Simulation of Building Energy Consumption for Different Design Features of Window Elements: Case Study in a Hot Climate Region

Abstract

Energy consumption in buildings plays a significant role in the global energy demand. The European Union has promoted different regulatory directives in the framework of energy efficiency to develop the construction of buildings with nearly zero energy consumption. The main objective of this paper is to simulate how the design characteristics of different factors of the window elements of buildings (frame, glass, and shading systems) located in a hot climate region affect their cooling primary energy consumption. For this purpose, a comparative analysis is carried out with multiple simulations of different types of single-family residential dwellings using the EnergyPlus energy model. From the results obtained, it can be deduced that, compared to the standard design configuration, the primary energy consumption for cooling of the buildings studied can be reduced by up to 12.7% and 29.5% by modifying the design characteristics of the frame–glass assembly or the shading system of the window openings, respectively. The conclusions drawn from this study can serve as a reference in normative and regulatory documents affecting the building sector for the establishment of minimum requirements for certain characteristics of the constructive design of buildings.

1. Introduction

One of the strategies of the European Union (EU) is to establish a common framework for the promotion of energy efficiency in all sectors. The general regulatory framework is defined in Directive 2023/1791 [1], which includes the establishment of the minimum requirements which need to be met by the member states. For the building sector in particular, the EU aims to achieve a zero-emission building stock by 2050 at the latest. To this end, through Directive 2024/1275 [2], it promotes the improvement of the energy efficiency of buildings, highlighting, in this regard, the importance of energy consumption of cooling. According to Annex I of this document, energy efficiency should be assessed by means of a numerical indicator of primary energy use per unit of floor area and year (kWh/m²·year). The Directive also refers to the integrated national energy and climate plans to be developed by the member states, indicating that they should include specific policies and measures to achieve the goal of climate neutrality by 2050 [3]. In the particular case of Spain, the Integrated National Energy and Climate Plan (PNIEC by its initials in Spanish) [4] proposes active and passive actions in the energy rehabilitation of residential buildings as a priority to achieve two of its main objectives established for 2030: (i) a 23% reduction in greenhouse gas emissions compared to 1990; and (ii) a 39.5% improvement in energy efficiency. With regard to the latter of these two objectives, the PNIEC prioritises actions on the thermal envelope of buildings, including façades, rooves, external joinery, glazing, and solar protection, with the aim of reducing the final energy consumption of the services which have the greatest weight in their energy consumption, one of which is cooling. In this sense, another of the strategies employed by the Spanish Government has been the establishment of a basic procedure to quantify the energy efficiency of buildings, with the ultimate aim of minimising their energy needs [5]. This procedure includes the technical and administrative conditions for the energy certification process of the buildings.

Improving thermal comfort may lead to increases in energy use [6]. Obtaining thermal comfort and a good energy performance at the same time is therefore a major challenge. Achieving the energy–thermal comfort binomial is even more complex when considering the nearly zero-energy building (nZEB) concept. Various assessments of both the energy and comfort levels of nZEBs can be found in the literature, many of which include the effects of climate change on both parameters. For example, in a long-term assessment developed for an nZEB in a Mediterranean climate, increases of cooling energy consumption were detected, as well as reductions in the hours of adaptive comfort [7].

The growing energy demand for cooling in the building sector is one of the most critical energy-related issues [8]. Currently, cooling appliances account for nearly 20% of global electricity use by buildings [8,9]. The International Energy Agency [8] estimates a more than three-fold increase in residential energy use for cooling by 2050. In this sense, the establishment of strategies to promote energy savings in regions with hot climates is a pressing priority [10]. At the European level, the purchase and use of air conditioning is increasing rapidly in Spain, Italy, Greece, and southern France [8]. This space-cooling technology dominates the market for residential and commercial buildings [11]. The increase in peak electricity demand, associated with cooling needs in particular, affects the stability of electricity systems [12], a factor which must be taken into account, especially in regions with weak electricity systems. This has already been identified by the PNIEC as a climate change-related threat to the Spanish energy system [4].

Various studies highlight the importance of studying the factors that influence cooling demand [13,14] and propose different solutions to reduce it. Larsen et al. [15] analysed the effects of changes in temperature trends and extremes on the energy demand of the residential sector in Europe. The results include the possible doubling of cooling demands by 2050 in some countries such as France, Spain, and Switzerland. According to [16], the projected space-cooling penetration rate [17] is equal to or higher than 75% in Spain, Portugal, Italy, Greece, and Bulgaria, with a notable escalation in electricity demand therefore predicted until 2100. As a solution, the authors propose the implementation of effective passive cooling measures. Such an implementation is recognised as being key to the framework of sustainable construction [18] and being able to improve indoor thermal comfort [19,20]. Gaglia et al. [21] calculated the specific energy consumption of the residential sector in Greece, reporting that electrical energy consumption increases in climate zone B, corresponding to the Prefecture of Attica, because of the higher cooling loads in urban regions. As energy-saving strategies, they propose the use of renewable energies, such as solar photovoltaic to cover energy needs, as well as low-cost interventions such as the regular maintenance of heating and air conditioning equipment and the installation of thermostats. Baglivo et al. [22] analysed the thermal behaviour of a multi-residential building in the southeast of Italy, characterised by a hot Mediterranean summer climate. They showed that buildings constructed within the legal limits will suffer from overheating, leading to more extensive use of cooling systems. More specifically, the thermal performance index, useful for cooling, would rise from 20.56 kWh/(m²·year) in 2010 to 45.11 kWh/(m²·year) in 2080. According to Salata et al. [23], in 2080, about 70% of Italy will reach levels of cooling degree hours not approached in 2000.

Bearing in mind the importance of the subjection of the roof of buildings to climatic excitations and its significant influence on the total cooling load of the building in hot climates, Chihab et al. [24] studied the dynamic thermal behaviour of different configurations of rooves built with hollow concrete blocks used in Morocco. They found that using a low-emissivity coating and inserting an insulating material between the outer surface of the block and the slab could reduce the total thermal load over 24 h by about 93.1% compared to the traditional configuration. Ragab and Abdelrady [25] investigated the effectiveness of the green roof concept in reducing energy consumption for cooling school buildings in three cities in Egypt. The results show that the proposed concept lowers the energy required for cooling by 32.31%, 34.89%, and 39.74% compared to traditional rooves in the three cities studied (Alexandria, Cairo, and Aswan, respectively). In another study [26], it was found that the application of reflective paint on the roof of a medical centre in a hot and humid region of Mexico obtained the best energy benefits. In [27], the suitability of using volcanic rock from Hainan (China) as a traditional building material was explored. According to the results, this volcanic rock possessed excellent thermal insulation properties, making it highly suitable for energy-efficient construction in hot climates.

Several studies have highlighted the importance of the choice of window frame, given its influence on the energy consumption of a building. In [28], a study was undertaken of the indoor thermal comfort and energy consumption of an office block in Jordan, characterised by its hot and dry climate. A comparison was made of the resulting energy consumption of three types of window frame: traditional aluminium, isolated aluminium, and PVC. The windows were double-glazed, and no consideration was given to the type of glass. Finally, the authors recommended the use of isolated aluminium or PVC window frames to achieve thermal comfort. Dąbrowski et al. [29] proposed the design of a novel system integrated in the window frame to increase net savings in terms of energy and CO₂ emissions. The results show that the integration of the system saved 50.3% of the energy that would be consumed by the basic window frame system without the integrated novel design to achieve the required indoor conditions. In the study, the authors analysed exclusively the effect of varying the geometry of different aluminium heat exchangers, without considering modifications to the window frame’s characteristics.

Onur and Nielsen [30] selected three types of glazing and shading systems to evaluate their effect on energy consumption of cooling. For the particular case of residential buildings in Dubai, the greatest energy efficiency was obtained with low-E-coated glass and external venetian blinds set at 45°. No consideration was given in this study to the modification of the material or dimensions of the window frame. Heydari et al. [31] investigated the effects of window configuration on energy consumption for a case study in Semnan (Iran), where the climate is hot and arid and the cooling load is more important than the heating load. For this purpose, various types of glass with different characteristics were considered, such as the filler gas, the thickness of the glass layer, and the thickness of the gap between two glass layers. Based on the thermal performance and an economic assessment, the authors recommended the use of double-pane windows with 4 mm thick tempered glass and an 8 mm inter-pane gap filled with air. It should be noted that the study did not consider modification of the window frame typology or the effect of shading elements. Pérez-Carramiñana et al. [32] determined the improvement in energy efficiency obtained through the incorporation of glass curtain systems in a common residential building located in Alicante (Spain). The glass curtain was frameless, which allows for increased solar radiation capture. Consequently, these systems were able to reduce heating energy needs by almost 60%. However, the use of shading systems and ventilation methods should be taken into account to reduce the risk of overheating in summer. As mentioned in [33], lowering the window-to-wall ratio is an effective way of reducing cooling loads and, therefore, the annual energy consumption of residential buildings in areas with hot climates.

In view of the gap found in the scientific literature review, the work carried out in this paper aims to simulate, jointly and comparatively, the effect that different factors involved in the design of window openings have on the primary energy consumption of cooling residential buildings. To this end, numerous simulations are carried out using the EnergyPlus energy model, combining three different residential building configurations with a wide range of designs and technical characteristics of window frames and glazing, as well as the shading systems implemented in them. The Canary Islands (Spain), which have a generally warm climate, was selected as the study region. The results obtained in the simulations are compared, and the effect and sensitivity of the studied factors on the primary energy consumption for cooling, as well as the subsequent impact on carbon dioxide emissions, are identified. The conclusions derived from this study can serve as a basis for the establishment of specific strategies in energy planning, as well as in normative and regulatory documents related to the construction of buildings.

2. Materials and Methods

Figure 1 shows the method developed to obtain the results and conclusions regarding the general objective of the research study carried out in this paper.

2.1. Study Climate Zone

As established in Directive 2024/1275 [2], EU Member States should consider outdoor climatic conditions and local specificities to promote the improvement in the energy efficiency of buildings. For the particular case of Spain, the Spanish Government defines, in the Technical Building Code (TBC) [39], six winter climate zones, with the codification α, A, B, C, D, E, and four summer climate zones, numerically codified as 1, 2, 3, 4. The codes, in both cases, are ordered from lowest to highest winter and summer climate severity, respectively. In this way, the climatic characteristics of each territory in Spain are referenced by combining a winter code with a summer code. The Canary Islands are characterised by many areas with low winter and high summer climatic severity, specifically with climatic conditions type α3 (see Figure 2). The α3 climate zone corresponds to sites located at an altitude below 350 metres above sea level.

The Spanish TBC provides in [34], for each climate zone, a file with the extension .met, which contains the hourly mean values of different climatological parameters for a representative year. The specific data for the climate zone studied in this paper can be found in the file zonaAlfa3c.met [34]. The interpretation of the information contained in the aforementioned file is explained in [40]. This climatological information was employed by the calculation model in the different simulations performed in the present study.

Table 1. Average monthly climate data for climate zone α3 in the Canary Islands. Note: the predominant wind direction is northeast.

Month	Dry Temperature (°C)	Effective Sky Temperature (°C)	Direct Solar Irradiance (W/m2)	Diffuse Solar Irradiance (W/m2)	Specific Humidity (kgH2O/kgdry air)	Relative Humidity (%)	Wind Speed (m/s)
January	17.23	6.31	105.33	49.14	0.0090	73.13	2.72
February	17.51	6.25	135.06	53.29	0.0087	69.55	2.97
March	18.46	7.72	157.32	66.58	0.0089	67.29	2.84
April	18.66	7.97	179.08	79.55	0.0095	70.33	3.03
May	20.15	9.55	200.65	79.42	0.0102	69.00	2.94
June	20.89	9.87	209.78	87.67	0.0114	73.53	2.94
July	24.21	13.44	214.96	80.68	0.0124	65.51	3.09
August	24.36	14.44	192.00	74.31	0.0128	67.30	2.88
September	22.85	13.72	149.43	68.02	0.0129	73.85	2.69
October	22.15	11.56	142.43	60.32	0.0120	71.85	2.52
November	20.12	9.24	114.02	50.68	0.0109	73.55	2.43
December	17.64	6.89	97.78	46.34	0.0100	79.42	2.62

contains the monthly mean values of the climatological parameters in climate zone α3, calculated on the basis of the 8760 records available in [34] for the different climatological parameters.

Figure 2 shows the total number of family dwellings [37] in each of the municipalities located in climate zone α3.

It should be noted that, according to the national report on the energy certification of buildings in Spain [41], 20.6% of existing buildings in Spain have the most unfavourable energy rating in terms of consumption. This problem is intensified in the particular case of the Canary Islands, where 60.6% and 52.4% of the buildings have the most unfavourable rating in terms of consumption and emissions, respectively. With regard to the latest registered certificates, in 2022, 1574 new buildings were constructed in Spain with the worst emissions rating, of which 952 were built in the Canary Islands.

2.2. Characterisation of the Dwellings

Three different types of single-family homes (SFHs) were modelled in this study to calculate their energy efficiency (see Figure 3):

• SFH-1: terraced;
• SFH-2: semi-detached;
• SFH-3: detached.

With regard to the openings in the main façade of SFH-1, the access door to the dwelling is on the ground floor, with a small window next to it along with the vehicle access door to the garage. Two windows are found on the first floor of the main façade, while on the rear façade, there are two more windows and a door leading to the terrace. The openings on the second floor comprise two windows on the main façade and another two windows on the rear façade. On the third floor, there is a small window in the attic.

The SFH-2 type has the same characteristics as described for SFH-1, except that, as it is only attached to one side façade, the other side façade is exposed to the surroundings and to all meteorological phenomena, with a small window on this façade in the bathroom on the second floor.

Structurally, SFH-3 is the same as SFH-1; however, as its name ‘detached’ indicates, it is not attached to any other dwelling. It has several openings on both side façades.

The basic construction design considered for the opaque part of the envelope of the buildings is based on the recommendations of the Spanish TBC and consists of a façade composed of a 1.5 cm layer of mortar, a layer of 20 cm thick cinder blocks, and a 1.5 cm thick plaster finish layer, with a thermal transmittance of 1.81 W/m²·K, and a roof composed of diverse layers of clay, mortar, bitumen, and plaster, in addition to a 30 cm thick unidirectional roof slab and a 10 cm thick layer of concrete, with a thermal transmittance of 1.73 W/m²·K. These base characteristics were considered in all of the simulations carried out according to the combinations of the different factors involved in the design of the window openings.

Table 2. External surface area of façades and window openings by dwelling type (in m²).

		Main Façade	Rear Façade	Right Façade	Left Façade
SFH-1	Total area (TA)	62.38	62.71	8.84	0.00
	Window area (WA)	10.01	9.76	0.25	0.00
	WA/TA (%)	16.0%	15.6%	2.8%	0.0%
SFH-2	Total area (TA)	62.38	62.71	86.11	0.00
	Window area (WA)	10.01	9.76	0.5	0
	WA/TA (%)	16.0%	15.6%	0.6%	0.0%
SFH-3	Total area (TA)	62.38	62.71	86.11	94.95
	Window area (WA)	10.01	9.76	9.32	8.82
	WA/TA (%)	16.0%	15.6%	10.8%	9.3%

shows a summary of the total and window areas by façade and dwelling type.

Of the total number of dwellings by municipality, an estimate was made of the distribution by type of construction according to the population density (PD) [37]. In this sense, the percentage of single-family dwellings increases as the PD decreases (see

Table 3. Relative distribution of dwelling type according to population density (PD).

PD (Inhabitants/km2)	SFH-1	SFH-2	SFH-3	Multi-Family Dwellings
PD ≥ 1000	10%	10%	10%	70%
500 ≤ PD < 1000	20%	10%	20%	50%
200 ≤ PD < 500	20%	20%	30%	30%
100 ≤ PD < 200	20%	20%	40%	20%
50 ≤ PD < 100	10%	20%	50%	20%
PD < 50	10%	10%	60%	20%

). The Canary Islands Institute of Statistics (ISTAC) [37] provides, for each municipality, the number of family dwellings with the following ranges of usable floor area: up to 30 m²; from 30 to 45 m²; from 46 to 60 m²; from 61 to 75 m²; from 76 to 90 m²; from 91 to 105 m²; from 106 to 120 m²; from 121 to 150 m²; from 151 to 180 m²; and more than 180 m². This information was used to estimate the mean area of the dwellings.

2.3. Window Frame Characterisation

Table 4. Technical characteristics of the different types of window frame.

Typology	Thermal Transmittance (W/m2·K)	Absorptivity	Air Permeability (m3/h·m2)
WFR1	2.2	0.75	50
WFR2	5.7	0.30	50
WFR3	4.0	0.30	20
WFR4	2.2	0.30	20
WFR5	2.2	0.75	20

shows the technical characteristics of the different window frame (WFR) types taken from the Catalogue of Building Elements of the Spanish Technical Building Code [38]. WFR1 is a wooden frame (configuration for simulation of the base case), WFR2 and WFR3 are metallic frames, and WFR4 and WFR5 are made of PVC. WFR2 has no thermal bridge break, whereas WFR3 does. WFR4 and WFR5 are, respectively, light- and dark-coloured double-chamber frames.

2.4. Glazing Characterisation

The technical characteristics of the different types of glass used are shown in

Table 5. Technical characteristics of the different types of glazing. Note: the thermal transmittance values shown are in vertical/horizontal position.

Typology	Thickness (mm)	Solar Factor	Thermal Transmittance (W/m2·K)	Emissivity
GL1	4	0.85	5.7/6.9	0.89
GL2	4-6-4	0.76	3.3/3.6	0.89
GL3	4-15-10	0.63	1.4/2.2	≤0.03
GL4	4-15-10	0.76	2.7/3.4	0.89

and were taken from [38]. GL1 comprises a single pane (configuration for simulation of the base case), whereas GL2, GL3, and GL4 are double panes. GL3 stands out for its low emissivity. The solar factor represents the fraction of thermal gain due to the solar radiation that the opening transmits directly, added to the portion that is absorbed and re-emitted by the opening itself into the building [42].

2.5. Window Shading System Types

The following types of window shading (WSHA) system were considered in the analysis:

• WSHA-1: without shading element (configuration for simulation of the base case);
• WSHA-2: 25 cm deep horizontal shading;
• WSHA-3: 75 cm deep horizontal shading;
• WSHA-4: 75 cm deep vertical shading (both sides of the window);
• WSHA-5: 75 cm deep horizontal and vertical shading;
• WSHA-6: fully closed blinds (from May to October).

2.6. Simulations

The simulations of the different configurations for the buildings, according to housing typology and the design characteristics of the study factors, were carried out using the EnergyPlus energy model [35] and the CYPETHERM HE Plus software (version 2024) [36]. The latter is recognised by the Spanish Government for normative justification of the basic energy saving requirements defined in the Spanish TBC [39].

The spaces of the thermal model are subjected to the operational conditions and occupancy profiles corresponding, in this case, to private residential use (see Annex D in [39]). For the operational conditions, the standardised setpoint temperature values given in [39] are used. In the summer period, the upper setpoint temperature is 27 °C between 23:00 and 6:59 and 25 °C between 7:00 and 22:59, while the lower setpoint temperature is 23.5 °C. In the winter period, the lower setpoint temperature is 17 °C between 23:00 and 6:59 and 20 °C between 7:00 and 22:59, while the upper setpoint temperature is 25 °C. In this regard, the CYPETHERM HE Plus software counts the number of hours in which the defined indoor comfort temperature is exceeded and the number of unmet load hours with and without occupancy (see [36], pp. 90–91). The Spanish TBC requires that the total of unmet load hours should not exceed 4% of the total occupancy time (see [39], p. 11). This rate is recognised in EN 16798 ([43]) as the percentage of time outside a specified operative temperature range. According to EN 16798-1, the highest expectation level of indoor environmental quality is achieved with a rate below 6%, which is classified as Category I (see Annex B of EN 16798-1:2019 ([43], p. 44). EN 16798 is included as a reference standard in the Spanish TBC with the Regulation on Thermal Installations in Buildings (see Appendix 2 in [44], p. 91). All of the simulations carried out in the present study comply with the Spanish TBC and, therefore, with the thermal comfort requirements for Category I according to EN 16798, presenting an optimal level in terms of thermal comfort for occupants.

With respect to the occupancy profiles, the following internal loads (in W/m²) are considered: sensible occupancy, latent occupancy, lighting, and equipment. The latter parameters are incorporated in the model with hourly values and for all months of a typical year [45]. The values are differentiated according to whether the day is a working day, a Saturday, or a public holiday.

The calculation model considers a standard pattern, established by the Spanish Government, for window opening patterns and building ventilation, differentiated according to the summer or winter period and time of day (see [45], pp. 21–23). This standard pattern was employed in all of the simulations.

The different configurations were additionally simulated according to eight different orientations of the main façade of the building from North (0°) to Northwest (315°). As a result, the use of primary energy for cooling per unit area and year, as well as the CO₂ equivalent emissions, were obtained, among other values.

Considering the 3 different dwelling typologies, the 8 potential orientations of the main façade, and the different technical characteristics for the design factors studied (glass, frame, and shading systems in windows), a total of 456 simulations were carried out.

2.7. Energy Savings Estimation

To estimate the energy savings, the primary energy obtained for the simulation with the design configuration of the base case was compared with the one whose design configuration minimises the demand. This gives the energy savings in kWh/(m²·year). The total annual energy savings that could be achieved were calculated considering the number of existing dwellings per building typology and their average floor area. This was finally compared with the current energy demand of the residential sector in the study areas.

3. Results and Discussion

Figure 4 shows the results obtained for the primary energy used for cooling according to dwelling type and orientation of the main façade. The results correspond specifically to the design features of the base case. It can be seen that, for all dwelling typologies, the primary energy consumption of cooling is minimal when the main façade is oriented to the north. The effect of orientation on energy consumption is greater for the SFH-1 housing type, where it is observed that the demand for the ‘East’ orientation (30.3 kWh/m²·year) is more than double that of the ‘North’ orientation. This is because this type of dwelling has two other dwellings attached to its side façades that provide shade, so the thermal gain in the main façade has a greater weight in the overall gain.

For the north and northeast orientations, the energy demand of the SFH-3 dwelling type is significantly higher than that of the other two types, indeed potentially more than double (from 14.2 to 30.9 kWh/m²·year). The primary energy consumption of dwelling types SFH-1 and SFH-2 have their maximum values with the east and west orientations, with these values very similar to those of the SFH-3 dwelling.

3.1. Use of Primary Energy Results by Window Frame and Glass Characteristics

Table 6. The results of primary energy consumption of cooling (in kWh/(m²·year)) for the different window frame and glass design configurations, differentiated according to the orientation of the main façade of the dwelling. Results for dwelling type SFH-1.

Configuration	North	Northeast	East	Southeast	South	Southwest	West	Northwest
WFR1/GL1	14.21	23.67	30.31	23.16	14.38	23.76	30.30	23.03
WFR2/GL2	13.30	22.33	29.01	21.85	13.47	22.42	29.00	21.73
WFR2/GL3	12.04	19.96	26.49	19.64	12.20	20.07	26.48	19.54
WFR2/GL4	13.89	23.37	30.31	22.87	14.07	23.46	30.30	22.74
WFR3/GL2	13.45	22.54	29.26	22.06	13.61	22.63	29.26	21.95
WFR3/GL3	12.18	20.15	26.73	19.83	12.33	20.26	26.72	19.74
WFR3/GL4	14.05	23.59	30.40	23.09	14.22	23.68	30.57	22.97
WFR4/GL2	13.54	22.66	29.38	22.16	13.69	22.75	29.40	22.07
WFR4/GL3	12.27	20.26	26.84	19.92	12.41	20.37	26.86	19.84
WFR4/GL4	13.99	23.54	30.51	23.03	14.16	23.64	30.51	22.91
WFR5/GL2	13.67	22.88	29.71	22.40	13.84	22.98	29.70	22.28
WFR5/GL3	12.41	20.50	27.20	20.18	12.57	20.61	27.20	20.07
WFR5/GL4	13.92	23.36	30.31	22.86	14.09	23.45	30.31	22.73

shows the results of the primary energy consumption of cooling for the particular case of the SFH-1 dwelling, according to the characteristics of the materials used in the window frames and glass and differentiated for the eight studied orientations. For all configurations, it can be seen that the consumption is the maximum for the ‘East’ orientation.

Table 7. Variation, relative to the base case (WFR1/GL1), in primary energy consumption for the different window frame and glass design configurations, differentiated according to the orientation of the main façade of the dwelling. Results for dwelling type SFH-1.

Configuration	North	Northeast	East	Southeast	South	Southwest	West	Northwest
WFR1/GL1 1	14.21	23.67	30.31	23.16	14.38	23.76	30.30	23.03
WFR2/GL2	−6.4%	−5.7%	−4.3%	−5.7%	−6.3%	−5.6%	−4.3%	−5.6%
WFR2/GL3	−15.3%	−15.7%	−12.6%	−15.2%	−15.2%	−15.5%	−12.6%	−15.2%
WFR2/GL4	−2.3%	−1.3%	0.0%	−1.3%	−2.2%	−1.3%	0.0%	−1.3%
WFR3/GL2	−5.3%	−4.8%	−3.5%	−4.7%	−5.4%	−4.8%	−3.4%	−4.7%
WFR3/GL3	−14.3%	−14.9%	−11.8%	−14.4%	−14.3%	−14.7%	−11.8%	−14.3%
WFR3/GL4	−1.1%	−0.3%	0.3%	−0.3%	−1.1%	−0.3%	0.9%	−0.3%
WFR4/GL2	−4.7%	−4.3%	−3.1%	−4.3%	−4.8%	−4.3%	−3.0%	−4.2%
WFR4/GL3	−13.7%	−14.4%	−11.4%	−14.0%	−13.7%	−14.3%	−11.4%	−13.9%
WFR4/GL4	−1.5%	−0.5%	0.7%	−0.6%	−1.5%	−0.5%	0.7%	−0.5%
WFR5/GL2	−3.8%	−3.3%	−2.0%	−3.3%	−3.8%	−3.3%	−2.0%	−3.3%
WFR5/GL3	−12.7%	−13.4%	−10.3%	−12.9%	−12.6%	−13.3%	−10.2%	−12.9%
WFR5/GL4	−2.0%	−1.3%	0.0%	−1.3%	−2.0%	−1.3%	0.0%	−1.3%

¹ Cooling primary energy consumption for the base case (in kWh/(m²·year)).

shows the results for the variations in this energy consumption, for all design configurations and orientations, compared to the base case (WFR1/GL1). For all orientations, the WFR2/GL3 design configuration presents the greatest relative reduction in demand with respect to the base case design. It also follows that, irrespective of orientation, combinations of any of the window frames with a GL4 type of glass do not practically improve the energy consumption of the base case design.

Figure 5 shows the results of cooling’s primary energy consumption, differentiated by single-family dwelling type, according to the characteristics of the materials used in the window frames and glass. As an example, the results for the ‘East’ orientation are presented.

From these results, it can be deduced that, in absolute terms, the reduction that can be achieved by modifying this type of design factor can vary, in the case of SFH-1, from a consumption of 30.31 kWh/(m²·year), corresponding to the case of the base design, to 26.49 kWh/(m²·year). For the SFH-2 dwelling type, the value of the base design (27.81 kWh/(m²·year)) can be reduced to 24.27 kWh/(m²·year), and in the case of the SFH-3 dwelling type, the 32.22 kWh/(m²·year) of the base design can be reduced to 28.19 kWh/(m²·year).

It is also observed that there are some design combinations, among those studied, that can worsen the base case consumption value or result in practically no improvement. This is of interest to consider in the initial design of the building envelope, as more expensive frame and glass configurations may not necessarily result in a reduced primary energy consumption.

Table 8. Variation in primary energy consumption of cooling by different window frame and glass types compared to the base case (WFR1/GL1).

Configuration	SFH-1	SFH-2	SFH-3
WFR1/GL1	30.31 kWh/(m2·year)	27.81 kWh/(m2·year)	32.22 kWh/(m2·year)
WFR2/GL2	−4.3%	−4.2%	−3.5%
WFR2/GL3	−12.6%	−12.7%	−12.5%
WFR2/GL4	0.0%	−0.3%	−1.3%
WFR3/GL2	−3.5%	−3.5%	−2.6%
WFR3/GL3	−11.8%	−12.3%	−11.7%
WFR3/GL4	0.3%	0.1%	−0.4%
WFR4/GL2	−3.1%	−3.2%	−2.3%
WFR4/GL3	−11.4%	−11.9%	−11.4%
WFR4/GL4	0.7%	0.3%	−0.1%
WFR5/GL2	−2.0%	−2.2%	−1.9%
WFR5/GL3	−10.3%	−10.8%	−10.8%
WFR5/GL4	0.0%	−0.4%	0.4%

shows the relative results of the comparative analysis of the different designs with respect to the base case (WFR1/GL1). It can be seen that, in relative terms, demand reductions in more than 12% can be achieved with the WFR2/GL3 configuration. In the rest of the simulations, combining the same type of glass (GL3) with the WFR3, WFR4, and WFR5 frames, reductions of more than 10% could be achieved.

In general, it can be concluded from the results shown in Table 6 that, while the type of frame influences the cooling primary energy consumption, the cooling primary energy consumption is much more sensitive to the characteristics of the glass installed in the windows.

Figure 6 shows the results obtained for the specific impacts of cooling’s primary energy consumption on CO₂ emissions. For all dwelling types, it is the WFR2/GL3 combination that minimises emissions. Compared to the base case (WFR1/GL1), for the SFH-1 dwelling type, emissions can be reduced by up to 12.6%, from 8.04 to 7.03 kg CO₂/(m²·year). For the SFH-2 dwelling type, the maximum reduction observed is 12.7%, and the value of 7.38 kg CO₂/(m²·year) obtained for the base case can be reduced to 6.44 kg CO₂/(m²·year). Finally, for the SFH-3 dwelling type, the base case value of 8.55 kg CO₂/(m²·year) can be reduced by 12.5% to 7.48 kg CO₂/(m²·year).

3.2. Use of Primary Energy Results by Window Shading System Characteristics

Table 9. Results of primary energy consumption of cooling (in kWh/(m²·year)) for the different shading system design configurations, differentiated by orientation of the main façade. Results for dwelling type SFH-1.

Configuration	North	Northeast	East	Southeast	South	Southwest	West	Northwest
WSHA-1	14.21	23.67	30.31	23.16	14.38	23.76	30.30	23.03
WSHA-2	12.76	21.04	27.34	20.91	13.32	21.53	27.50	20.62
WSHA-3	11.74	18.14	23.11	18.51	12.48	19.07	23.46	17.99
WSHA-4	12.64	20.20	28.34	19.86	12.76	20.39	28.31	19.62
WSHA-5	10.51	15.24	21.36	15.70	11.12	16.21	21.69	15.07
WSHA-6	16.26	25.98	32.26	25.61	16.44	26.08	32.28	25.49

shows the results, in absolute terms, of primary energy consumption of cooling, according to the characteristics of the materials used in the window shading systems, differentiated for the eight studied orientations. The results are those for dwelling type SFH-1. For all shading system configurations, it can be seen that the primary energy consumption is higher for the case of the ‘East’ orientation.

Table 10. Variation, relative to the base case (WSHA-1), in primary energy consumption of cooling for the different shading system design configurations, differentiated by orientation of the main façade. Results for dwelling type SFH-1.

Configuration	North	Northeast	East	Southeast	South	Southwest	West	Northwest
WSHA-1 1	14.21	23.67	30.31	23.16	14.38	23.76	30.30	23.03
WSHA-2	−10.2%	−11.1%	−9.8%	−9.7%	−7.4%	−9.4%	−9.2%	−10.5%
WSHA-3	−17.4%	−23.4%	−23.8%	−20.1%	−13.2%	−19.7%	−22.6%	−21.9%
WSHA-4	−11.0%	−14.7%	−6.5%	−14.2%	−11.3%	−14.2%	−6.6%	−14.8%
WSHA-5	−26.0%	−35.6%	−29.5%	−32.2%	−22.7%	−31.8%	−28.4%	−34.6%
WSHA-6	14.4%	9.8%	6.4%	10.6%	14.3%	9.8%	6.5%	10.7%

¹ Primary energy consumption for cooling the base case (in kWh/(m²·year)).

shows the results for the variations relative to the base case (WSHA-1) of this energy consumption for all design configurations and orientations. For all orientations, the WSHA-5 shading system achieves the greatest relative reduction in demand with respect to the base case design. It can also be deduced that, regardless of the orientation, the WSHA-6 system always increases the energy demand of the base case.

Figure 7 shows the results of the building’s primary energy consumption for cooling differentiated by the type of WSHA system used. As with the WFR and GL configurations, the results shown are relative to the specific case of the ‘East’ orientation. The WSHA-6 system is the only one with which the primary energy consumption is higher than the base case system (WSHA-1), increasing by more than 6% for the SFH-1 and SFH-2 dwelling types and by almost 10% for SFH-3 (see

Table 11. Variation in primary energy consumption for cooling by window shading system compared to the base case (WSHA-1).

Configuration	SFH-1	SFH-2	SFH-3
WSHA-1	30.31 kWh/(m2·year)	27.81 kWh/(m2·year)	32.22 kWh/(m2·year)
WSHA-2	−9.8%	−9.7%	−8.4%
WSHA-3	−23.8%	−23.2%	−19.6%
WSHA-4	−6.5%	−6.5%	−6.3%
WSHA-5	−29.5%	−28.7%	−24.9%
WSHA-6	6.4%	6.1%	9.9%

). It follows that extreme shading systems can impede the transfer of heat generated in the dwelling itself from the inside to the outside, thus requiring a higher cooling energy. However, this primary energy consumption can be significantly reduced with the other WSHA systems studied. For the specific case of the WSHA-5 system, the primary energy consumption of cooling is minimal for all of the types of dwelling studied. The reductions with respect to the base case are 8.95, 7.97, and 8.02 kWh/(m²·year), for dwelling types SFH-1, SFH-2, and SFH-3, respectively (see Figure 7). These reductions correspond to 29.5%, 28.7%, and 24.9% (see Table 11).

Comparing the results of energy savings according to shading system type with those obtained previously in relation to the characteristics of the window frame and glass, it is concluded that the primary energy consumption of cooling is significantly more sensitive to the shading system installed in the window openings than to the type of frame and glass chosen for the windows.

Figure 8 shows the results for CO₂ emissions due to primary energy consumption of cooling for each dwelling type and are differentiated according to the shading system installed in the building’s window openings. For all dwelling types, emissions are minimal for the case of the WSH-5 system, with reductions of 2.37, 2.11, and 2.15 kgCO₂/(m²·year) for the dwelling types SFH-1, SFH-2, and SFH-3, respectively.

3.3. Results of Overall Energy Savings Achieved with the Optimal Shading System

Our study shows that, in hot climate regions, the primary energy consumption for cooling is significantly sensitive to the shading systems installed in the window openings. For the case study, the implementation of the WSHA-5 system in residential buildings minimised cooling’s primary energy consumption in all cases. In this sense, an estimation of the overall energy savings in the residential building sector, differentiated for each of the study islands, was made for the particular case of the scenario in which the optimal shading system is implemented in the SFHs. Figure 9 shows the results obtained for each of the islands of the Canary archipelago. The current primary energy consumption in the residential sector was calculated based on the latest official data on energy consumption in the residential sector in the Canary Islands for 2022 [46]. In absolute terms, the islands of Gran Canaria, Tenerife, and Lanzarote have the highest energy reduction potential, while El Hierro has the lowest. This is mainly due to the fact that the former have the largest total surface area of SFHs in hot climate areas. In relative terms, energy savings of more than 9% can be achieved on the islands of La Gomera and Fuerteventura (see Figure 10).

It is concluded that, for the general case study, the implementation of the optimal shading system in the SFHs achieves a total energy saving of 151.23 GWh/year, equivalent to a reduction of 5.40% of the total energy consumption of the residential building sector of the archipelago. Considering the latest official emissions factor for electricity generation in the archipelago (0.575 tCO_2-eq/MWh [46]), this energy saving would avoid emissions of 86,956 tCO_2-eq/year. Bearing in mind that the average price for CO₂ emission allowances in 2022 was 80.87 EUR/tCO_2-eq [47], the potential economic saving for this reason would be 7,032,131.7 EUR/year.

The insular and isolated nature of the Canary Islands’ electricity systems implies very high electricity generation cost overruns, which are borne by the national electricity system operator. Taking into account the latest official data for the year, the average cost of electricity generation in the archipelago was 224.81 EUR/MWh [46]. In this sense, the energy savings achieved according to the above example represent an economic saving for Spain’s electricity system operator of 33,998,016.3 EUR/year. If to this is added the potential economic saving from emission allowances, the total economic savings could amount to 41,030,148 EUR/year.

Thus, it is considered to be of interest in regions with hot climates to promote and/or legislate actions within the framework of sustainable energy strategies that promote the modification of existing housing and/or that impose, for new housing, the obligation to implement systems in the window openings that optimise the primary energy consumption of cooling, all within the framework of achieving the strategic objectives for energy savings established by the EU.

3.4. Final Discussion of Key Findings

From the analysis developed in Section 3.1, it follows that the best results, in terms of energy consumption reduction, are achieved by double-glazed windows with metal or PVC frames. At the same time, these configurations meet the requirements for indoor comfort. Consequently, they contribute to attaining the energy–thermal comfort binomial within the framework of sustainability in nZEBs.

Indoor air temperature is the main parameter involved in human comfort assessment and, therefore, this has been taken as the only control parameter in numerous thermal comfort assessments and microclimate analyses [48]. There are very few evaluations that consider both indoor temperature and energy consumption as control parameters [48]. One example can be found in [49], where a comparison is made of the two parameters obtained for the same building model by two different Building Energy Performance Simulation (BEPS) tools (Design Builder/Energy Plus, and IDA ICE). This study considers a scenario with a small window (4.9 m²) and another with a large one (7.4 m²).

The present research work compares the results of primary energy consumption for cooling within the operating temperature ranges corresponding to the optimal indoor comfort level for different building typologies and from multiple simulations carried out for different configurations in the design of frames, glass, and shading systems in window openings.

While an experimental investigation on thermal comfort in an office building located in a hot climate [28] provides a general recommendation to use isolated aluminium window frames, from the results obtained in the present study, it is determined that the specific configuration to minimise energy demand is a metal window frame with no thermal bridge break combined with double-pane glass. However, it should be noted that significant energy savings, slightly lower than the optimum configuration, are also achieved with both light- and dark-coloured double-chamber PVC frames in combination with double-pane glass. In this sense, PVC window frames are also competitive with other window frame types in terms of achieving the energy–comfort nexus, and their choice would be justified if other factors are also taken into account, such as economic costs or the degree of resistance to corrosion and oxidation.

In view of the results of this study, the greatest energy savings are achieved with GL3 glass, which is characterised by its low emissivity (see Table 5). While it is true that low-emissivity glass is highly recommended for the optimisation of both cooling and lighting efficiency in buildings located in hot climates [30], this study specifies the characteristics of the glass, such as the thickness of the layers, the solar factor, and the thermal transmittance in both vertical and horizontal position (see Table 5), as well as the type of frame with which it should be combined (see Table 6) to minimise primary energy demand and meet indoor comfort conditions. In this sense, the use of low-E double pane glass is recommended in single-family dwellings located in hot climates.

As shown in Section 3.2, energy consumption is lowered with the different types of fixed shading systems considered, except for the extreme system WSHA-6, based on fully closed blinds during the summer period, which increases consumption, as it hinders the expulsion of the heat generated inside the living space. One solution found in the literature to reduce cooling energy needs is to use mobile shading devices with seasonally adaptable solar transmission coefficients [32]. However, these devices are not considered to be the most suitable solution, as they need to be modified depending on the weather conditions. In this sense, the results obtained in the present study show that the fixed 75 cm deep horizontal and vertical shading system is able to minimise energy consumption and ensure comfortable indoor conditions.

4. Conclusions

In this paper, the effect of different factors related to the design of window openings on the primary energy consumption of cooling residential buildings was studied jointly and comparatively. Factors such as the technical characteristics of the frame and glass, as well as the shading systems implemented in the windows, were considered. Multiple simulations were carried out using the EnergyPlus calculation engine, combining three different configurations of residential buildings and a wide range of characteristics for the different factors. This study was particularised for territories with hot climate characteristics in the different islands of the Canary archipelago (Spain).

Simulations were carried out for thirteen combinations with different insulation characteristics for window frames and glass in each of the three housing typologies. The results were all compared with each other, as well as with a ‘base case’ combination with standard frame and glass characteristics. From the results obtained in the simulations, it is observed that the potential reduction, with respect to the base case, in the cooling primary energy consumption due to the window frame and glass factors can be as high as 12.7%, and this energy consumption is significantly more sensitive to modifications of the characteristics of the glass than to those of the window frame.

In addition, simulations were carried out for six shading systems for the window openings, with different design characteristics, implemented in each of the three building configurations studied. It can be deduced from the results that the primary energy used for cooling savings for this factor can be as much as 29.5% (i.e., 2.3 times higher than those obtained for the window frame and glass factor). In this sense, it was found that the primary energy consumption for cooling is significantly more sensitive to modification to the characteristics of the window shading system than to those of the frame–glass assembly.

From the analysis of the results for all the simulations, it was observed that the degree of improvement in primary energy consumption depends on the configuration of the residential building. For all the design factors studied, combinations were found for which the reduction in primary energy consumption with respect to the base case was negligible, or for which the primary energy consumption was even higher than for the base case. In other words, consideration should be given to the fact that possible modifications in design factors could lead to an economic cost overrun and may not result in energy savings.

The results and conclusions obtained in this study can be used to establish strategies for action in potential modifications to existing buildings or in the design of new constructions, and are of general interest to the framework of sustainable building. They can also serve as a basis for the definition of strategic lines in energy plans corresponding to regions and/or countries, as well as to establish minimum requirements for certain construction design characteristics, within the framework of normative and regulatory documents associated with the building sector.

This study was carried out for different types of single-family homes; whereas they generally have a more direct exposure to the outside environment than dwellings located in multi-family buildings, the external weather conditions have a greater effect on the primary energy demand for cooling. Although some of the results obtained in this study could be generalised to the particular cases of dwellings located on the top floor of multi-family buildings, as these can be assimilated to SFH-1- and SFH-2-type dwellings depending on their position, a specific study for dwellings in multi-family buildings would be required, especially for the particular cases of dwellings located in other positions, such as those located on the ground floor and/or intermediate floors.