Study of Various Types of Glazing in a Building Constructed Using Hybrid Technology with a Large Window Area

Abstract

Hybrid building construction, in which the steel frame is filled with modular panels made of wood, is a relatively new technical solution. This type of structure allows the integration of large window surfaces. The aim of this study is to indicate the optimal glazing system, taking into account energy consumption, thermal comfort and economic indicators. A house made using new hybrid technology with an area of 152.4 m², located in Bialystok (Northeastern Poland) and in Kiruna (Northern Sweden), was selected as the reference object. Energy simulations of this building were performed with DesignBuilder v. 6.1.8.021 software. Due to the large format of the glazing, the assessment of the thermal environment was performed using the PMV index. An economic analysis aimed at selecting the optimal type of glazing was carried out. It was based on the most commonly used indicators such as LCC, NPV and IRR. The results of this study indicated that the selection of triple-glazed windows in the reference house reduced energy demand by over 22% for Bialystok and about 24% for Kiruna compared to double-glazed windows. Even greater effects can be achieved by using quadruple-glazed windows, as they provide energy savings of 36% and 39%, respectively, for these locations. The results of the analysis performed for a 2% increase in energy prices showed that triple and quadruple windows had a similar LCC value when the discount rate was lower than 2.5% for the Bialystok site. Quadruple-glazed windows were the best option for the Kiruna site when the discount rate was less than 5%. This research study found that, assuming a stable financial situation and a small increase in energy prices, it is recommended to use triple-glazed windows in the climate of Northeastern Poland. In more severe weather conditions, for example those characteristic of the area of Northern Sweden, quadruple-glazed windows are recommended.

1. Introduction

Increasing the share of window area in the envelope of both office and residential buildings is becoming a determinant of the modern way of designing them. It would be worth mentioning here that the concept of “glass houses” appeared 100 years ago in the outstanding novel by Polish writer Stefan Zeromski entitled The Coming Spring. This symbol represented a vision of a land of prosperity. Coming back to present times, the undoubted advantage of large-format windows is in achieving greater living comfort. On the other hand, a large glazing area increases the energy consumption associated with heating or cooling the conditioned spaces, depending on the climatic conditions. The scientific literature on the impact of glazing systems on the energy performance of buildings is very extensive. Scientists exploring this topic use various methods and research tools.

1.1. Scientific Studies Using the Building Energy Simulation Method

The issues related to window-to-wall ratio (WWR) were considered in detail by Phillips et al. [1]. The aim of this analysis was to assess the impact of different levels of WWR on the environment, economic indicators and occupant satisfaction metrics. For this purpose, the authors used the triple bottom line (TBL) method [2], performed energy simulations using EnergyPlus v. 8.9 software and estimated the net present value (NPV), the life cycle costing (LCC) and the life cycle assessment (LCA). A 12-storey office building located in different climatic zones of the USA represented by San Francisco, Boston and Miami was an example research object. Energy simulations and economic calculations were performed, assuming a 60-year lifespan for the building and the replacement of windows after 40 years. The general conclusion that emerged from the analysis was that an increase in the WWR coefficient increases the environmental impact and life cycle costs. In addition, the results showed an increase in occupant dissatisfaction in all three climate zones.

A multi-variant analysis of glazing selection was performed by Jaber and Salman Ajib [3]. A single-family building with an area of 154 m², located in Jordan and Germany, was analysed using TRNSYS software. The heat transfer coefficients of single-, double- and triple-glazed windows and climatic conditions, as well as the area and orientation of the glazing, were used as variables in multi-aspect energy simulations. The results of the calculation of the annual energy consumption costs for the heating and cooling of the reference building indicated that, in the optimal variant, the savings amounted to 21% for the climatic conditions of Amman, 20% for the location in Aqaba and 24% for Berlin.

Tokarik and Richman [4] performed a multi-criteria analysis of a reference residential building located in Toronto (Canada). The aim of this study was to minimise the life cycle costs of the case study house. The authors developed an energy model of this building in EnergyPlus software and then validated it based on actual energy consumption. Double, triple and quadruple glazing packages were considered in this study. It turned out that energy consumption can be reduced by up to 33% compared to the current energy efficiency requirements contained in the building code.

A study of the influence of large glazing areas in buildings in hot climate zones on the increase in solar heat gains and cooling loads was carried out by Elkhayat et al. [5]. Three types (passive, active and BIPV) of high-performance glazing systems were analysed as an alternative to traditional double-glazed windows in a seven-storey office building located in New Cairo (Egypt). Energy consumption costs for multiple options were estimated based on energy simulations performed in DesignBuilder software. As a result of the economic analysis, it turned out that glass with a metal oxide layer (Low-E) was the most effective, because the Payback Period (PBP) was the shortest and amounted to 15 years. Low-E glazing also had the highest NPV and the lowest LCC, primarily due to low disposal costs. The electrochromic glazing system, on the other hand, had the highest LCC value and a thirty-year Payback Period, which made it economically unprofitable.

Saadatian et al. [6] determined the parameters allowing for the optimal selection of a building glazing system. Based on the life cycle assessment, an assessment of the impact of the window characteristics on the environment, this study was carried out. In addition, investment costs were estimated and already taken into account at the building design stage. Unlike in previous studies, the entire building was not considered. Only a room [7] with an area of 5.50 m × 3.60 m and a height of 2.80 m was taken as a case study. Based on the results of multi-variant calculations, it was found that for all orientations and climatic conditions, the window to wall area ratio had the greatest impact on both the environment and investment costs. The U-value was found to also have a significant environmental impact for larger windows in cold climates and smaller windows in warm climates. Additionally, larger windows or smaller windows that have a high solar heat gain coefficient (SHGC) are recommended in colder climates. On the contrary, in warmer climates, smaller windows or larger windows with a low solar gain are recommended.

The University of Victoria (Canada) is renovating 49 non-residential buildings to reduce their energy use and greenhouse gas emissions. The campus buildings were largely constructed before building energy regulations were introduced. Mahmoodzadeh et al. [8] developed a methodology to assess the possibilities of achieving potential energy savings and performed energy simulations of three selected buildings in EnergyPlus version 8.8. The thermal characteristics of the partitions based on the UA coefficient (U-value multiplied by the partition area A) was the main element that was analysed first. The scope of the computer simulations also included the characteristics of double-, triple- and quadruple-glazed windows. The main conclusion from this study was that the significant impact on the thermal performance of buildings came from the increase in the WWR value.

Buildings with large-format glazing provide their occupants with much greater access to daylight. However, this is very often associated with reduced energy efficiency. Poirazis et al. [9] examined a six-storey office building with a height of 21 m and a usable area of 6177 m² located in Gothenburg (Sweden). IDA ICE 3.0 dynamic energy simulation software was applied as the research tool. Three WWR values of 0.3, 0.6 and 1.0 were analysed to determine the influence of the glazing area on energy consumption. Below are some of the most important and rather obvious conclusions from this computational analysis:

• It is recommended to use glazing with a low heat transfer coefficient, because it significantly reduces energy consumption for heating and at the same time does not significantly change the energy demand for cooling;
• The SHGC coefficient, which determines how much solar radiation falling on a window is transmitted into the room, has a key influence on the energy demand for cooling;
• The most effective way to reduce cooling demand is to use shading devices.

Kim et al. [10] estimated the service life and the amount of CO₂ emission reduction of apartment buildings located in South Korea. The aim of this investigation was to select low-emission technologies for residential buildings using so-called green technological solutions. In this study, 36 types of alternative durability strategies were considered based on a 100-year building service life. The analysis included three types of glazing: metallic double-glazed windows, plastic double-glazed windows and metallic quadruple-glazed windows. The results showed that the quadruple-glazed windows with a 6 mm air gap between the panes achieved the greatest reduction in carbon dioxide emissions.

An examination of the differences between the energy demand of a building using a simplified and a detailed method of calculating the optical–thermal parameters of windows was performed by Thalfeldt et al. [11]. The reference object of this study was a single-family house located in Tallinn (Estonia). The calculations were made for windows with triple and quadruple glazing and assumed different glazing areas on the west and south elevations. It turned out that the energy demand for heating was lower by up to 7%, while the demand for cooling was higher by 23%, when simplified models were used. It was also proposed to increase the U-value of triple windows by 15% when using simplified methods in order to reduce errors in calculations of heating energy demand.

Van Gulck et al. [12] determined the interrelationships of the economic methods LCA and LCC based on the analysis of multiple strategies for the renovation of the façade of the Drie Hofsteden Building IV located in Kortrijk (Belgium). The baseline scenario considered in this study assumed a cavity brick wall with a U-value of 1.76 W/m²-K and single-glazed windows with a timber frame with a U-value of 5 W/m²-K. In the subsequent simulation variants, double- and triple-glazing windows with PVC frames were examined. The energy consumption calculations of the reference house were performed using software developed at Ghent University [13]. It turned out that capital expenditure had the greatest impact on the result of the LCC method, while energy consumption costs were decisive in the method taking into account the environmental assessment. Moreover, it was found that the use of a triple-glazed window in both the environmental and cost methods was the optimal solution.

Issues related to the selection of building façades and glazing with regard to sustainable development aspects were considered by Feehan et al. [14]. The reference research object was a seven-storey office building located in Dublin (Ireland). The OpenStudio program was used to create a model of this building. The energy consumption was estimated using EnergyPlus software. The authors carried out a life cycle assessment of a reference building over a period of 50 years, considering different building façade systems, changing the window to wall area ratio and using double- and triple-glazed windows. An analysis of calculation results correlated to the replacement of double-glazed windows with triple-glazed ones showed an 8.3% reduction in operational demands across all environmental categories with an increased embodied burden from 2% to 11%.

Jezierski and Zukowski [15] proposed four deterministic mathematical models that can be used to determine the annual energy consumption for heating and cooling purposes, as well as the CO₂ emissions associated with the operation of an HVAC system. This study employed mathematical modelling and energy simulations using DesignBuilder software. The calculations included changing the glazing area; double-, triple- and quadruple-glazed windows; three levels of external wall insulation; and three climate zones. As the results of this analysis indicated, the heat transfer coefficient of the windows was the only factor that had a significant impact on reducing the annual energy consumption of the reference house.

1.2. Analyses Based Only on Modelling of Heat Transfer Through Window

A similar type of study was conducted by Zavala-Guillén et al. [16] in three different Mexican locations: Mérida, Yucatán, Mexico. However, in this case no specific building was considered, but only heat transfer through single-pane, double-pane and triple-pane window configurations was taken into account. The calculations were performed based on computer code written and validated by the authors. Based on the analysis of the simulation results, it was found that triple-glazed windows reduced energy consumption by up to 33% and 15.5% compared to single- and double-glazed windows, respectively.

Windows consisting of six panes were constructed and tested by Kralj et al. [17]. The analysed structure was composed of four chambers containing argon and one open chamber filled with air. Two panes (outer and inner) were 8 mm thick, three panes were 2.1 mm thick and one was 4 mm thick. The interior panes were sprayed with low-emission filters. This type of technical solution was characterised by a very low U-value of 0.24 W/(m²-K), a direct solar energy transmittance coefficient of 0.17 and an SHGC value of 0.24. The case study glazing was used to renovate the Wergelandsveien 7 building located in Oslo (Norway). It turned out that replacing the existing triple-pane glazing with a 6-pane glazing system resulted in a significant reduction in annual energy consumption per m² from 220 kWh to 110 kWh.

The issues of the evaluation and selection of optimal building glazing using the Pareto optimal frontier method were considered by Saadatian et al. [18]. This research study included estimating the investment costs and environmental life cycle of various window systems and was performed for 32 variants, taking into account eight types of glazing and four types of frames. The reference object [8] was a window measuring 1.23 m × 1.48 m installed in the front wall, with dimensions of 3.60 m × 2.80 m, of an office room with a total area of 19.8 m². The general conclusion of the analysis was that in the case of warm climate conditions, windows should be characterised by the lowest possible solar gains. The optimal solution for glazing installed in buildings located in cold climate zones should be the highest possible thermal resistance.

An evaluation of thermal and optical characteristics of windows consisting of three and four panes was carried out by Sadooghi and Kheran [19]. They developed a new mathematical method to investigate the effect of blinds and shutters on the thermal performance of the glazing. Various inter-pane spacings, louvre angles and gas fill types were subjected to parametric analysis. The results of numerical simulations indicated that the convective heat transfer coefficient is largely dependent on the angle of the blinds. When the blinds were fully open, the value of the window’s heat transfer coefficient was the highest.

A numerical simulation of the heat transfer phenomenon occurring in double-, triple- and quadruple-glazed windows was carried out by Arıcı et al. [20]. Multivariate calculations, taking into account a change in the gap width between the panes from 6 mm to 21 mm and a change in outside temperature from −28 °C to 5.2 °C, were performed. A window height of 1 m and an air temperature in the room of 20 °C were assumed as constant parameters. Based on the simulation results, it was estimated that the energy savings resulting from replacing a double-glazed window with a triple-glazed or quadruple-glazed window were approximately 50% and 67%, respectively. The authors of this numerical simulation recommended the use of multi-glazed windows in regions of Turkey that are characterised by cold climatic conditions.

Transparent façade elements have a significant influence on the energy efficiency of buildings, as well as on aesthetics. Salazar et al. [21] investigated the impact of different types of windows on energy consumption and CO₂ emission reduction in two locations: Merida and Yucatan (Mexico). Calculations of heat transfer in transient states through multi-glazed windows were performed based on a numerical model using the control volume method. It took into account heat conduction, free convection and radiation. The results of multivariate numerical simulations showed that the use of triple-glazed windows instead of single-glazed windows reduced heat losses under the design conditions by over 56% and 31% during the coldest and warmest day, respectively. The reduction in energy consumption and CO₂ emissions for this type of window installed in the north façade was 32.8%; in the south façade it was 29.2%, in the east façade 31.3%, while in the west façade the result was the smallest and amounted to 29%.

Tükel et al. [22] investigated the value of the heat transfer coefficient of double-glazed windows placed in a roof. The calculations were performed using computational fluid dynamics algorithms in the Ansys/Fluent software environment. The numerical model was prepared for double-, triple-, and quadruple-pane window configurations. The results of the computer simulations indicated that the highest thermal resistance was obtained with an air gap width of 9 mm, regardless of the number of panes. The lowest heat transfer coefficient (U-value = 0.77 W/m²-K) was achieved in the case of a four-glazed window with a low-emission filter. Moreover, such a high thermal resistance increased the thermal comfort in the room, because the temperature of the inner surface of the glass was close to the internal air temperature.

A very similar analysis to that described in article [22] was also performed by Arıcı and Kan [23]. The difference was that the windows were positioned vertically, and argon was additionally taken into account as a gas filling the space between the panes. The results of comprehensive numerical simulations performed in Ansys/Fluent software indicated that the optimal gap width was 12 mm regardless of the glazing unit. In addition, the four-glazed window filled with argon and covered with low-emission materials achieved a very low heat transfer coefficient of 0.4 W/m²-K. This study presented a correlation enabling the calculation of the U-value of a window depending on the number of panes, the emissivity of the coatings covering the panes, the width of the space between the panes and the type of gas filling this space.

Huang et al. [24] developed a comprehensive literature review that included a discussion of glazing system technologies and an assessment of opportunities to reduce energy consumption under various climatic conditions. Inter-pane fillings with gases, liquids, phase-change materials (PCMs) and aerogels were discussed in detail. The article was written in order to select the appropriate glazing system and the possible further direction of development of this type of technology.

1.3. Summary of Literature Reviews

To sum up, it can be noted that issues related to the influence of the type of glazing on the energy performance of a building are quite a popular topic of scientific research. However, only a few studies have examined quadruple-pane windows. One of them was conducted by Mahmoodzadeh et al. [8], but it concerned non-residential buildings. Further research related to quadruple-glazed windows was performed by Kim et al. [10]. This time, the case study was apartment buildings. Tükel et al. [22] and Arıcı and Kan [23] are the authors of articles on the characteristics of multi-glazed glazing. However, it should be mentioned that their main emphasis was on performing numerical simulations of heat transport only through windows. A similar scope of research to that proposed in this article can be found in the co-authored publication by Jezierski and Zukowski [15]. In this case, we are dealing with a different type of building, a lack of economic analysis related to the selection of the optimal glazing option and a limited discussion of the impact of the glazing type on thermal comfort conditions.

1.4. Scientific Objective of the Current Analysis

The main objective of the current study is to select the type of glazing for hybrid buildings located in a cold climate zone that will be optimal in terms of energy consumption and at the same time economically justified. Three types of glazing (double, triple and quadruple) were analysed in terms of their impact on the building’s energy consumption and thermal comfort, as well as capital expenditure and operating costs. The novelty includes the selection of a modular reference house, the construction of which is hybrid, i.e., a combination of a wooden structure and steel beams. The technology of these modular houses is based on a spatial structure made of steel. An example of this type of steel frame is shown in Figure 1. The distance between load-bearing pillars should not exceed 4 m. It is required that the load-bearing columns align axially on both floors. Wooden panels fill the interiors of the steel frames. The wall panels with optional window openings are made of two oriented strand boards with thermal insulation at the centre of this sandwich structure. The main advantage of this type of construction is the lack of load-bearing walls and beams. This allows for great freedom in creating the interior architecture and integrating large glazing areas. These types of houses can be made in single- or two-storey designs. Of course, it is also possible to create a building with a variable shape, i.e., 1.5 storeys.

The main issue in this type of house construction with a large area of window openings is the selection of an appropriate glazing system. An extensive review of the literature allows the author to conclude that no similar analysis has been performed so far for the discussed hybrid structure of a single-family house in cold climatic conditions. It was decided to implement a complete solution to this problem. First, an energy analysis of the reference building combined with the HVAC system was performed. The energy simulation results were then used to evaluate the three glazing systems using basic economic indicators. The influence of large-area multi-glazed windows on the thermal environment inside the case study house was also determined. The current analysis focused on cold climate conditions. For this purpose, the reference house was located in Bialystok (Northeastern Poland) and Kiruna (Northern Sweden).

2. Materials and Methods

The selection of the optimal glazing system for the reference house was carried out with the building energy simulation method performed in the DesignBuilder software environment. Thermal comfort was assessed based on the commonly used Predicted Mean Vote (PMV) index, with particular consideration of the impact of radiation temperature on it. LCC, NPV and internal rate of return (IRR) were used in the economic analysis.

2.1. Key Characteristics of Test Building and Weather Conditions

A single-family, two-storey, set-on-the-ground building was chosen as the study house. It was made of an innovative hybrid construction, and its total area was 152.4 m², including 5.82 m² of unconditioned area. The model of this house (Figure 2) with the HVAC system was created with the DesignBuilder energy simulation tool. The external dimensions of the building in the outline were 11.4 m by 10.0 m, and the height was 6.67 m.

Three types of windows, i.e., double-, triple- and quadruple-glazed, were examined. Details of their thermal and optical parameters are provided in

Table 1. Basic parameters of the windows used in the analysis.

Window Parameters	Double-Glazed Window	Triple-Glazed Window	Quadruple-Glazed Window
U-value (ISO 10292 [25], EN 673 [26]) [W/m2-K]	1.634	1.072	0.806
Total solar transmission (SHGC) [-]	0.691	0.579	0.466
Direct solar transmission [-]	0.624	0.458	0.338
Light transmission [-]	0.744	0.698	0.624

.

Examples of the three types of windows that were used in this study are presented in Figure 3. The differences between these glazing technologies are not only in the number of panes but also in the thickness, thermal insulation of the frame and window casing.

As mentioned earlier, the hybrid structure allows for an increase in the glazing area.

Table 2. Wall area and window–wall ratio.

	Total	North	East	South	West
Gross Window–Wall Ratio [%]	38.87	28.49	50.23	33.85	45.07
Ground Wall Area [m2]	261.08	69.54	61.00	69.54	61.00
Window Opening Area [m2]	101.48	19.81	30.64	23.54	27.49

shows the WWR values and the window area in the reference building.

A case study house was made of partitions with the following heat transfer coefficients, taking into account thermal bridges (BS EN ISO 6946 [27]): external wall—0.105 W/(m²-K), flat roof—0.109 W/(m²-K), floor on the ground—0.136 W/(m²-K), internal floor—0.232 W/(m²-K). Due to the innovative nature of the hybrid construction used in this reference house, the manufacturer did not allow the disclosure of the structure of individual partitions.

Meteorological data were taken from Weather Data by Location [28]. The climatic conditions for the Bialystok location are classified as ASHRAE climate zone—6A and Koppen classification—Dfb. The design temperature used to calculate the heat load of a building is −22 °C. Kiruna’s climatic conditions are more severe, as the temperature assumed for the design of the heating system is −29.8 °C. This climate zone is classified as Dfb according to Koppen–Geiger and 7A according to ASHRAE.

The basic weather parameters that characterise both locations, divided into winter, summer and year-round periods, are presented in

Table 3. Monthly averages of meteorological data describing the climatic conditions.

Parameter	Year	Winter Period	Warm Period
Bialystok
Outside dry-bulb temperature [°C]	6.87	0.36	13.38
Wind velocity [m/s]	2.53	2.94	2.12
Wind direction [°]	175.00	188.51	161.49
Direct normal solar irradiance [W/m2]	38.95	16.38	61.52
Diffuse horizontal solar irradiance [W/m2]	51.63	23.61	79.66
Kiruna
Outside dry-bulb temperature [°C]	−1.15	−8.44	6.15
Wind velocity [m/s]	3.79	3.88	3.69
Wind direction [°]	187.19	187.10	187.28
Direct normal solar irradiance [W/m2]	45.58	21.30	69.85
Diffuse horizontal solar irradiance [W/m2]	41.56	9.16	73.96

.

The author has also been conducting experimental studies of basic meteorological parameters on the campus of the Bialystok University of Technology since 2014. A detailed discussion of these results and their comparison with the typical meteorological year (TMY) can be found in [29].

2.2. Assumptions Made in Building Energy Modelling

As previously mentioned, energy simulations of the reference house with the HVAC system were performed in DesignBuilder software. Below are selected assumptions related to modelling the building and indoor environment:

• The simulation period was one year, and the calculations used meteorological data for TMY—POL_BIALYSTOK_IMGW and SWE_KIRUNA_IWEC—Kiruna [28];
• It was assumed that 4 people lived in the reference building;
• Schedules for occupant behaviour were implemented as typical for residential spaces;
• Blinds with highly reflective slats placed inside provided shading for the windows;
• Activity in the form of light manual work, clothing thermal resistance in summer—0.5 clo, and in winter—1 clo were assumed in order to estimate the PMV index;
• Linear controlled lighting was used. It consisted of a linear change in artificial lighting intensity depending on the change in daylight intensity;
• Energy-saving lighting was used: “surface-mount” type with a visible fraction of 0.84 and a normalised power density of 2 Wm²-100 lux;

In the case of HVAC system modelling, shown in Figure 4, the following assumptions were made:

• Mechanical ventilation with heat recovery with an efficiency of 0.7 at design flow and 0.75 at 75% airflow;
• A ground-source heat pump was used to heat the building with an average COP of around 4;
• The rooms were cooled using a single-speed DX cooling coil with a gross rated COP coefficient of 3;
• The set air temperature during heating was 20 °C in the rooms and 24 °C in the bathroom, while the cooling process started when the air temperature exceeded 25 °C;
• Underfloor heating system made of pipes with an internal diameter of 13 mm and regulated by variable flow.

Figure 4. Diagram of the HVAC system used in the reference building (screenshot from DesignBuilder software).

Figure 4. Diagram of the HVAC system used in the reference building (screenshot from DesignBuilder software).

2.3. Key Assumptions Used in the Financial Cost Analysis

The following financial analysis was based on three indicators: LCC, NPV and IRR. According to the ISO 15686-5 standard [30], each LCC analysis should take into account the costs of four components, i.e., construction/initial (material, transport to site, labour), operation, maintenance and disposal (disassembly, transportation, treatment/destruction). The maintenance cost was omitted because in this analysis a single-family house was assumed as the reference building, and no glazing repairs were planned. Of course, in the case of office buildings, such assumptions should not be made. Therefore, the LCC value can be calculated from the following relationship:

$$LCC=IC+{\sum }_{i=1}^{i=T}OC\left(i\right)·{\displaystyle \frac{1}{{\left(1+r/100\right)}^{\mathrm{i}}}}+DC·{\displaystyle \frac{1}{{\left(1+r/100\right)}^T}},$$

(1)

where

• IC—initial cost [EUR/m²];
• T—life cycle period [year];
• OC—annual operational costs [EUR/m²];
• r—discount rate [%];
• DC—disposal cost [EUR/m²].

At this time, in conditions of high economic variability, the discount rate is assumed in a wide range from 2% (optimistic variant) to 7–10% (pessimistic option). In the current analysis, it was decided to perform calculations over the entire range of variability of this parameter, i.e., from 2% to 10%. Based on data obtained from leading manufacturers, it can be stated that the life cycle of PVC windows is between 30 and a maximum of 40 years. Due to the continuous progress in glazing technology, an analysis period of 30 years was assumed. There is a high probability that after this period it will be profitable to replace existing windows with those made to a much higher energy standard. The energy cost associated with heat transport through the glazing system was calculated based on building energy simulations and treated as operating costs. All three cost categories were determined for each window type and converted per m² of glazing to make this analysis more universal.

The initial costs (ICs) for quadruple-glazed windows were assumed to be 331.3 EUR/m², those for triple-glazed windows—276.1 EUR/m² and for double-glazed windows—225.4 EUR/m². In the case of disposal costs (DCs) for quadruple-glazed windows, 100 EUR/m² was assumed, for triple-glazed windows—200 EUR/m² and for double-glazed windows—200 EUR/m². Operating costs were additionally increased by the costs related to replacing seals and adjusting windows. This maintenance activity should be carried out on average once every 10 years. It was assumed that the value of these costs are 14.5 EUR/m², 13.5 EUR/m² and 10.2 EUR/m² for quadruple-, triple- and double-glazed windows, respectively.

The NPV indicator is usually used to assess the feasibility of a specified investment proposal. A positive NPV value indicates the profitability of the project from a financial perspective. The formula for calculating NPV is presented below:

$$NPV=-∆IC+{\sum }_{i=1}^{i=T}∆CF\left(i\right)·{\displaystyle \frac{1}{{\left(1+r/100\right)}^i}},$$

(2)

where ∆CF—cash flows assumed as an annual reduction in energy costs [EUR/m²].

The internal rate of return determines the real rate of return from an investment. An investment is profitable if the IRR value is greater than the assumed limit value. Calculating the IRR involves finding the value of the discount rate that meets the following relationship:

$$-∆IC+{\sum }_{i=1}^{i=T}∆CF\left(i\right)·{\displaystyle \frac{1}{{\left(1+r/100\right)}^i}}=0.$$

(3)

3. Results

The thermal building simulation results in terms of the influence of glazing types on energy consumption, solar heat gains and thermal comfort are presented below. In addition, the results of the economic analysis focused on the selection of the optimal window system are also discussed in this part of the article.

3.1. Results of Building Energy Simulations

Energy calculations, including the determination of the heating and cooling loads of the building, were performed in three variants differing in the type of glazing:

• Variant A—double-glazed system;
• Variant B—triple-glazed system;
• Variant C—quadruple-glazed system.

The impact of the factors influencing energy consumption in individual months can be followed in

Table 4. Monthly energy consumption divided into individual components.

Variant	Month
	1	2	3	4	5	6	7	8	9	10	11	12	Sum
Bialystok
Heating energy [kWh]
A	646	470	327	166	43	17	12	28	74	232	442	580	3037
B	452	328	221	108	24	9	7	15	43	160	311	407	2085
C	376	273	184	90	20	8	6	12	35	135	260	339	1738
Cooling energy [kWh]
A	1	3	17	94	211	282	326	184	126	21	0	0	1266
B	1	2	15	81	185	250	289	158	106	17	0	0	1104
C	1	1	9	59	142	198	232	119	78	12	0	0	851
Energy used by fans [kWh]
A	136	123	136	132	136	132	136	136	132	136	132	136	1606
B	117	106	117	113	117	113	117	117	113	117	113	117	1381
C	98	88	98	94	98	94	98	98	94	98	94	98	1149
Energy used by pumps [kWh]
A	13	11	11	9	7	5	5	7	8	11	12	13	112
B	12	11	11	9	7	5	5	7	8	11	12	13	109
C	12	11	11	9	7	6	5	7	8	11	12	13	111
Kiruna
Heating energy [kWh]
A	1100	757	562	293	118	31	17	70	246	423	899	1026	5543
B	796	514	368	183	66	15	9	36	160	286	640	734	3807
C	651	423	304	151	53	13	9	29	132	238	522	595	3119
Cooling energy [kWh]
A	0	1	25	74	143	256	305	122	23	9	0	0	959
B	0	1	23	66	123	227	270	102	20	8	0	0	839
C	0	0	15	46	88	172	207	73	13	4	0	0	618
Energy used by fans [kWh]
A	145	131	145	140	145	140	145	145	140	145	140	145	1709
B	129	116	129	125	129	125	129	129	125	129	125	129	1516
C	103	93	103	100	103	100	103	103	100	103	100	103	1216
Energy used by pumps [kWh]
A	16	13	12	9	8	5	5	7	10	11	14	15	126
B	14	11	11	9	8	5	5	7	10	11	13	14	118
C	14	11	11	9	8	6	5	8	10	11	13	13	118

. As might be expected, in cold climate conditions the dominant component is the energy demand for heating the building. The smallest share in the energy balance, which could practically be ignored, is the consumption of electricity for pumping.

The total energy consumption for the location of the house in Bialystok, shown in Figure 5, was 6021 kWh in Variant A, 4678 kWh in Variant B and at least 3849 kWh in Variant C. For the Kiruna site, the total final energy demand was 8336 kWh in Variant A, 6280 kWh in Variant B and 5072 kWh in Variant C, which is about 25% higher on average.

Assuming Variant A as the base, it can be stated that the use of triple-glazed windows allows for a reduction in energy demand of over 22% for Bialystok and about 24% for Kiruna. The installation of quadruple-glazed windows provides energy savings of 36% for Bialystok and about 39% for Kiruna. The lowest energy consumption can be observed in late summer and early autumn, while the highest energy demand occurs in January.

Double-glazed windows are characterised by the highest heat loss compared to other multi-glazed systems. However, as can be seen in Figure 6, this type of glazing allows for the highest passive solar gains. An increase in the number of panes causes a decrease in the solar heat gain coefficient, which is calculated as a fraction of solar radiation transmitted through the window to the interior of the building. In the case of a window consisting of three panes, there is a decrease in heat gains by 32%. In turn, four-pane windows reduce these solar transmission by over 50% compared to double-pane windows in both locations. As can be seen in Figure 6, the heat gains from solar radiation in December and January for the house located in Kiruna are very small due to the high value of the geographical latitude. During the summer, it is necessary to cool occupied spaces, and the priority is to limit the amount of radiation that enters the rooms. In the period from April to September, quadruple-glazed windows reduce solar gains by 6574 kWh for Bialystok and 8056 kWh for Kiruna, and triple-glazed windows by 4242 kWh for Bialystok and 5198 kWh for Kiruna, compared to double-glazed windows.

3.2. Thermal Comfort Assessment Results

Air parameters such as temperature, humidity and velocity, as well as the radiation temperature of partitions, types of clothing and work intensity, have a direct impact on thermal comfort. In order to estimate the thermal satisfaction of occupants in a building with large windows, the PMV comfort model was used. All the above-mentioned parameters are taken into account when determining the PMV index.

Glazing system have a significant impact on the thermal sensations of residents. Window surface temperatures vary much more than wall temperatures. This affects the heat exchange through radiation, which determines the feeling of comfort. Increasing the number of panes causes the temperature on the internal surface to be higher in winter and lower when cooling the rooms in summer.

Table 5. Average monthly value of the PMV index.

Variant	Month
	1	2	3	4	5	6	7	8	9	10	11	12	Sum
Bialystok
A	−0.99	−0.89	−0.65	−1.21	−0.55	−0.06	0.01	−0.51	−0.91	−0.61	−0.91	−1	−0.69
B	−0.97	−0.87	−0.63	−1.2	−0.55	−0.07	0	−0.51	−0.92	−0.6	−0.9	−0.98	−0.68
C	−0.98	−0.89	−0.67	−1.29	−0.68	−0.23	−0.15	−0.63	−1.02	−0.63	−0.91	−0.99	−0.76
Kiruna
A	−1.32	−1.08	−0.79	−1.46	−1	−0.29	−0.16	−1.02	−1.82	−0.83	−1.22	−1.27	−1.02
B	−1.21	−1.03	−0.76	−1.42	−0.98	−0.27	−0.13	−0.99	−1.79	−0.81	−1.13	−1.15	−0.97
C	−1.2	−1.05	−0.79	−1.51	−1.1	−0.43	−0.29	−1.08	−1.82	−0.82	−1.12	−1.14	−1.03

presents the results of the evaluation of thermal comfort depending on the type of glazing. It turned out that the PMV index values averaged for the entire reference building are similar to each other. This is primarily due to the fact that the simulation included follow-up regulation of the indoor air temperature. In addition, underfloor heating was provided, which increased the perceived radiant temperature T_R. This limited, to some extent, the impact of the low temperature of the window surfaces on thermal comfort.

In order to more accurately assess the radiation temperature of the inner surface of the windows, an open-plan living room on the ground floor of the building was selected. Figure 7 shows the change in the monthly average T_R value during the year. Throughout the heating season, the radiant temperature in a house with triple- and quadruple-glazed windows is rather similar, and on average about 0.4 °C higher than the temperature in the room with double-glazed windows, for Bialystok. It should also be noted that there is a radiative heat exchange between the warm floor surface and the cooler window surfaces, causing the T_R value to increase. Temperature differences in glazed surfaces become more visible during the cooling period. In the months with the highest intensity of solar radiation, i.e., June and July, it can be observed that the surface temperature of four-glazed windows is 0.7 °C lower. This can be considered an evident advantage of this system. However, in the case of the house located in Kiruna, a significantly lower temperature occurs in December and January when using double-glazed windows.

3.3. Discussion of the Influence of Glazing Type on Artificial Lighting

The level of daylight intensity in living spaces depends on the following factors: the degree of cloudiness, the position of the sun above the horizon, the geographical location of the building, the surface and optical characteristics of the windows, the shading system and the light reflection coefficient from internal surfaces. In this analysis, the influence of glazing type on the electricity consumption required for artificial lighting was considered. The decisive element was the light transmission coefficient of the glazing given in Table 1. Based on the simulation results presented in

Table 6. Monthly energy consumption for artificial lighting.

Variant	Month
	1	2	3	4	5	6	7	8	9	10	11	12	Sum
Electricity consumption for artificial lighting [kWh]
Bialystok
A	73.8	65.1	67.3	52.9	49.9	43.8	48.4	51.8	59.0	69.6	70.5	74.3	726.4
B	74.1	65.4	67.6	54.2	50.3	44.6	48.9	52.7	59.6	70.4	70.9	74.7	733.3
C	74.3	65.7	68.2	55.0	50.9	44.7	49.5	52.9	60.0	71.2	71.3	75.0	738.7
Kiruna
A	80.04	67.87	64.57	43.3	31.83	21.17	23.51	39.89	54.43	72.46	76.04	80.49	655.60
B	80.11	68.04	65.29	44.28	33.26	21.56	24.83	41.01	56.19	73.07	76.18	80.49	664.31
C	80.13	68.24	66.23	45.4	34.47	22.98	25.22	41.69	57.49	73.37	76.33	80.49	672.04

, it can be seen that the type of windows has little impact on the total electricity consumption for artificial lighting purposes. The use of double-glazed windows reduces energy demand by about 2% and triple-glazed windows by about 1% compared to a four-glazed system. In the case of the building’s location in Kiruna, these differences are slightly higher and amount to approximately 2.5% and 1.2%.

3.4. Results of the Economic Analyses and Their Discussion

The LCC provides a comprehensive assessment of the costs of a glazing system from the moment of purchase to the moment of disposal, throughout its entire life cycle. Using this method, the most cost-effective option was identified. To enable this task, a graph (Figure 8) showing the change in the LCC value depending on the discount rate was created. It turned out that the lowest total cost in the long term (30 years) was for Variant B, i.e., windows made in a triple-glazed system in the case of a house located in Bialystok. If inflation constantly remains above 5.2%, choosing Variant A would become a more profitable solution. This is a rather unrealistic scenario for the development of global business and the situation in financial markets. Additionally, a new and quite realistic variant was considered, in which a 2% increase in energy prices is predicted (Figure 9). In this case, triple- and quadruple-glazed windows have a similar LCC value provided that the discount rate is lower than 2.5%. Within the range of 2.5% to 7.5%, triple-glazed windows are characterised by definitely the lowest LCC. In the highly unlikely event of an increase in the return on invested capital to over 7.5%, triple-glazing windows become less profitable.

Slightly different LCC analysis results are obtained for the house located in Kiruna. If the discount rate is less than approximately 3%, triple- and quadruple-pane windows will have a similar LCC value. Above this value of r, a triple-glazed system will have the lowest life cycle costs. And after taking into account a 2% increase in energy prices, quadruple-glazed windows are the best choice, provided that the discount rate is not higher than about 5%. Therefore, we can see that in more severe climates it is profitable to use windows with the highest thermal resistance.

The study considered three variants to estimate the NPV, assuming a 5% discount rate:

• Variant I—replacing a double-glazed window with a triple-glazed one;
• Variant II—replacing a double-glazed window with a quadruple-glazed window;
• Variant III—replacing a triple-glazed window with a quadruple-glazed window.

In the case of determining NPV, it was assumed that the financial flows were defined as the reduction in energy consumption caused by replacing windows with more energy-efficient ones. A positive NPV value indicates the profitability of the project from a financial perspective. The calculation results showed that the value of cash inflows (denoted as ΔCF in Equation (2) is greater than the costs only in the case of Variant I (NPV = 2.95 EUR/m²), i.e., when double-glazed windows were replaced with triple-glazed ones in the case of a house located in Bialystok. The negative net present value indicates that Variant II (NPV = −19.1 EUR/m²) and Variant III (NPV = −22.08 EUR/m²) are unprofitable. Assuming a 2% increase in energy prices, the NPV value increases in each case considered and amounts to NPV = 15.9 EUR/m² for Variant I, NPV = 1.31 EUR/m² for Variant II and NPV = −14.58 EUR/m² for Variant III. Therefore, only replacing a triple-glazed window with a quadruple-glazed window can probably not be economically justified.

As expected, the NPV values for the Kiruna site show that projects involving the use of energy-efficient windows are more cost-effective. The values of this indicator are 29.35 EUR/m², 20.45 EUR/m² and −8.89 EUR/m² for Variant I, Variant II and Variant III, respectively. So, simply replacing a triple-glazed window with a quadruple-glazed window will not provide a profit.

Window replacement should be assessed as a low-risk project. In this case, it is usually assumed that the rate of return should be of around 5% to 10%. Figure 10 shows the change in the IRR value in individual years of the life cycle for the three options described above. As we can see, only Variant I meets the low-risk criterion, because after 26 years the IRR value exceeded 5% in the case of a house located in Bialystok. The project of replacing a double-glazed window with a triple-glazed one (Variant I) becomes even more profitable with a 2% increase in energy prices (Figure 11). In this case, the time to achieve a 5% discount rate is reduced by 6 years. When considering the Kiruna location, Variant I and II meet the criterion of r > 5% after only 14 and 21 years, respectively. The situation is even better if we assume an increase in energy prices, because all variants can be treated as low-risk projects. Variant I meets the criterion after 12 years, Variant II after 18 years and Variant III after 29 years.

To sum up the above analysis, it can be concluded that triple glazing is the optimal solution in terms of economic assessment in the climatic conditions of Bialystok. In the case of a more severe climate and at a low discount rate, both triple- and quadruple-glazed windows are a cost-effective solution.

4. Summary and Conclusions

The aim of the scientific study presented in this article was to determine the optimal glazing system for a building with large-format windows. The need to solve this type of issue appeared at the early stages of designing a new generation of modular houses built using hybrid technology. It consists of connecting a steel supporting frame with panel walls made in a wooden structure. This technical solution allows the design of large-surface windows.

DesignBuilder was applied as one of the most popular and validated research tools to predict energy consumption and thermal conditions in occupied spaces. A two-storey house with an area of 152.4 m² placed in Bialystok (Poland) and Kiruna (Sweden) was selected as the reference building for this study. These geographical locations may be representative of regions with long cold winters and short warm summers.

The multi-variant analysis based on energy simulations and economic calculations provided a large number of conclusions. The most important findings of this paper were divided depending on the issues examined and are presented below.

4.1. Conclusions Related to the Energy Efficiency of the Building

The results of energy simulations indicated that the annual energy consumption per square meter of conditioned area of the reference building located in Bialystok was 41.08 kWh/m², 31.92 kWh/m² and 26.26 kWh/m² in the case of the installation of double-, triple- and quadruple-glazed windows, respectively. In the case of the Kiruna location, the energy demand was higher by about 25%; therefore, the corresponding indicators had higher values, i.e.,: 56.87 kWh/m², 42.84 kWh/m², and 34.60 kWh/m², compared to the previous ones.

As is commonly known, an increase in the number of panes results in a decrease in the heat transfer coefficient but at the same time limits heat gains from solar radiation. Thus, a triple-glazed window reduced heat gains by 32% compared to a double-glazed window. An even greater reduction of over 50% occurred in the case of quadruple-glazed windows.

A separate issue is the decrease in the amount of daylight associated with the increase in the number of panes in the window. Light transmission affects both occupant comfort and the additional consumption of electricity for artificial lighting. The simulation results showed that the use of double-glazed windows reduces the energy demand by about 2%, with about 1% for triple-glazed windows, compared to windows with four panes. Slightly higher differences appeared in the case of the building location in Kiruna and amounted to about 2.5% and 1.2%. Therefore, it can be concluded that an increase in the number of glass panes affects the increase in electricity consumption for artificial lighting, but this effect is insignificant.

4.2. Conclusions Related to Thermal Comfort

The influence of the glazing type on the thermal environment inside the building was assessed by determining the PMV index. Generalising the results from both locations, the radiation temperature in rooms with triple- and quadruple-glazed windows was approximately 0.4 °C higher than the temperature of double-glazed windows throughout the heating season. During the warmest period, the radiation temperature when using four-pane windows was 0.7 °C lower than that of double-pane windows. It was found that in the case of a house located in Kiruna, significantly lower temperatures can be observed in December and January when using the least energy-efficient windows.

Despite this, it turned out that in all cases thermal comfort was within the recommended limits. Underfloor heating in winter, cooling in summer and a mechanical ventilation system contributed to this positive result.

4.3. Conclusions Related to Economic Analysis

A comprehensive analysis revealed that the optimal type of glazing system in the location of the reference building in Bialystok was triple-glazed windows. Financial indicators were the main determinant. This type of window was characterised by the lowest LCC value after the end of the 30-year life cycle, assuming that the discount rate would not exceed 5.2%. However, if this value were higher, then four-pane windows would have the lowest LCC value. However, one should hope that this is an unrealistic scenario for the development of the financial situation in the world.

The balancing of cash inflows and costs was also performed in order to compare the case of replacing windows with a lesser number of panes with windows with one more panes. It turned out that only one situation, i.e., replacing double-glazed windows with triple-glazed ones, would be a profitable undertaking. This was evidenced by the positive value of the NPV indicator, equal to 2.95 EUR/m² for the Bialystok site and 29.35 EUR/m² for the Kiruna. The IRR value was equal to 5% after 26 years and 14 years for Bialystok and Kiruna, respectively.

In the case of an annual increase in energy prices by 2% for the Bialystok site, triple and quadruple windows had a similar LCC value when the discount rate is lower than 2.5%. The house equipped with triple windows was characterised by the lowest LCC in the range r from 2.5% to 7.5%.

In the case of a house site in a slightly colder climate (Kiruna), triple- and quadruple-glazed windows had similar LCC values when the discount rate was less than about 3%. When r values were greater, only the triple-glazed system had the lowest life cycle costs. Quadruple-glazed windows were the best choice when the discount rate was less than 5%, and a 2% increase in energy prices was taken into account.

To summarise, assuming a stable financial situation and a small increase in energy prices, it is recommended to use triple-glazed windows in the location of the house in the climatic conditions of Northeastern Poland. Quadruple-glazed windows are suggested in more severe climatic conditions, for example similar to those prevailing in Northern Sweden.

It is important to note that the high initial cost of quadruple-glazed windows currently results in very little interest from investors. However, it should be noted that the constant increase in energy prices, as well as rapidly developing technological progress, can reduce production costs and increase the profitability of this technology in the future.

Future works: The author plans to perform a detailed experimental and CFD simulation analysis of thermal bridges occurring in houses made of a hybrid construction in order to precisely determine the heat transfer coefficients of individual building partitions. In addition, it is planned to carry out a similar analysis but for the location of a reference house in a warm climate zone, for example Southern Italy or Spain.