Energy and indoor thermal performance analysis of a glazed façade high-rise building under various Nordic climatic conditions

Research has shown that glazed buildings can have higher energy use and are more prone to overheating than other types of buildings. However, few studies have explored the performance of glazed buildings in cold climates. This article aims to evaluate the energy and indoor thermal performance of a high-rise residential building with glazed façades and balconies under Nordic climatic conditions, through a parametric study. Dynamic, whole-year simulations are used to evaluate the impact of four design parameters (with and without glazed balconies, type of balcony glazing, window to wall ratio, and building location within the Nordic region) on the energy and indoor thermal performance of the building. The results show that the building without glazed balconies outperformed that with glazed balconies. Changing from single-to double-pane glazing also helped to reduce energy use and overheating, as did lowering the window-to-wall ratio. Overheating of apartments was found to occur during the summer in five of the six locations simulated, which suggests that solar control strategies might be needed for glazed buildings even in a Nordic climate. This study highlights the importance of further research on glazed residential buildings, which are becoming more common in contexts subject to such climates.


Background
Glazed buildings are regarded as architecturally attractive because they provide ample daylight (Friess and Rakhshan, 2017;Oliveira and Pedrini, 2023), create aesthetic value and visual screening between indoors and outdoor (Rezaei et al., 2017), and promote acoustic comfort (Bulut et al., 2021).In Nordic countries, there is growing interest among architects and other building professionals to design buildings with glazed balconies and façades to achieve transparency, visual connectivity, and individual identity in the buildings (Ribeiro et al., 2020;Grynning et al., 2013).These buildings are also popular among occupants in the region because they provide better views and exposure to daylight, as well as additional living space, such as glazed balconies in residential buildings, which are usable even in winter (Ribeiro et al., 2020;Hilliaho et al., 2015b).However, compared to other buildings, glazed buildings generally have a greater sensitivity to outdoor climatic conditions due to their increased exposure to solar irradiation, which affects the thermal indoor environment (Hwang and Shu, 2011;Oliveira and Pedrini, 2023).Additionally, they generally have lower thermal resistance than conventional building envelopes, which can result in increased heat loss and energy use during the winter, despite the passive solar gains (Ulpiani et al., 2017).In Nordic countries where long and cold winters mean that heating demand dominates over cooling demand, the higher thermal transmittance of glazed buildings may increase heat loss beyond the level that is offset by passive solar gain, making such buildings unfavourable in terms of energy use.This study was therefore conducted to improve understanding of the energy and indoor thermal performance of buildings with glazed balconies and façades under Nordic climatic conditions.Hereupon, the study contributes to the overarching goal of United Nations Sustainable Development Goal 11, which strives to foster the development of inclusive, resilient, and sustainable cities and human settlements.

Glazed balconies
Several studies have reported that glazed balconies (also referred to as sunspaces) can have a greenhouse effect, achieving temperatures some 5 • C higher than the outdoor air temperature due to the admission of solar irradiation (Hilliaho et al., 2015a(Hilliaho et al., , 2016;;Mihalakakou, 2002).Researchers have therefore investigated how this greenhouse effect might affect residential buildings' energy performance.For instance, Asdrubali et al. (2012) found that an attached sunspace could reduce the winter heating demand of a flat in central Italy by about 20%.Suárez et al. (2011) showed that a sunspace could reduce a single-family house's energy demand on a clear winter day in northern Spain by 15%-32%.Similarly, Babaee et al. (2016) identified the potential to reduce up to 46% of the heating demand in an apartment block in a cold region of Iran by incorporating an optimally designed sunspace.Afshari et al. (2023) showed that a glazed balcony of a two-storey building in Turkey created a temperature difference of 6-9 • C with the outdoor environment in the cold season, which resulted in reduced heat loss.Zhang et al. (2023) found that an attached sunspace helps raise winter thermal comfort for a Tibetan dwelling in a cold region of China.However, despite the energy performance benefits of glazed balconies during winter, research has suggested that their greenhouse effect may lead to severe indoor overheating in warmer seasons (Ribeiro et al., 2020;Saleh, 2015;Mihalakakou, 2002).For instance, it has been reported that the air temperature in buildings with glazed balconies could exceed comfortable levels during the summer in the Middle-East (Ribeiro et al., 2020;Saleh, 2015), southern Europe (Mihalakakou, 2002;Oliveti et al., 2012;Fernandes et al., 2020), and in cooler, heatingdominated climates (Ribeiro et al., 2020;Mihalakakou, 2002).Although there are numerous studies into the impact of glazed balconies on buildings' energy and indoor thermal performance, few of these explore this issue in the context of Nordic climates.

Glazed façades
Numerous studies have emphasized the important impact of glazed façade components on the energy and indoor thermal performance of buildings in temperate and tropical climates.For instance, Raji et al. (2016) assessed the impact of energy-saving envelope design solutions for a high-rise glazed office building in a temperate climate, finding that glazing type and window-to-wall ratio were among the most impactful parameters investigated.In a recent review of indoor thermal comfort and energy savings in residential buildings in hot climates, Hu et al. (2023) found controlling the window to wall ratio to be one of the most effective passive cooling strategies.Hwang and Chen (2022a;2022b) conducted two recent studies on the impact of façade design parameters in office buildings located in Asian hot-humid climates.In their first study (Hwang and Chen, 2022a), they created a performance map for glazed facades based on building energy use and thermal comfort conditions in an Asian hot-humid climate zone.In their second study (Hwang and Chen, 2022b), they investigated the effect of window to wall ratio, glazing type, orientation, and overhangs on cooling loads and thermal comfort.Singh et al. (2022) performed a sensitivity analysis to investigate the impact of various glazing and shading design parameters on the energy and indoor thermal performance of a glazed façade office building in Amritsar, India.However, only a few studies have tackled the impact of façade glazing on buildings' energy and indoor thermal performance under Nordic climatic conditions, mainly with a focus on office buildings.For instance, Dubois's (1998) study of office buildings in the various heating-dominated climates of five cities in Canada, Sweden, and Norway showed that larger areas of glazing were more energy efficient in the colder cities, where there is more potential for using passive solar gain in the winter.Poirazis et al. (2008) energy simulation of a typical Swedish office building indicated that a fully glazed building (i.e.100% window to wall ratio (WWR)) is likely to lead to higher heating and cooling demand than a building with 30% WWR.In contrast, Persson et al. (2006) investigated the energy balance of twostorey low-energy residential houses in southern Sweden.They found that window sizes have no significant impact on heating demand during the winter but are relevant to the need for cooling in the summer.

Research gap, objective and scope
Although the studies mentioned above provide insights into the energy and indoor thermal performance of buildings with glazed balconies and façades, they have some limitations that create a need for further exploration: • Some of these studies are outdated and present contradictory results, particularly in relation to Nordic climatic conditions; for instance, compare findings of Persson et al. (2006) with Poirazis et al. (2008).• None of these studies have investigated the performance of glazed buildings in different inhabited parts of the Nordic region, which experience a range of climatic conditions from oceanic to subarctic.• Most of the previous studies focus on office buildings and do not consider the performance of high-rise residential buildings with glazed balconies and façades in the Nordic region.
The objective of this study is to evaluate the energy and indoor thermal performance of a specific high-rise residential building design of this type, through a parametric study based on dynamic, whole-year simulations under the climatic conditions in a range of Nordic locations.The study considers variations in four design parameters (with and without glazed balconies, type of balcony glazing, window to wall ratio, and building location) to evaluate and suggest strategies that improve the building's energy and indoor thermal performance.The study is based on a high-rise residential building with glazed façades and balconies built in a subarctic location in northern Sweden.Analysis of its energy performance considered the annual heating demand while cooling demand was excluded due to the absence of specific cooling systems in Nordic residential buildings.The indoor thermal performance was analysed by accounting for the simulated indoor operative

Method
Major components of the workflow of the simulation study are shown in Fig. 1.The case study building and the simulation method are further described in this section.

Characteristics of the case study building
The case study building is a recently constructed (2017) high-rise multi-family residential building with glazed façades and balconies in Piteå, northern Sweden, which has a subarctic climate with long, cold, dark winters and short, mild summers.The building was designed for good environmental performance based on the criteria for energy use, indoor environmental quality, and hazardous materials specified under the Swedish environmental certification system Miljöbyggnad (Sweden Green Building Council, 2014).An exterior view and a typical floor plan of the building are presented in Fig. 2. The building is not typical of residential buildings in northern Sweden but is rather an epitome of modern architecture.Each apartment has a curved glazed balcony with a single clear glass (6 mm) placed up to two meters from the building envelope.The glazed balconies are positioned on all floors and in all cardinal directions, and cover about 70% of the façade.The glazing gives the building an oval shape which gives it an iconic feature and provides sound insulation  from external noise.Furthermore, it adds attractive space that can be functional to occupants for larger parts of the year than open balconies would be.
The building has sixteen floors with a total heated area of about 6600 m 2 , accommodating 60 apartments of between 46 and 148 m 2 living space.The primary active heating system comprises hydronic radiators in each apartment connected to the city's district heating grid through heat distribution units.The building has a mechanical ventilation system with a heat exchanger to ensure good indoor air quality.The structure is a concrete frame with prefabricated concrete sandwich wall panels and precast concrete slabs.The building envelope is well insulated with an average thermal transmittance of 0.4 W/m 2 K. Table 1 shows its construction elements and their thermal properties.

Development of simulation model
A model of the case-study building created in the dynamic simulation software IDA Indoor Climate and Energy (IDA ICE version 4.8, Equa, Sweden) was used to simulate the building's energy use and indoor thermal climate.Previous studies have validated the accuracy of IDA ICE in simulating energy use and indoor thermal climate (La Fleur et al., 2017;Rohdin et al., 2014) across a wide range of building types, including buildings with glazed spaces (Hilliaho et al., 2015a,b).The simulation model was developed based on a model made by the building contractor during the design stage and data assembled from as-built drawings and documents.Fig. 3 shows a schematic view of the model.As can be seen in Fig. 3, the glazed balcony zones were modelled as a single zone for each cardinal direction and floor level.The glazed balconies are naturally ventilated unheated spaces and were, therefore, not connected to the building's central heating and ventilation unit in the IDA ICE model.In addition, no shadings were considered in the simulations.
Key input data for the simulation are summarized in Tables 1, 2 and 3. Table 1 presents building envelope thermal properties.Table 2 shows HVAC (heating, ventilation, and air conditioning) system properties, domestic hot water usage, occupancy and internal heat gains.Table 3 presents the properties of glazed elements (windows, glazed balcony doors and balcony glazing).
Simulations of the energy use and thermal indoor climate were performed for the period June 2018 to May 2019.Before the simulations started, the simulation model was calibrated and validated by comparing its outputs with actually-measured monthly energy use and indoor temperature data for the case study building over the same period.

Simulation output parameters
The output parameter used for the energy performance analysis was the energy use for space heating (hereafter referred to as heating energy use).The energy use for domestic hot water was excluded.The indoor thermal performance was evaluated using two output parameters: operative temperature (T op ) and the number of overheating hours (OH) for the whole study period.
Generally, T op is defined as the uniform temperature of an imaginary black enclosure, and the air within it, in which an occupant would exchange the same amount of heat by radiation plus convection as in the b Standard values according to the Swedish industry standard for energy use in buildings (Sveby, 2020).

S. Bhattacharjee et al.
actual non-uniform environment (Fanger, 1970).In IDA ICE, the T op is calculated in an enclosure with the dimensions 4 × 4 × 3 m and thermal properties as specified by CEN 15217 (European Committee for Standardisation, 2007).The enclosure's distance from the wall was set to 2 m, and the height above the floor was set to 0.6 m.The equation for calculating T op , which is validated numerically by the finite difference method in IDA ICE, is expressed as the arithmetic average of air temperature (T air ) and mean radiant temperature (T mrt ): The T mrt between a person and the surrounding surfaces is defined as a uniform temperature in an imaginary enclosure where the radiant heat transfer between the human body and the non-uniform enclosure is equal.The calculations of T mrt in IDA ICE follow ISO 7726 (International Organization for Standardization, 1998) except in one regard: according to Eq. ( 2), the view factor in a principal direction is <1 as it only considers the surface parallel to the radiating surface.The T mrt is obtained as an average of the mean radiant temperatures calculated for surfaces in six principal directions (denoted by 6): F i→j refers to the view factor for each surface i, and T j is the surface temperature radiating in each principle direction j.The view factors for each cube surface are calculated for the walls, windows, and room units such as radiators or cooling devices attached to the walls.The temperatures of the surfaces are weighted using the view factors to get the T mrt .
T op and OH were evaluated for apartments situated in all the cardinal directions, to see the impact of solar irradiation for different façade orientations.According to guidelines by the Swedish HVAC Association (Ekberg, 2013), the desired T op in dwellings is 20-24  2005), according to which the T op should be 20-24 • C in winter and 20-26 • C in summer.Therefore, the threshold for overheating was set at 26 • C, and any hours exceeding this were registered as overheating hours.To analyse seasonal variations, the T op data were grouped according to the calendrical definition of seasons generally recognized in Nordic countries: winter (December-February), spring (March-May), summer (June-August), and autumn (September-November).

Parametric analysis
The impact of the four design parameters on the building's heating energy use, operative temperature and overheating hours was evaluated through parametric simulations.In doing so, each parameter was varied individually while all other parameters were kept fixed to their basecase value.The parameters, their base-case setting (corresponding to the as-built design of the case study building) and the examined alternative settings are as follows: • Building with and without glazed balconies: to investigate the effect of the glazed balconies on the building's energy and thermal performance, the performance of the as-built case was compared to a simulated case without the glazed balconies.• Type of balcony glazing: to investigate how balcony glazing with lower thermal transmittance could affect the building's energy and indoor thermal performance, the analysis considered both singlepane (as per the as-built case, see thermal properties in Table 3) and double-pane clear glass (thermal transmittance of 2.9 W/m 2 K, solar heat gain coefficient (g value) of 0.76, solar transmittance (T e ) of 0.7 and visible transmittance (T vis ) of 0.81).• Window to wall area ratio (WWR): to see whether less glazing could produce heating energy savings and reduce the risk of overheating, the effect of lowering the WWR from 70% in the as-built case to 50% was tested.Therefore, in addition to the base case location of Piteå(with a subarctic climate), five other cities, representing climatic conditions ranging from Nordic oceanic to subarctic, were chosen.Table 4 shows the locations and their corresponding climate classification according to the Köppen-Geiger system (Peel et al., 2007), annual outdoor air temperatures and global solar irradiation on a horizontal  surface.Records of their climatic conditions were retrieved from the national meteorological institutes' weather stations in the respective city, as per availability of data.For the analysis period June 2018 to May 2019, monthly average temperature and solar irradiation values were retrieved (or calculated based on hourly values) and used for analysis purposes (see Section 3.2.4).

Results and analysis
This section presents the simulation results regarding the effect of varying each investigated design parameter one by one on the heating energy use (Section 3.1) and indoor thermal climate (Section 3.2) of the case study building.

Heating energy use
The case study building's actually-measured heating energy use from June 2018 to May 2019 was 50.6 kWh/m 2 , while the calibrated model prediction for the as-built case was 52.8 kWh/m 2 (a deviation of less than 5%).Monthly measured and simulated heating energy use during the analysis period are presented in Fig. 4. The heating energy use shows the pattern typical of residential buildings in cold climates, with the highest values during the winter months (December-February) and the lowest values during the summer months (June-August).For a high-rise building situated in a subarctic climate, the heating energy use of the glazed case study building is relatively low.In comparison to the energy performance criteria of the Swedish building code in effect when the building was built (BFS, 2014:3), the building used 44% less energy for space heating, domestic hot water production and facility electricity (90 kWh/m 2 y compared to the maximum allowed 130 kWh/m 2 y).
Table 5 presents the four investigated parameters and their settings, alongside the predicted heating energy saving relative to the as-built base case for each parameter setting.

Parameter 1: With and without glazed balconies
As shown in Table 5, the simulated heating energy use for the case study building without glazed balconies was 16% lower than for the base case with glazed balconies.The glazed space functions as a heat buffer, particularly during the long period when the indoor temperature is higher than the outdoor temperature (which can be seen by comparing the temperatures presented in Figs. 5 and 6).This reduces heat losses from the part of the exterior wall facing the glazing.However, at the same time, the balcony glazing helps shade and shield the apartments from incoming solar irradiation and thus reduces passive solar heat gains.Therefore, the lower heating energy use under the case without glazed balconies could be explained as the loss in solar heat gain outweighing the decrease in heat loss from the exterior wall.

Parameter 2: Type of balcony glazing
Compared to the base case with single-pane glazing, heating energy use decreased by 10% when double-pane glazing was applied (see Table 5).The double-pane glazing provides better thermal insulation and prevents heat loss more effectively than the single-pane glazing.On the other hand, the lower solar energy transmittance of the double-pane glazing means that it transmits less solar irradiation to the apartments.This suggests that, for the case with double-glazed balconies, the penetration of solar irradiation through the balcony glazing was less important for saving energy than the glazing with higher thermal resistance.

Parameter 3: Window to wall area ratio (WWR)
A reduction of the WWR from 70% to 50% led to a heating energy saving of 6.2% (see Table 5).The thermal transmittance of the windows (0.81 W/m 2 K) is lower than the thermal transmittance of the exterior wall (0.21 W/m 2 K), which means that a higher WWR increases the  overall thermal transmittance of the building envelope.At the same time, more glazing allows more solar irradiation into the apartments and greater passive solar heat gains, which can help to reduce the heating energy use.However, the sunshine hours are very limited during the winter in this Swedish subarctic town.In fact, the high heating demand, which results from the combination of low outdoor temperatures and a higher WWR, leads to comparatively higher heating energy use in the studied building.

Parameter 4: Building location
The simulated annual heating energy use is higher in the more northerly locations (Tromsø, Reykjavik, and Piteå) than in the more southerly ones (Copenhagen and Stockholm), with Helsinki falling between these locations (see Table 5).This pattern of variation in heating energy use values matches the pattern of the respective annual outdoor temperatures and solar irradiation values in the different locations, as presented in Table 4, with higher outdoor temperatures and passive solar gains in the more southerly locations leading to less heating demand to maintain the same indoor temperature.

Indoor thermal climate
To evaluate how parametric variations can influence the indoor thermal climate, two floors were selected (see Fig. 5): floor 9 in the middle of the building (24-meter level) and floor 15 at the top (41-m level).The 9th floor has four apartments, representing the typical building layout as shown in Fig. 2b (the 9th to 14th floors are identical), while the 15th floor contains two larger apartments (penthouses).The apartments on the 9th floor range from 55-90 m 2 , while the apartments on the 15th floor are 128 m 2 and 148 m 2 .The two apartments on the 15th floor have larger glazed areas than those on the 9th floor and are thus more exposed to incoming solar irradiation.
The simulated operative temperatures (T op ) and corresponding overheating hours (OH) for the base case and the alternative settings for parameters 1-3 are presented in Table 6.

Parameter 1: With and without glazed balconies
For both the cases with and without glazed balconies, the T op in the apartments during winter remained within the range of 20-24 • C (Table 6) as recommended by the Swedish HVAC Association and the Public Health Agency of Sweden.As can be seen from Table 6, the difference in T op between the cases with and without glazed balconies was larger during summer than during winter, with higher maximum temperatures obtained during summer with glazed balconies.In the case with glazed balconies, the T op during summer generally exceeded the recommended maximum of 26 • C, except in the N-oriented apartment (Table 6).The highest T op and OH were obtained for the S-, W-and SEoriented apartments during July and August.By contrast, the T op did not exceed 26 • C during summer in any of the studied apartments without glazed balconies.In addition to the results shown in Table 6, simulations were performed for the spring (March to May) and the autumn  (September to November) seasons to evaluate the temperature fluctuations throughout the year.According to the simulations, the T op varied between 20.4-23.0 • C during both seasons, and only a few OH were obtained for the S-oriented apartment during spring.

Parameter 2: Type of balcony glazing
During winter, the T op for the double-pane glazing case remained between 20.7-23.5 • C, which is only slightly warmer than for the base case (see Table 6).However, during summer, the T op in the S-and Woriented apartments was reduced by 1-2 • C when applying double-pane glazing.The OH was also reduced in these apartments.This indicates that the double-pane balcony glazing reduced the passive heat gains by allowing less admission of solar irradiation indoors.
The simulated T op in the glazed balconies shows that, during winter, the double-pane glazing seems to have retained the heat better than the single-pane glazing (see Table 7).However, the level of solar irradiance received by the glazed balconies, and therefore the simulated T op , varied greatly according to their orientation and was lowest in the NE-oriented balconies.During the subarctic winter in Piteå, there is little to no sunshine, particularly from a north-easterly direction, which means that the temperature drops to a larger extent.In contrast, during summer, OH in the NE-oriented balcony on the 15th floor were higher than in the SWoriented balcony on the same floor.This is probably explained by the lower sun angle from the northeast than from the southwest, meaning that solar irradiation can penetrate more directly through the glazing and heat the balcony space more.
During summer, the T op was highest in the SE and SW-oriented balconies with single-pane glazing (see Table 6).Generally, there was more OH for the single-pane case than for the double-pane case, which means that the lower thermal transmittance of double-pane glazing helped moderate heating in the glazed balconies.
During autumn, no OH was found for the balconies studied.However, during spring, 96 OHs were projected for the SW-oriented balconies when they were equipped with single-pane glazing.

Parameter 3: Window to wall area ratio (WWR)
The results show that a WWR of 50% reduced OH compared to the asbuilt case with a WWR of 70% for all apartments studied (see OH for the base case in Table 6).This implies that if the admission of solar irradiation to the indoor space is reduced through lower WWR, the number of OH also decreases.However, although OH decreased considerably by lowering the WWR, the number of OH in the S-oriented apartment remained relatively high.In a similar pattern, the maximum T op decreased when the WWR was reduced from 70% to 50%, except for the SW apartment on the 15th floor, for which the maximum T op remained similar under both the 70% and 50% WWR cases.

Parameter 4: Building location
Monthly average outdoor air temperature and global solar irradiation during the analysis period (June 2018 to May 2019) for the different building locations, subject to various Nordic climatic conditions, are presented in Fig. 6.
Temperatures in the apartments are affected by temperatures in the glazed balconies, which can vary substantially between different cardinal directions (see Section 3.2.2) and building locations, particularly during winter and summer.Therefore, the T op in the balconies and how it is affected by the outdoor air temperature was evaluated first.Fig. 7 shows the simulated average monthly T op in the 9th floor balconies for each location studied.
As shown in Fig. 7, SW-oriented balconies generally had the highest T op , followed by SE and NW-oriented balconies, probably due to the higher transmission of incident solar irradiation through the glazed surfaces in these cardinal directions.The T op in the glazed balconies decreased towards the heating season as the outdoor temperature dropped, and gradually increased towards the warmer months.The

Table 6
Simulated operative temperatures (T lowest average monthly T op was found in January in the base case location Piteå, which experienced the lowest monthly mean outdoor winter temperatures of all the locations studied (see Fig. 6).
As can be seen by comparing the temperatures in Figs. 6 and 7, the average monthly balcony T op was generally higher than the average monthly outdoor temperature in all locations.However, there were some notable exceptions, particularly in winter.For example, in Piteå and Stockholm in January, the coldest month of the year, the T op in the NE-oriented balcony was slightly lower (<1 • C) than the outdoor temperature, which indicates the low thermal transmittance through the building envelope and the negligible solar irradiation received from the NE.Notably, although the average monthly outdoor temperature in Piteå in July was only 19 • C (see Fig. 6), excessive T op (>26 • C) in the SW balcony was obtained (see Fig. 7).
During summer, the average T op in the glazed balconies for the more southerly locations of Copenhagen, Stockholm, and Helsinki was be-tween 21-29 • C, while the balcony T op remained between 13-22 • C for the more northerly locations of Tromsø and Reykjavik.In Piteå, summer temperatures in the glazed balconies were 18-27 • C, putting them in a similar range to those for the southerly locations.Similarly, as shown in Fig. 6, the global solar irradiation values for Piteå in summer are closer to those obtained for the more southerly locations than the northerly ones.Tromsø and Reykjavik received lower solar irradiation and experienced lower outdoor temperatures during summer than the other locations, which explains the milder balcony temperatures obtained in these locations.
The temperature fluctuations in the two glazed balconies on the 15th floor, for each building location, showed the same trend as identified for the 9th floor.Therefore, these results are not presented.
Table 8 shows the simulated T op and OH for the different apartments and locations.The T op in winter remained within the recommended range (20 and 24 • C) in all locations.Although the global solar  irradiation values are slightly higher for the more southerly locations of Copenhagen, Stockholm and Helsinki (Fig. 6), the values were not high enough to cause overheating.The apartments oriented towards the S, W, SE, and SW have T op higher than the recommended maximum of 26 • C during summer in all the locations studied except Reykjavik (see Table 8), even though the average outdoor temperatures ranged between 7.3 • C and 22.5 • C (see Fig. 6).Since the simulation did not consider shading, excessive incident solar irradiation was admitted, generating higher indoor temperatures, particularly in SW and SE-oriented apartments.For instance, the simulated T op peaked at 33 • C in the SE-oriented apartment on the 15th floor at 6 pm on 3rd August, when the façade is likely to have received direct sunlight and the outdoor temperature was high, making these zones more prone to overheating.
The results of the simulations performed for the Spring and Autumn seasons show that the T op remained within a moderate range (21-24 • C) in the N-och E-oriented apartments in all the locations studied.However, the T op for the W, S, SE and SW-oriented apartments exceeded the recommended maximum of 26 • C. In Helsinki, 288 and 264 OH occurred during Spring and Autumn, respectively, in a W-oriented apartment where the T op varied between 27-28 • C. In contrast, there were just 24 OH in the more southerly location of Copenhagen in May, and the T op remained within a moderate range (21-26 • C) during September.
While investigating the data in detail, it was noticed that although Copenhagen is further south than Stockholm and Helsinki, the T op there reached a lower summer maximum.Thus, a higher T op seems not entirely dependent on a location's latitude.Other factors, such as heat from direct and diffuse solar irradiation, heat from circulated air flows, and heat from windows or openings, can also influence the indoor thermal climate.Three examples of the influence of such other factors can be found in Appendix.

Discussion
Much of the previous research has considered the performance of glazed buildings in hot climates and primarily focussed on office buildings.This study contributes to the literature by evaluating how the energy and indoor thermal performance of a residential building with glazed façades and balconies under the cold climatic conditions found in Nordic countries are affected by variations in four key design parameters: • With and without glazed balconies The results showed that the balcony temperature was generally higher than the outdoor temperature (cf.Figs. 6 and 7), indicating that the glazed spaces can capture and store solar irradiation and reduce heat losses from the building during the long cold periods of the year.In this sense, the results confirm previous research findings regarding the greenhouse effect of glazed balconies (see e.g. the review by Ribeiro et al., 2020).However, while earlier studies found that this greenhouse effect could lower residential buildings' heating demand (Afshari et al., 2023;Asdrubali et al., 2012;Suárez et al., 2011), the results presented here showed that the case with no glazed balconies outperformed the base case with single-glazed balconies (16% heating energy saving; Table 5).Although the single-glazed balconies could reduce heat losses, they could also shade and shield the apartments from incoming solar irradiation (c.f., for example, Asdrubali et al., 2012), reducing passive solar heat gains and, thereby, increasing the need for active heating.The case with no glazed balconies performed better than the base case with single-glazed balconies also in terms of indoor thermal climate (no overheating hours; Table 6).These results align with previous studies of residential buildings in other heating-dominated climates (Ribeiro et al., 2020;Mihalakakou, 2002), which found that glazed balconies could cause overheating indoors during summer.• Type of balcony glazing In contrast to the base case with single glazing, the results for double-glazed balconies indicated that their potential to reduce transmission losses could offset the lower level of passive solar heat gains during cold periods.Thus, the case with doubleglazed balconies outperformed the base case with single-glazed balconies in terms of energy performance (10% heating energy saving; Table 5) and indoor thermal climate (fewer overheating hours; Table 6).Hilliaho et al. (2015b) showed similar results for a multi-family building in Helsinki, southern Finland, where changing from single-to double-pane glazing resulted in 14.8%-16% energy savings and less solar irradiation penetrating the glazing.This implies that low-transmittance balcony glazing can play an important role in improving the energy performance of buildings with glazed balconies in a subarctic climate.• Window to wall area ratio: The results showed that reducing the WWR from 70% to 50% could improve the building's energy performance (6.2% heating energy saving; Table 5) and reduce overheating (Table 6).These results indicate that the reduction in window transmission losses could outweigh the loss in passive solar heat gains, improving the overall energy performance of a residential building in a subarctic climate.This contrasts with the findings of Persson et al. (2006) who found that window sizes had an insignificant impact on winter heating demand in residential houses in the continental climate of Gothenburg, Sweden, and Dubois (1998) who showed that larger glazing to wall area ratio resulted in lower annual energy use in office buildings in cold cities with a continental climate, such as Montreal, Canada, or a subarctic climate such as that of Luleå, Sweden.However, from an indoor thermal comfort perspective, the results align with previous studies performed in hot climates (Hu et al., 2023;Hwang and Chen, 2022b), showing that reducing the WWR can reduce the risk of overheating.• Building location: Regarding heating energy performance, the results showed the expected pattern; the building performed better when located in the more southerly Nordic cities with sunnier and warmer climatic conditions than the subarctic base case location (Table 5).
Conversely, from an indoor climate perspective, the building showed the highest thermal comfort, with fewer or no overheating hours, in locations with less solar irradiation, cooler outdoor temperatures and, consequently, lower temperatures in the glazed balconies than the base case location (see Reykjavik and Tromsø in Fig. 7 and Table 8).Notably, overheating was found to occur in all of the building locations investigated.Thus, in line with previous research which has detected overheating in residential buildings in cold climate regions (Lundqvist et al., 2019;Wang et al., 2017), these results highlight that overheating in glazed buildings can occur during summer, even under cold Nordic climatic conditions, unless appropriate strategies to ensure indoor thermal comfort are implemented.This is especially true for apartments that face south and west which, as expected for locations in the northern hemisphere, were those most exposed to overheating (see Tables 6 and 8).However, it should be noted that shading and natural ventilation (e. g. manual airing by window opening), which were not accounted for in the performed simulations, could help reduce the excessive heat gains during summer; for example, Fernandes et al. (2020) illustrated the importance of occupants' actions in regulating thermal comfort conditions by circulating air through windows and activating/deactivating shading in buildings with glazed balconies.Additional test simulations performed indicated that allowing the case study building to capitalize on natural driving forces can help mitigate overheating during summer without increasing the heating energy use.For example, the performed tests indicated that applying a hybrid ventilation mode could reduce the overheating hours for the most overheated apartments (Apt.3 and Apt. 4 on the 9th floor, see Table 6) by 17%-38%, and save about 6% heating energy compared to the base case with a mechanical ventilation system.Similarly, Tognon et al. (2023) found that the heating energy demand of a residential apartment remained similar between mechanical and natural ventilation-dominated cases when performing simulations for three different European climates (Rome, Venice and Helsinki).However, purely natural ventilation, without heat recovery, is not preferred in subarctic or other Nordic climates where the spaceheating season is long (c.f.Fig. 4) since it can yield high energy use for heating and, thereby, reduce the overall building energy performance.In the case of solar protection for the investigated building, adding exterior shading devices is not feasible due to the constraints of the structure being a glazed high-rise building.Still, interior blinds could be used, in combination with passive ventilation, to reduce high summer temperatures while ensuring ample access to daylight indoors.

Conclusions
A simulation-based parametric analysis was conducted to evaluate the energy and indoor thermal performance of a residential building with glazed façades and balconies under Nordic climatic conditions.Based on the results, the following conclusions can be drawn: • The presence of single-glazed balconies in a Nordic subarctic climate can negatively affect the building's whole-year energy performance and summer indoor thermal climate, at least when the glazed balconies cover as much as 70% of the façade, as was the case for the investigated building.• The use of double-pane instead of single-pane glazing on the balconies can help counteract the increased energy use, particularly in a Nordic subarctic climate where the temperature difference between outdoor and indoor is significant.Double-pane glazing can also help reduce the risk of indoor overheating during summer.• The use of highly glazed façades, with a large window to wall area ratio, can be unfavourable for the energy and indoor thermal performance of a residential building in a subarctic climate.• Overheating can occur during the summer in climatic conditions ranging from oceanic to subarctic.This suggests solar control strategies might be needed for glazed residential buildings even in the northern Nordic region.
Our results suggest that abundant glazing of façades and balconies can negatively affect the energy and indoor thermal performance of a residential building in subarctic Nordic locations.However, it should be noted that the simulations performed may not fully represent real-life conditions.For example, overheating hours might be reduced by passive ventilation or internal shading devices, which were not considered in the simulations.This is a topic for further investigation.In addition, future studies may provide additional insights into energy use and indoor thermal conditions in glazed residential buildings under Nordic climatic conditions if they include measurement data from other real-life cases.An acknowledged limitation of this study is the access to measured data for only one real-life case building, which restricted the possibility to explore additional real-life cases.Extending the research scope to include other real-life cases and typologies of residential buildings is therefore suggested to help develop more generalizable conclusions and recommendations regarding glazing design strategies for Nordic climate conditions.
Nevertheless, the results emphasize the importance of considering the impact of the level of glazing of façades and balconies on the energy and indoor thermal performance of residential buildings in Nordic climates.Contrary to earlier research studies of glazed buildings in cold climates, our results show that reducing the abundant glazing of façades and balconies can improve the overall energy performance of a residential building in a subarctic climate.For building designers, this implies that disregarding the glazing of balconies to save time and effort in design phase simulation work can lead to underestimating rather than overestimating the heating energy use of buildings subject to such a climate.However, the modelling and simulation approach used here is time consuming, particularly for complex glazing geometries (such as the curved shapes of the investigated building) and multi-zone spaces, highlighting the need for research to develop approaches that are accurate as well as efficient and practically feasible for energy and indoor thermal performance analysis of glazed buildings.

Fig. 1 .
Fig. 1.Major components of the workflow of the simulation study.

Fig. 2 .
Fig. 2. (a) Exterior view and (b) 2D drawing of a typical floorplan of the case study building and a close-up of a glazed balcony.

Fig. 3 .a
Fig. 3. 3D model of the simulated case study building in IDA ICE.

Fig. 4 .
Fig. 4. Monthly measured versus simulated heating energy use for the as-built case study building over the course of the analysis period (June 2018-May 2019).

Fig. 6 .
Fig. 6.(a) Monthly average outdoor air temperature and (b) global solar irradiation over the course of the analysis period (June 2018-May 2019) for the different simulated building locations.
overheating hours (OH; h of Top > 26 • C) for the base case and the alternative settings for parameters 1the base case are presented in Table 5. b N, E, S and W denote the cardinal directions North, South, East and West.
Fig. A.1.Simulated heat balance for the W-oriented apartment (Apt.4) on the 9th floor of the case study building during July 2018 in Copenhagen.The X-axis shows the days and the Y-axis shows the heat generated from different sources.

Fig. A. 3 .
Fig. A.3.Simulated heat balance for the W-oriented apartment (Apt.4) on the 9th floor of the case study building during July 2018 in Helsinki.

Table 1 Key construction and thermal characteristics of the building envelope.
• C in winter and 23-26 • C in summer.The Public Health Agency of Sweden (FoHMFS, 2014:17) also advises that the T op should not exceed 24 • C (in summer 26 • C) for long periods.The Swedish recommendations were developed based on ISO 7730 (International Organization for Standardization,

Table 3
Model input data regarding the windows, glazed balcony doors and balcony glazing.

Table 4
Characteristics of the different evaluated Nordic locations.

Table 5
Investigated parameter settings, corresponding heating energy use and savings relative to the base case.

Table 7
Simulated operative temperatures (T op in • C) inside balcony zones and corresponding overheating hours (OH) with single-pane and double-pane balcony glazing.
a N, E, S and W denote the cardinal directions: North, South, East and West.

Table 8
Simulated operative temperatures (T