1. Introduction
The energy improvement of building stock is among the main challenges for architects and engineers due to deficient energy performance resulting from building standards without energy efficiency goals [
1]. Consequently, building energy consumption plays a significant role in the energy consumption generated by anthropogenic activities and thus in greenhouse gas emissions. According to quantified data, the building sector is responsible for 40% of the energy consumption and 25% of the greenhouse gas emissions globally [
2,
3]. This deficient energy performance of buildings faces demanding goals by international bodies, such as the European Union, to reduce greenhouse gas emissions. By 2050, European buildings should reduce greenhouse gas emissions by 90% [
4].
The adoption of energy conservation measures (ECMs) in buildings is required to achieve these goals. ECMs are understood as any type of modification which allows the building energy performance to be improved [
5]. Those ECMs focusing on improving various building elements could be adopted. However, envelope elements are the main element to be treated because of their narrow relationship with energy demand [
6,
7,
8,
9]. The effectiveness of applying ECMs to building envelopes has been widely analyzed, particularly for the energy improvement of buildings located in cold climate zones. Aksoy and Inalli [
10] analyzed the influence of passive design parameters, such as form factor and orientation, in a building located in a cold region of Turkey. Similarly, Güçyeter and Günaydin [
11] assessed the improvement of the envelope of an office building located in Turkey using six ECMs. Invidiata et al. [
12] analyzed the influence of six ECMs in a residential building located in the north of Italy in order to select the best option to improve building sustainability. A greater scale study was conducted by Qian et al. [
13], who assessed the savings achieved at a national scale by combining various ECMs in commercial buildings.
Most studies have focused on the analysis of ECMs of the envelope on the improvement of thermal transmittance. However, the effect of thermal bridges could significantly vary the energy consumption of the building being improved. Some studies have quantified the importance of thermal bridges with regard to energy consumption. Theodosiou and Papadopoulos [
14] assessed the influence of thermal bridge designs on the energy demand of representative buildings of the Greek building stock and the importance of correct characterization. Later, Theodosiou et al. [
15] determined that thermal bridges should also be quantified in metal claddings of the ventilated façades. In addition, other studies quantified the effect of thermal bridges on the energy demand. Ramalho de Freitas and Grala da Cunha [
16] assessed how the impact of the thermal bridges of reinforced concrete structures of a Brazilian building could vary its energy performance up to 20%. Ge et al. [
17] also analyzed reinforced concrete elements. Their study assessed how the thermal bridges of balconies could vary the energy demand of Canadian residential buildings between 1 and 13%. Evola et al. [
18] studied the effect of thermal bridges in two semi-detached houses located in a mild Mediterranean climate, determining that the improvement of thermal bridges resulted in a 25% reduction in the heating load and an 8.5% reduction in cooling load. Finally, Bienvenido-Huertas et al. [
19] analyzed the energy improvement achieved with patents on the thermal bridge of the slab front, obtaining overall reductions of heating energy demand greater than 18%, and reductions greater than 2.80% for cooling energy demand.
However, no existing studies have analyzed the effect of focusing ECMs on the improvement of buildings or a simultaneous assessment of the effect of thermal bridges and how their energy performance could be influenced. Likewise, the economic and environmental profitability of the analyzed ECMs is crucial in this analysis because of the energy consumed and the CO
2 emissions produced by the ECMs during the stages of manufacturing materials, transport to the construction site, and placement or installation in the existing building [
20]. Assessing the true profitability of these ECMs is fundamental to achieving the goal of reducing greenhouse gas emissions. In addition, tests to assess the energy performance in current and future climate change scenarios are required using available estimations of how building energy performance could vary throughout the 21st century [
21,
22,
23]. The research by Bienvenido-Huertas et al. [
19] is the sole study partially analyzing the possible performance of the thermal bridge in future years, although this study focuses on new buildings and very specific designs of slab fronts. Thus, the knowledge gap that is discussed in this work is the reliability of the improvement of façades considering the effect of thermal bridges and the expected building performance throughout the 21st century. Likewise, the payback period of both consumed energy and CO
2 emissions in the manufacturing phases, as well as the reliability of these types of measures to achieve the low-carbon goals by 2050, are also analyzed. These aspects are studied from the perspective of existing climates in Spain, a country with the largest surface area in southern Europe. This work is important as few studies have analyzed the importance of improving façades and the enhancement of the effect of thermal bridges from a varied perspective. Furthermore, this study also aims to improve the energy performance of existing buildings. In this regard, the design of new energy-efficient buildings is a complex task for engineers and architects, which is more difficult in the case of existing buildings [
24]. In addition, the long useful life of buildings implies the need to analyze the building performance over a longer period of use compared to other sectors, such as the automotive industry.
This study employs a multidisciplinary perspective to analyze the possibilities of using ECMs to improve façades to reduce the energy consumption and greenhouse gas emissions of existing buildings, assessing the effect of thermal bridges simultaneously. For this purpose, an existing building in Spain was selected and analyzed in various climate zones of the country. Spain was selected because of the difficulties found by some studies to implement nearly zero energy buildings (NZEB) correctly in the southern countries of Europe [
1]. NZEB is included in the Energy Performance of Buildings Directive (EPBD) [
25], and there have been significant difficulties achieving this category in existing buildings [
24]. Thus, this study’s orientation to existing buildings aims to address the limitations of reaching NZEB in such buildings. Furthermore, this work adopts new operational patterns that users could employ as a result of the COVID-19 pandemic [
26]. The study shows that the application of ECMs focused on reducing the thermal transmittance of façades and the linear thermal transmittance of thermal bridges has a more significant energy and environmental effect than other more economical energy conservation measures that do not improve the effect of thermal bridges. Likewise, the application of ECMs to improve façades, although in energy terms clearly reduces greenhouse gas emissions and other problems such as energy poverty [
27], is far from achieving the goals set by the European Union to reduce these emissions by 90%. As such, these measures should be combined with other measures.
The structure of the article is as follows:
Section 2 describes the methodology,
Section 3 presents and discusses the results of the research. Finally,
Section 4 summarizes the main conclusions of the study.
3. Results and Discussion
Thus study first assessed the linear thermal transmittance of the thermal bridges in the building. For this purpose, two-dimensional transitory simulations of the thermal bridges of both the slab front and the pillar located at the corner were performed, both with the original design of the building and with the two ECMs presented, thus obtaining the linear thermal transmittance.
Figure 6 includes the results obtained from the simulation process of the thermal bridges. The building presented a major heat transmission in the thermal bridges that could contribute to a diminished energy performance of the building. The linear thermal transmittance of the slab front was 0.855 W/(mK), and that of the protruding corner with pillar, which would be a negative value in a case where there was no material of other thermal properties, was 0.155 W/(mK). These junctions therefore contributed to a greater building energy demand. If one of the two presented ECMs were used, their effects on the linear thermal transmittance of junctions would be different. On the one hand, the implementation of an ETICS in the building was an optimal solution to improve the thermal transmittance of the façade and to mitigate the linear thermal transmittance of the thermal bridges, obtaining a value of 0.124 W/(mK) in the slab front and −0.001 W/(mK) in the corner. On the other hand, the internal plasterboard had different tendencies in the two thermal bridges: the lowest linear thermal transmittance value was obtained in the corner, and the linear thermal transmittance value was increased in the slab front in comparison with the value obtained in the existing building. Although ETICS and the plasterboard resulted in a similar improvement of the thermal transmittance of the façade, their effects on the thermal bridges could lead to different energy performances.
The energy performance obtained in the building was analyzed by improving the façade. However, the energy performance of the building was first assessed in the climate zones in Spain, both in the current scenario and throughout the 21st century, with the A2 climate change scenario.
Figure 7 includes the yearly results of the current scenario, and
Figure 8 and
Figure 9 show the yearly results of the climate change scenario. Two operational patterns were considered according to the new working tendencies as a result of the COVID-19 pandemic: working outside the home and teleworking. According to this operational pattern, the intensity of the energy demand of the building will be greater with teleworking users. An analysis of the results determined how the energy performance of the building in the current scenario was different according to the climate zone, with greater energy consumption in the coldest climate zones. The climate zones with a greater WCS were characterized by obtaining the greatest energy consumption because of the high heating energy consumption. However, the combination of the winter climate zones C and D with zones of high SCS obtained the highest combinations of energy consumption in the country, except zone E1. The climate classification in Spain is made independently for winter and summer, so there could be very cold regions in winter and very hot regions in summer. The climate zones C3, C4, and D3 obtained a greater energy consumption following zone E1. In some regions, high cooling energy consumption could also contribute to high energy consumption at the end of the year. In this regard, zones with a low severity in winter, such as climate zones B3 and B4, obtained high energy consumption values. In general terms, this energy consumption value surpassed 10 MWh, which corresponded to emission values greater than 3.57 t equivalent CO
2. Likewise, the significant rise in users teleworking in their dwellings considerably increased the energy consumption of the building. The increase generated in energy consumption with respect to an operational pattern in which individuals worked outside was between 63.5 and 339.8% for cooling energy consumption, between 39.1 and 46% for heating energy consumption, and between 46 and 78.6% of the total energy consumption. Thus, the influence of new working conditions established as a result of the COVID-19 pandemic could significantly influence the energy performance of building stock and the possibility of energy poverty. These results showed that greater use of the building could increase its energy consumption by up to 78.6%, thus generating greater energy expenditure for family units.
Regarding the future tendency of the energy performance of the building, the climate evolution expected throughout the 21st century will progressively reduce heating energy consumption and increase cooling energy consumption. Consequently, the relationship between climate zones and the greatest energy consumption will vary throughout the 21st century, and at the end of the century, the hot zones B3 and B4 will obtain similar values of energy consumption to that of zones D3 and E1, which are the zones with the greatest value in the current scenario. This aspect increased the energy consumption of the hottest zones between 30% and 33%, while reductions between 5% and 17% were obtained in the coldest zones. Thus, it is obvious that the energy performance and impact of Spanish building stock will vary throughout the century, with the buildings located in the hottest climate zones being more important. This aspect will also depend on the operational pattern of users. In general terms, teleworking was found to increase energy consumption between 51.4% and 66.1% by 2050, and between 51.9% and 68.6% by 2100. Thus, new use tendencies could generate a greater environmental impact and greater greenhouse gases emitted to the atmosphere, increasing the effect of climate change.
For this reason, the improvement of the energy performance of the building stock with ECMs is crucial. Both ECMs analyzed in this study significantly reduced energy consumption.
Table 6 and
Table 7 present the savings in energy consumption obtained with the ECMs when users worked at home, and
Table 8 and
Table 9 present the savings in energy consumption when users worked outside the home. The application of the ECMs clearly improved the energy performance of the building. In this regard, the application of ETICS with teleworking users obtained savings oscillating between 26.61 and 115.96 MWh in the current scenario, between 21.68 and 104.58 MWh in 2050, and between 29.83 and 93.47 MWh in 2100. This reduction in energy saving was different according to the climate zone, obtaining a greater saving in the coldest zones. Thus, the climate zones D1, D2, and E1 obtained a greater saving in energy consumption, and the hot zones A3 and A4 obtained a lower saving in energy consumption. Another aspect to be emphasized is the differences in the energy saving obtained with plasterboard in comparison with that obtained with ETICS. The energy saving obtained with the plasterboard was lower than that obtained with ETICS, with a percentage reduction in energy savings between 37.5% and 43.7% in the current scenario, between 32.8% and 43.3% in 2050, and between 33.5% and 41.9% in 2100. One of the reasons for this lower energy saving could be the increase generated in the linear thermal transmittance of the slab front with the plasterboard, although both solutions implied a similar improvement in the thermal transmittance of the façade (0.371 W/(m
2K) with ETICS and 0.364 W/(m
2K) with the plasterboard). The linear thermal transmittance of the slab went from 0.855 W/(mK) in the existing building to 0.124 W/(mK) with ETICS, and with the plasterboard, a value of 0.881 W/(mK) was obtained, resulting in an increase of 0.026 W/(mK) in comparison with the building. Thus, the implementation of ECMs inside, which could be cheaper, could also be less advantageous because of their low effectiveness and could even increase the effect of certain thermal bridges of the building, such as slab fronts. Finally, the possible effect of teleworking in comparison with working outside the home is worth emphasizing. The energy consumption of the building with teleworking was greater than when users worked outside the home, and this aspect also implied that the energy saving obtained in the building was greater when users worked at home. In this regard, teleworking implied that the energy savings presented a percentage increase in comparison to that obtained when users worked outside the home between 66.1% and 145.9% with ETICS and between 15.4% and 209.5% with plasterboard with insulation.
These variations in the savings of energy consumption obtained with the ECMs and the operational patterns affected the payback periods expected with the measures and savings in CO
2 emissions. To assess this aspect, the payback period required to recover the economic cost of the implementation of the ECMs was analyzed, as well as the recovery of the energy and CO
2 emissions produced by the ECMs in the stages of manufacture, transport, and implementation in the building. As the energy simulation data corresponded to the EPW files generated and were coincident with each decade of the 21st century (e.g., 2030 or 2040), linear interpolations between each decade were conducted to obtain the economic, energy, and CO
2 emission savings achieved in each year.
Figure 10 shows the number of recovery years required. Recovery periods presented different values according to the climate zone, the type of ECM, and the operational pattern. In general terms, the determinants that allowed greater energy savings to be obtained (cold climate zones and teleworking) contributed to shorter recovery periods. As for the economic recovery, plasterboard was the ECM with the greatest facility of economic recovery due to its low price; thus the years of economic recovery oscillated between 13 and 55 years with teleworking, and between 23 and 71 years with working outside the home (economic recovery was not possible in zone A4). These data contrasted with those of ETICS, which presented longer recovery periods (between 27 and 71 years with teleworking and between 46 and 77 years working outside the home), with difficulties for the economic recovery in many climate zones. However, the possibilities of financing through aid, such as the financing program for energy rehabilitation in the State Housing Plan in Spain, reduced the payback periods of the economic cost of the ECMs. This plan achieved reductions of 40% in the economic cost of the investment, so the amount that users should amortize is 60%. With this reduced value of the economic cost of the ECMs, the payback period obtained with ETICS presented values similar to those obtained with the plasterboard without financing. However, in the plasterboard, the financing reduced the number of years required and the possibility of economic recovery in all the combinations of climate zones and operational patterns. Based on these results, the plasterboard constituted an interesting ECM from an economic point of view because of its possibility of rapid economic recovery in comparison with ETICS. Nevertheless, the recovery of the energy and the CO
2 emissions generated in the manufacture, transport, and installation of the ECMs was lower with ETICS. The reasons were as follows: (i) the greatest value of energy and emissions generated by the plasterboard was mainly due to the manufacture of the laminar plaster plate; (ii) the greatest energy saving was achieved with ETICS. The combination of these two aspects meant that the recovery period of CO
2 emissions oscillated between 3 and 24 years with ETICS, and between 6 and 44 years with the plasterboard. Finally, and regarding this last aspect, the percentage saving in CO
2 emissions achieved with both ECMs in 2050 with respect to emissions in the current scenario was notable. As
Table 10 shows, the emission savings obtained with ETICS oscillated between 6.9% and 36.7%, so these values were far from the general goals of 90% set in the roadmap towards a low-carbon economy by the European Union [
4]. These results are in accordance with the indications included in various works related to limitations in the modernization of façades [
29]. Although the improvement of the envelope clearly reduces the percentage of CO
2 emissions of the building, combining this measure with others, such as a more intelligent use of HVAC systems [
40], self-consumption [
41], and a greater generation of electrical energy with renewable sources at a national scale [
42], is required. The combination of these measures, together with the improvement of the envelope, would more easily achieve a low-carbon building stock. Nonetheless, the role of the thermal bridges should be considered in these energy improvements. Based on the results obtained, an improvement of the envelope focusing only on obtaining a low thermal transmittance value could be an inappropriate option if, due to design reasons, the linear thermal transmittance of the thermal bridges is not reduced or is considerably increased. Both ECMs analyzed in this study obtained similar thermal transmittance values, but their effects in terms of energy saving and reduction of CO
2 emissions were clearly different because of the effects generated by the thermal bridge of the slab front. For this reason, it is necessary to develop decision-making support systems that consider the possible building improvements from an interdisciplinary perspective [
43,
44]. These support systems could help architects and engineers to determine the most suitable ECM to achieve the sustainability goals foreseen for the entire building stock of the European Union by 2050.
4. Conclusions
The achievement of a low-carbon building stock by 2050 is among the main challenges of today’s society. For this purpose, one of the lines of action to improve the building stock is dealing with envelopes, which could be significantly influenced by thermal bridges, together with other aspects, such as users’ operational patterns and the climate zone in which the building is located. To study these aspects, the performance of a case study representing a large percentage of the Spanish building stock was assessed in 12 climate zones. Performance was assessed in both the current scenario and throughout the 21st century by using the A2 climate change scenarios. The building was assessed with its original envelope design and two different ECMs: ETICS and plasterboard with insulation.
The results showed that the energy consumption presented by the building with its original design was greater in the coldest climate zones in Spain, and the energy consumption was lower in the other zones. However, these energy consumption values in the current scenario presented variations throughout the 21st century that tended to reduce the heating energy consumption and to increase the cooling energy consumption, so at the end of the 21st century, less cold climate zones (e.g., B3 or B4) achieved similar or greater values than those obtained in the coldest zone in Spain (zone E1). In addition, these energy consumption values could be mitigated by improving the envelope. Important energy saving values were achieved using the two ECMs analyzed in this work. However, the saving values were different between these ECMs, although both achieved similar values in the thermal transmittance of the façade. In quantified data, ETICS achieved reductions between 21.08 and 115.96 MWh, and the plasterboard achieved reductions between 13.87 and 66.24 MWh. This variation was mainly based on the effects generated by both ECMs on the linear thermal transmittance of the slab front of the building. The buildings built with the first energy efficiency regulations of the country were characterized by having thermal insulation in the façade, but their design implied that the linear thermal transmittance of the façade junctions (particularly significant in the slab front) was high. As for the two ECMs presented, ETICS significantly reduced the linear thermal transmittance, but the plasterboard increased the linear thermal transmittance in comparison with the value obtained with the original design of the façade. Although both ECMs could similarly improve the thermal transmittance of the façade, there were different effects on the energy improvement achieved. Therefore, it is crucial to focus on the ECMs presented in the existing buildings, not only on the improvement of the heat transmission of the façade but also on the treatment of the various thermal bridges. Additionally, two other aspects related to the effectiveness of the ECMs should be emphasized: the climate zone of the building and users’ operational pattern. On the one hand, the cold zones obtained a greater energy saving, and the hot zones obtained a low energy saving. On the other hand, users’ operational patterns played an important role in the quantification of the energy saving achieved and the reduction of the environmental impact. The COVID-19 pandemic has led to a new working model through teleworking, thus generating a greater use of buildings in comparison to those users working outside the home. As a result, the energy consumption and the energy savings significantly increased, so the energy savings obtained by improving the envelope could be between 15.4% and 209.5% greater compared to that obtained with teleworking users.
These aspects became more important when the periods were analyzed not just in relation to economic terms, but also in relation to the emissions and the energy consumed by the ECMs in the manufacture, transport, and installation stages. The energy savings obtained with ETICS achieved rapid recovery periods of emissions and energy consumed during the A stages of the life cycle of the ECM, and the recovery periods were greater with the plasterboard. This aspect was opposed to the economic recovery period, in which the plasterboard had a lower economic recovery time. However, if the government establishes financing programs, then an appropriate economic feasibility for the most expensive ECMs, such as ETICS, could be ensured. Thus, the relationship of economic-environmental amortization stresses the importance of ETICS to the detriment of other ECMs, such as plasterboards. The significant reduction of the thermal transmittance and the linear thermal transmittance of thermal bridges considerably influenced the reduction of the energy consumption, thus treating aspects such as CO2 emissions or the energy poverty risk. Nonetheless, the improvement achieved by ETICS was insufficient to achieve the goals of a low-carbon building stock established by the European Union for 2050. In this regard, the reduction percentages achieved in 2050 oscillated between 6.9 and 36.7%, which were far from the goal established by the European Union (90%). The difficulties related to the implementation of nearly zero energy consumption buildings in the south of Europe require the combination of traditional ECMs, such as the improvement of the envelope, with other ECMs, such as the intelligent use of HVAC systems or self-consumption. Further studies will be focused on the analysis of the possibilities of energy savings achieved by combining the improvement of the envelope, the appropriate use of HVAC systems, and self-consumption. In this regard, it is crucial to analyze the importance of photovoltaic systems to achieve a low-carbon building stock in southern European countries.