wellbeing at the Business Center Strojarska in Zagreb, many respondents oriented to all cardinal directions stated that there was sufficient daylight. However, a third of users (33%) noted that there can be excessive light in the east and a quarter of users (25%) in the south from time to time. When asked the same question, users surveyed in the Westgate Tower in Split indicated that the overall level of daylight was adequate. However, one-third of users (33%) reported excessive sunlight in areas with a west-facing orientation.

The response of users to heat transfer differed between Zagreb and Split. The glass façade in Zagreb allowed excessive heat to enter from the east (67%) and west (43%), while the north (72%) and south (75%) received sufficient heat. According to the user responses in Split, the majority found the amount of heat transfer to be satisfactory in the north (86%), east (80%), and south (100%). However, one-third of users (33%) reported experiencing excessive heat in the west-facing offices. Surprisingly, in both cities, only 14% of respondents in the north-facing offices reported an insufficient amount of heat.

During the summer period in Zagreb, all users facing east reported experiencing overheating (100%). More than half of the users facing west (57%) also reported overheating, as well as a quarter of those facing south. In Split, users facing east reported overheating during the summer season (80%), as well as many west-facing users (67%) and half of the south-facing users (50%). During the summer period, a quarter of the north-facing users in both cities reported experiencing overheating.

The Business Center Strojarska respondents rated the impact of the façade on their productivity with an average rating of 3.48. Additionally, they rated their experience working in the office space as an average of 3.91 out of 5. Many users have reported that the workspaces were well-lit with natural light, making it unnecessary to turn on artificial lighting when using sun protection devices, except in the south-facing areas where one-quarter of the respondents (25%) reported that they did not have enough daylight with interior shading devices in use. During winter, 86% of users in the west-facing offices turned on their lights, while in the south, half of the users regularly switched on their lighting, and the remaining individuals did so occasionally.

Based on the survey responses from the Westgate Tower, the average rating for the impact of the building’s façade on productivity was 3.91. Additionally, the average score for working in the office space was 4.01 on a scale of 1 to 5. According to users, the workspaces were well lit with natural light, and users did not require additional lighting when using interior shading devices, except in the west-facing offices where it might be necessary to use lighting. During winter, only 25% of respondents in the north, east, and south reported regularly using electrical lighting. However, in the west-facing offices, there was an even split with 50% using it regularly and 50% not using it regularly. A summary of users’ responses to the Case Study 1 questions are shown in Figure 13. Each of the pie charts, graphs, and bars below graphically shows the percentage of individual degrees of answers in relation to the orientation for 13 aspects of indoor performance indicators.

Open-Ended Response Results

In addition to closed questions, at the end of a questionnaire survey, an ‘open’ question was included, where respondents were invited to write comments or suggestions. Open-ended questions are free-form survey questions that allow respondents to answer in an open-text format based on their complete knowledge, feelings, and opinions [74]. Some selected responses for Case Study 1 received via questionnaire survey regarding the glass façade-related issues are shown in

Table 1. Selection of open-ended responses for Case Study 1 [60].

Case Study 1/Users’ open-ended responses regarding the impact of the glass façade on indoor comfort. Survey question: Do you have any comments for the glass façade or suggestions for improving it?
Zagreb   Business Center Strojarska	the west-facing workspace ‘The space is always perfectly lit with natural light’. ‘Sunsets in the summer shine too brightly, but that is easily solved with blinds’. ‘We rarely regulate the temperature, due to good insulation in winter, and the windows are opened in summer. There is always a light wind’. ‘In winter, the situation is mostly good. In summer, not really. The space is exposed to direct sunlight and heats up very fast. It is necessary to aggressively turn on the air-conditioning (18°C) to cancel these influences’. ‘The office space doesn’t have an effective supply of clean air through the ventilation system, so windows that open only 15 degrees must be used. The space then heats up even faster’. ‘We must use thick blinds to be able to read from the monitor’.
Split   Westgate Tower	the south-facing workspace ‘My workplace is situated on the southern façade, but I face the west because the desk is leaning against the façade. The constant reflection bothers me the most, so I use blinds. Colleagues on the other side of the office space do not use the blinds, so they have an open view’. ‘The temperature is pleasant, except in the summer. It would be very hot if the air conditioning was not working all the time. Some coworkers don’t like cold air conditioning temperatures, so they open the windows’. ‘There is a problem with the windows: they can’t open when strong wind blows outside. The wind is often strong and slams the windows. We even have instructions not to open the windows in the case of strong wind’.

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Open-ended response results generate subjective information from the users. Well-constructed arguments are based on personal experience of the office space; so, there are positive and negative responses about the impact of the glass façade on indoor workspace comfort. Responses of the employees in Zagreb and Split confirmed the most frequent topics such as the indoor air temperature and overheating, glare issues, window opening and sun-shading use, and the air-conditioning of the space. Direct sunlight in office buildings, especially in hot and sunny climates, may cause discomfort due to more overheating than diffuse light and glare. The open-ended responses showed that users were not satisfied when the windows could not be opened. Users preferred an open view and visual contact with the outdoor environment while working in a high-rise building. At the same time, they preferred the prevention of glare and direct sunlight; so, the sun blinds must be in use. For a better user experience, the possibility of a slightly opened window (when weather conditions allow) that can introduce outdoor air is beneficial.

5.2. Results for Case Study 2

The second case study survey received feedback from 27 respondents. Twelve employees from the Eurotower in Zagreb (higher tower) and fifteen employees from the administration building at the University Library in Split completed the survey. The number of survey respondents in relation to the workspace orientation for Case Study 2 is shown in Figure 14.

To collect and analyze the data, the perimeter office area was divided into four zones: north, south, east, and west. The analysis of the selected buildings of Case Study 2 shows that the façades of both examples were treated uniformly, regardless of their orientation. The Eurotower workspaces are continuously distributed along the façade, regardless of orientation, while at the university administration building in Split the workspaces are southwest and southeast oriented, and communication hallway is located in the northeast along the façade.

Both analyzed buildings feature shading devices located in the air gap of the double façade on the external side of the inside façade. Both buildings received similar high user ratings for their shading effectiveness. The Eurotower scored 3.83 out of 5, while the administrative building in Split scored slightly higher with a rating of 3.86. Most of those workers in the Eurotower in Zagreb (75%) reported that micro-climatic conditions impacted their productivity. Additionally, all respondents reported that they consistently use control systems devices to manage these conditions (Figure 15).

In the administration building at the University Library in Split, a considerable proportion of users (87%) reported that the micro-climate conditions had an impact on their productivity. Additionally, half of these users (53%) occasionally utilized control system devices to regulate the indoor climate. The air quality in both buildings was satisfactory and had a rating of 3.33 out of 5.

Users sometimes opened the windows in both buildings. In Eurotower, all users in the south (100%) constantly opened the windows. In Split, 40% of users in the southwest opened windows frequently. When asked about the sufficient daylight illuminance levels and impact on their wellbeing in the Eurotower in Zagreb, users stated sufficient daylight illuminance in the north and east (100%) and occasionally too much light in the south (50%) and west (67%). To the same question, survey respondents in Split answered that generally the amount of daylight illuminance level was sufficient, except for the southeast orientation, where half of the respondents reported too much sunlight (50%). In Zagreb, the glass façade provided enough heat in the north (100%) and in the east (75%). There was too much heat in the south (100%) and in the west (67%). In Split, the glass façade let too much heat in the southwest (60%). There was sufficient heat transfer in the southeast (60%). All respondents in the south and west reported that overheating occurred during the summer period in Zagreb, while in the north (67%) and east (75%), there was generally not too much overheating during the summer period. In Split, many respondents reported that overheating occurred in the summer. There was more overheating in the southwest (80%) than in the southeast (60%).

In Zagreb, the respondents’ average score for the impact of the building’s façade on productivity was 3.58, the average score for the comfort of the workspace was 4.17, and the average score for air quality was 3.33 (these scores range from 1 to 5). The users also state that the working areas were mostly illuminated in full depth. In the south (100%) and west (67%), it was necessary to turn on the lighting while shading devices were used, contrary to the north- (67%) and the east- (50%) oriented workspaces, where additional lighting was not needed. In winter, the lights were switched on regularly in the north (100%), mostly regularly in the east (75%) and west (67%), and sometimes in the south.

In Split, the respondents’ average score for the impact of the building’s façade on their productivity was 3.20. The average score for the comfort of their workspace was 3.53, and the average score for air quality was 3.33 (these scores range from 1 to 5). According to 60% of users, the workspaces were well-lit in sufficient depth. In the southeast, there was a higher demand for turning on the lights while having sun protection (80%) than in the southwest. During winter, turning on the lighting was necessary regularly in the southeast (60%), whereas it was only needed occasionally in the southwest (30%) (Figure 16).

6. Discussion

6.1. Evaluating the Different Climate Contexts

This study examined four office buildings located in Croatia: two office buildings in Zagreb and two in Split. The analysis of the climate characteristics of the two cities reveals that the office buildings in Split are exposed to significantly higher solar radiation and higher temperatures compared to the buildings located in Zagreb, especially during the summer period. The mentioned parameters can affect the selection of the type and material of the façade, the energy efficiency of the building [75], and the comfort of staying in the interior space of the building.

Key Lessons Learned

• Buildings are subject to climatic and environmental elements; so, building design and construction methods must vary in different climate contexts to accommodate different challenges.
• More attention should be paid to specific bioclimatic parameters.

6.2. Evaluating the Different Types of Glass Façades of Buildings and Environmental Parameters

The analyzed office buildings are characterized by different types of glass façade: a single-unit system glazed façade (Case Study 1) and a double-ventilated glazed façade (Case Study 2).

Important environmental parameters can be drawn from the urban setting: the surface-to-volume (A/V) ratio, building depth, ground plan, natural light, and shadows. The building’s volume and form have a direct influence on the spatial quality, use, and comfort of the interior space. The arrangement of spaces inside the building responds to the environment. The envelope surface-to-volume ratio of Case Study 1 is almost equal for both analyzed buildings (the A/V ratio amounts to 0.22 m⁻¹ for both towers of the Business Center Strojarska in Zagreb and 0.20 m⁻¹ for the analyzed lower Westgate tower in Split) even though the buildings are in two different climatic contexts and despite the difference in the total annual radiated solar energy. The surface-to-volume ratio of Case Study 2 amounts to 0.14 m⁻¹ for Eurotower (higher building) in Zagreb and 0.21 m⁻¹ for the administration building at the University Library in Split.

As for Case Study 1, the location and design of the buildings resulted in 20% of the Business Center’s total façade and 24% of the Westgate Tower’s total façade being shaded from sunlight (Figure 11). As for Case Study 2, due to its location and design, 18% of the Eurotower façade area remains in shadow. The shorter sides of the Eurotower are oriented north–south, and the longer sides are oriented east–west, while the volume of the University library is rotated to the urbanistically set axis of the university campus, with all the façades being sunny (Figure 14).

The design of all the analyzed buildings’ façade is treated uniformly, regardless of their orientation. The workspaces are arranged equally regardless of their orientation. The analyzed offices are oriented to the north, west, south, and east, except for the administrative building at the University Library in Split, where the offices are oriented to the southwest and southeast.

By analyzing the evidence gathered from the case studies collected, the architects did not consider the outer sun protection of the building envelope when designing the single glass façade. The studied examples of the double glass façade show a sun protection system located in the façade air gap, but it was designed uniformly on differently oriented facades.

Key Lessons Learned

• The design of all analyzed buildings’ façade is treated uniformly, regardless of their orientation and insolation.
• Design decisions are not guided by bioclimatic parameters and orientation but rather by design and aesthetic references.

6.3. Evaluating the User Experience

This study utilized surveys to obtain a better understanding of users’ viewpoints regarding the workplace layout and the design elements of glass building envelopes. Existing research on user satisfaction at the workplace has shown that users’ perceived comfort depends on several factors. The survey results showed that respondents working in offices on different sides of the world have very different experiences. Primarily, experiences with solar energy transmission are less favorable in the east, south, and west; in other words, the east-, south-, and west-oriented offices are at risk of overheating in the summer period of the year.

To maximize the daylight and the passive heating, the building must be in correct alignment with the sun. During the summer months, in hot and sunny climates (such as coastal), solar energy transmission creates major difficulties for both users and energy consumption. Solar energy transmission at midday and in the afternoon between June and August can become undesirable in buildings with glass facades (increased cooling energy consumption). The orientation of the building is of utmost importance. It is necessary to design the indoor space according to the level of heat and light required, depending on the purpose of each room. Likewise, there are different potential solar gains in summer and winter, precisely because of the incidence angle of the sun and the time of exposure to sunlight.

Given the drastically different user experiences depending on the orientation of the workspace, the building itself, i.e., the façade, cannot be analyzed uniformly, but it was necessary to separate the user experiences regarding the orientation of their workspace. In general, user experiences of the workspace comfort in Zagreb and Split are comparable. As expected, there is more overheating in office buildings located in Split than in Zagreb. Both office buildings in Zagreb and the Westgate Towers in Split are cardinally oriented (with north, south, east, and west façade), but users have different experiences at different orientations. Users of the Business Center in Strojarska and of the Westgate Tower (Case Study 1) facing the east and west have more difficulties with workspace overheating and heat loss. The survey has also shown a slight overheating in south-facing workspaces. Users of the Eurotower experience summertime overheating in the south and west. The lowest average rating for workspace comfort is for the south-facing workspace. The University Library in Split is rotated to intermediate directions; so, negative experiences are distributed from southwest to southeast. Addressing overheating in office buildings therefore plays an important role in the indoor comfort and environmental performance of buildings.

The survey results also showed that the possibility of opening windows and the way the windows are opened is important for users. In general, users are not satisfied when the windows cannot be opened or can be opened minimally to increase outdoor airflow (because of ventilation standards, security, strong wind, etc.).

East- and west-facing glass façades proved to be the most unfavorably oriented in Zagreb. They are often exposed to more intense sunlight because they have a lower incidence angle. East-facing workspaces have a very low average rating for workspace comfort in the Business Center Strojarska. Consequently, glass façades of the eastern and western sides should be designed more carefully. South-positioned workspaces are also less favorable, especially in Split. North-facing users are mostly satisfied because the northern side is without direct sunlight but therefore potentially colder.

Key Lessons Learned

• The relationship between indoor environmental factors and complex human behavior is not simple. It is thus important to know the needs of users to be able to set the performance criteria of such an environment.
• Users working in offices on different sides of the world have very different experiences.
• Addressing overheating in office buildings plays an important role in indoor comfort.
• Users are not satisfied when the windows cannot be opened or can be opened minimally to increase outdoor airflow.
• Various strategies such as appropriate building alignment and shape, glazing characteristics, shading devices, operable windows, and user-friendly technology tools in workplaces should be used to control the sun’s penetration into the building and improve indoor comfort.

6.4. Limitations of the Study

This study also has some limitations. The impact of the time constraints for the survey negatively affected the study. The duration of the winter semester did not allow us to repeat the survey questionnaire. The final deadline for the completion of the project was strictly determined and respected. The findings of some studies show that timing, i.e., seasonal difference, also affects survey responses [76]. To be able to generalize the collected data, a larger sample size is needed. The low response rate of online surveys has been a concern for many researchers in the last few years [77,78]. A statistically significant sample size of respondents is needed to be able to use descriptive statistics, which was usually employed in analyzing all the survey items. The number of answers collected by the survey was not enough to subject the collected data to control.

Second, unknown users, i.e., all existing users of the buildings and employees of different companies and service activities, were approached for data collection. Presenting surveys to a clearly defined and refined population positively affects the online survey response rate [79]. Also, we did not implement demographic segmentation of the respondents. Using the most common demographic survey questions (sex, age, employment status, educational level, etc.) can help researchers to have a better understanding of the users participating in the research [80]. Understanding user behavior is a challenging task, but demographic data can shed light on the factors influencing it.

Third, we have not anticipated all the features of a workplace environment; in the future, other types of workplace environment parameters could also be surveyed (acoustic comfort, glare-related effects, etc.).

Another limitation of this study was that the indoor temperature and humidity were not monitored before and after the occupants’ control of heating, cooling, ventilation, and sun-shading [14]. Therefore, it is difficult to compare the impact of users on the indoor environment. However, this study only focused on user comfort and satisfaction using subjective evaluations (by users) and qualitative assessments (of the façade’s resistance to environmental influences) without experimental data produced by a measurement, test method, or experimental design.

6.5. Further Research Directions

For future studies, it would be useful to carry out the survey in different seasons of the year, for example in winter and in summer, to gain a better understanding of user experience. Future studies should include acoustic comfort analysis of indoor space as well to comprise the complete user experience research. Also, a larger sample size can provide more detailed and deeper insights regarding the relationship between the workplace environment and employee performance. The study could also be limited to a certain category of users, for example, banking sector employees. In addition, an online survey could be developed especially for potential users (for example state companies) and provide useful insights. Obtaining the demographic information of respondents is also important in helping researchers to have a better understanding of the users and employees in office buildings.

End users need to be involved in the design process to make their wishes and demands clearer. Eventually, their feedback must be translated into real buildings (building geometry and building envelope) by the architects and processes (maintenance, energy use, services) by the stakeholders involved in the whole lifecycle of the indoor environment.

The findings from this study may contribute to raising awareness and informing students, architects, designers, and relevant stakeholders, e.g., property managers, of the options available for improving indoor environments.

This paper focuses on the qualitative features of the indoor environment. The future research directions should implement real-time monitoring of the temperature and relative humidity, illumination levels, and indoor air quality to provide data on the overall (both qualitative and quantitative) aspects of the indoor environment. Also, dynamic simulations using software could be used to balance the requirements of energy efficiency and users’ satisfaction in a specific climate context. In the future, researchers should focus on other research designs to address overheating and promote energy-efficient designs that minimize heat gain in indoor workspaces in different contexts and with different user behavior patterns.

7. Conclusions

Based on the detailed analysis, comparison, and interpretation of data from the two case studies and the results of the subjective methods employed, this paper now answers two research questions that address the impact of glass façades on users’ workspace comfort in different climate contexts:

(1)

Is a fully glazed office building pleasant to work in?

Fully glazed façades allow significant levels of daylight into the building and connect users with the external environment. The exposure to natural light and the opportunity of having an open view of the surroundings are well-known driving factors of productivity improvements in office buildings.

To achieve the satisfactory indoor comfort level of users in their offices, it is necessary to audit the appropriateness of the suggested façade design according to the orientation. However, current evidence suggests that decision makers do not consider the orientation while designing the glass facade. Equal treatment of the glass façade on all orientations results in uneven user experiences. East- and west-facing glass façades proved to be the most unfavorable orientations in Zagreb because of overheating in the summer. In Split, overheating of the workspace is present in the south, west, and east.

In addition, when users have the possibility to adjust inside environmental factors such as temperature and airflow speed, they feel more satisfied with their environment, which is often neglected in the design and management of buildings. In conclusion, it is evident that users are working in offices with different orientations and have different impressions about their place of work and the façade; so, it is impossible to produce a uniform solution for the whole building.

(2)

Is a fully glazed façade an appropriate solution for office buildings in hot and sunny climates?

User experiences of the workspace comfort in Zagreb and Split are comparable. In Zagreb, users reported overheating in the east and west orientations. In Split, users reported overheating in the east, south, and west orientations. Addressing overheating in office buildings in the east, south, and west orientations of the workspace plays a significant role in the comfort and environmental performance of a building. Using curtains or blinds (especially located on the outer side of glass façade) can help reduce the excessive amount of light and heat that enters the workplace.

Design decisions are often not guided by bioclimatic parameters and orientation but rather by design and aesthetic references. More attention should be paid to specific bioclimatic parameters such as orientation and insolation. Architects and designers should understand how their designs of glass building envelopes (especially in terms of glazing and sun protection) can affect users’ productivity and the comfort of staying in their workplace in a specific climate context. To achieve a comfortable indoor environment for buildings’ users, it is necessary to design the façade according to the orientation, especially in terms of the glazing characteristics, sun protection system, and the way that the windows are opened.