**Life-Cycle Assessment of Alternative Envelope Construction for a New House in South-Western Europe: Embodied and Operational Magnitude**

#### **Helena Monteiro 1,2,\* , Fausto Freire <sup>2</sup> and John E. Fernández 3,4**


Received: 30 June 2020; Accepted: 6 August 2020; Published: 11 August 2020

**Abstract:** The building envelope is critical to reducing operational energy in residential buildings. Under moderate climates, as in South-Western Europe (Portugal), thermal operational energy may be substantially reduced with an adequate building envelope selection at the design stage; therefore, it is crucial to assess the trade-offs between operational and embodied impacts. In this work, the environmental influence of building envelope construction with varying thermal performance were assessed for a South-Western European house under two operational patterns using life-cycle assessment (LCA) methodology. Five insulation thickness levels (0–12 cm), four total ventilation levels (0.3–1.2 ac/h), three exterior wall alternatives (double brick, concrete, and wood walls), and six insulation materials were studied. Insulation thickness tipping-points were identified for alternative operational patterns and wall envelopes, considering six environmental impact categories. Life-cycle results show that, under a South-Western European climate, the embodied impacts represent twice the operational impact of a new Portuguese house. Insulation played an important role. However, increasing it beyond the tipping-point is counterproductive. Lowering ventilation levels and adopting wood walls reduced the house life-cycle impacts. Cork was the insulation material with the lowest impact. Thus, under a moderate climate, priority should be given to using LCA to select envelope solutions.

**Keywords:** LCA; environmental impact; house; building envelope; thermal performance

#### **1. Introduction**

Households represent around 27% of the European Union's (EU) final energy consumption. To address this, EU regulatory efforts have been enacted to promote energy efficiency, and the new EU Green Deal roadmap aims to encourage that EU building stock (new and existing buildings) become energy and resource efficient. To support new building developments, a life-cycle perspective is recommended since reducing operational energy through improved building envelopes is likely to affect the impact of other life-cycle phases of new buildings.

Life-cycle assessment (LCA) has been extensively used to study residential buildings [1,2], building options [3,4], and building construction [5]; however, most studies have focused on primary energy and/or CO2 emissions, disregarding other environmental impacts. Review articles on LCA of buildings [6,7] agree that comparing different studies is not linear because building characteristics (size, shape, construction, and occupation) vary with location and climate, and the studied methodological

assumptions (functional unit, lifespan, life-cycle impacts, exclusions) widely vary [8,9]; although some trends can be climate specific, each study helps to explain a climatic and regional context.

Studies covering cold climate houses in developed countries have concluded that the operational phase has a preponderant weight in the total life-cycle of the building [1,10]. Moreover, studies of conventional buildings in different countries (Sweden [11], Kazakhstan [12], Alaska, USA [13], Spain [14], Portugal [15]) have showed that operational energy is dominant, representing 60–90% of the total environmental impacts. Thus, reducing heating and cooling is essential. Interestingly, a study that provided an LCA benchmark for dwellings in North Italy and Denmark [16] showed that, in North Italian case studies, operational impacts accounted for 69–76%, and embodied impacts accounted for 24–31% of the overall impact, whereas in Danish cases, the impacts per life-cycle phase are reversed due to the low impact of the future Danish energy grid. This shows that life-cycle results are also highly sensitive to specific regional conditions other than climate, such as the energy mix.

Dylewski [17] studied the environmental impact of diverse thermal insulation materials for exterior walls in Poland, considering alternative heat sources, in order to find the optimal insulation thickness considering both economic and ecological net present value of insulation (as an investment). Results showed that significantly higher thicknesses were recommended when considering environmental data as compared to economic data—for instance, 0.46–0.52 m of expanded polystyrene (EPS) insulation considering brick walls and a heat pump system. Some LCA studies assessed low energy and passive houses [18–20], in which operational energy is substantially reduced. Generally, when operational energy is reduced, the relative contribution of embodied impact rises and therefore a life-cycle perspective is essential [21].

In low energy houses, embodied energy can amount to 50–70% of the life-cycle energy [22], and the building envelope is accountable for a significant share of embodied impacts. Consequently, alternative building options must be carefully assessed in new dwellings and, again, a life-cycle assessment study to support decision-making at an early building design stage is desirable.

Some studies covered South European dwellings [14,21,23–27]. However, a trend regarding which life-cycle phase has the most impact in new houses located in South Europe under a mild climate was not determined. Embodied and operational impacts are both significant, but their life-cycle contributions appear to be highly sensitive to construction options, energy systems, operation/occupation behavior, and regional aspects. The electric production mix (share of renewable) is essential to characterize the environmental impacts of the use phase [14,28,29]. Additionally, operational heating and cooling behavior can significantly affect a study outcome [23,30].

Thus, the prevailing strategies for cold climate houses should not be directly transposed onto other building or climatic contexts [31] because, depending on the local context, the embodied impact may surpass the operational impact. Studying alternative passive architecture measures and their influence on operational energy of buildings in Spain, a recent study [32] concluded that, for some climate regions, a few passive strategies could reduce operational energy to the passive house level: north–south orientation, small window-to-wall ratio (<20%), insulated envelope (U = 0.35 W/m2K). Nevertheless, the authors recognize that user behavior remains unaddressed. Furthermore, as a life-cycle perspective was not considered, the embodied energy of the building measures was not assessed.

In South-Western Europe, many houses are exposed to a moderate Mediterranean warm climate, and interior comfort (operational patterns) may be dependent on user behavior (influenced by cultural heating habits and economic constraints). Thus, typical operational energy levels of these houses are lower than in most North and Central European countries [28]. According to Lavagna et al. [10], a considerable part of the environmental life-cycle impacts of EU building stock is associated with single family houses located in moderate climates, and new houses in this climate have not been widely assessed considering user behavior.

Regarding building components, exterior walls comprise a significant part of the construction embodied impact [29,33], and roofs were also identified as significant [34], especially for top-floor dwellings. A recent life-cycle study has assessed 114 flat roof alternatives for a Portuguese apartment

located in Lisbon considering environmental, energy, and economic criteria [34]. The functional unit assessed was 1 m<sup>2</sup> of roof used during a 50-year lifetime. The study concluded that, with an identical insulation layer, the roof impacts can vary widely among alternatives (e.g., the best non-accessible roof can lower CO2 emissions by 30%).

The goal of this study is to assess the life-cycle environmental influence of key building envelope options (with varying thermal performance) for a South-Western European compact house located in Portugal in a moderate Mediterranean climate. This research investigates how operational and embodied impacts of a house vary with building envelope alternatives in order to identify the alternatives with the lowest impacts. LCA and building dynamic simulation were integrated to assess the following envelope construction options throughout the walls and roof: five insulation thickness levels (0–12 cm), four total ventilation levels (0.3–1.2 ac/h), and six insulation materials. In addition, since exterior walls represent most of the building envelope area, three exterior wall construction alternatives (double brick, concrete, and wood walls) were also considered.

#### **2. Materials and Methods**

LCA methodology [35] was used to assess the environmental impact of building envelope alternatives for a new South-Western European house located in a mild Mediterranean warm climate in Coimbra, Central Portugal (1460 heating degree days). An attributional LCA approach [36] was selected since it was not expected that the flows within the supply chains would change as a consequence of the adoption of the alternatives assessed. A process-based life-cycle inventory was built based on previous research [23,28,37] and using average background data. The functional unit selected was to build and use a house (for a 4-person family) during its lifespan. A lifespan of 50 years was assumed since it is a common lifespan considered for buildings in the literature [8,37]. The life-cycle study included three life-cycle phases: construction, operation (heating and cooling), and maintenance of the building and envelope alternatives. Furthermore, these phases are considered the most significant and amount to the majority of a building's life-cycle impacts (82–98%, based on [16]).

The construction phase included material production, transport to the construction site, and on-site construction processes (considering a 5% material waste factor). Materials and techniques commonly used in Portugal during the last few decades were assumed. The environmental impacts of building material production and transport were aggregated by average construction product or process and assessed based on European background data from ecoinvent v3.2 [38], using SimaPro 8.3 software [39].

Maintenance activities that preserve the physical characteristics of the building during its lifespan (painting, vanishing, and roof water-proof layer replacement) were taken into account based on data from local construction material producers [40,41]. Detailed information regarding the maintenance activities schedule can be found in [28]. Their environmental impact was assessed based on background data from ecoinvent v3.2.

The annual heating and cooling loads for the house and the various building alternatives were obtained by thermal simulation in DesignBuider © v3.0 [42], which is a dynamic thermal simulation tool based on the Energyplus calculation engine (tested and validated under the comparative standard method of test BESTEST and ANSI/ASHRAE Standard 140–2011). Operational patterns were considered to better represent mild climate modest energy (heating and cooling) use, typical of Portuguese dwellers. In the LCA, the heating and cooling electric energy requirements obtained by thermal simulation were converted to life-cycle environmental impacts using inventory data for the Portuguese electricity generation mix in 2012 [43]. In the last few years, Portugal has consistently had a large share of electricity generated from renewable energy sources when compared to other European countries, which influences the operational life-cycle impact.

Information regarding the case study definition, namely construction and alternative construction scenarios considered, can be found in Section 2.1, while operational phase details and the operational patterns considered are presented in Section 2.2.

In the life-cycle impact assessment (LCIA) stage, two well-known LCIA methods were used. These were the cumulative energy demand (CED) method, to account for the non-renewable primary energy (NRPE), and the CML 2001 method, to account for the following environmental impacts [44]: abiotic depletion, global warming potential (GWP), acidification, eutrophication, photochemical oxidation, and ozone layer deletion (OLD).

Given the comparative nature of this LCA study, the life-cycle model implemented assumed a few simplifications, which are identified and explained in Table 1.



#### *2.1. Construction: Base Case House and Envelope Alternatives*

The house under study is a Portuguese household occupied by a 4-person family. A single-family house was selected because it is the most common residential building type in Portugal. The compact building shape, typology, and area are representative of an average Portuguese house based on statistical data [46,47]: it has two floors, 133 m2, and a 3-bedroom typology. Table 2 describes the main building components of the base case house; axonometric drawings of the building can be found in [37].




**Table 2.** *Cont*.

A parametric analysis of the alternative construction options studied (presented in Table 3) was performed for the following: five envelope insulation levels (0–12 cm), five insulation materials, four total ventilation levels (including infiltration), and three exterior wall systems.


**Table 3.** Envelope construction alternatives and base case.

<sup>1</sup> Measures applied both to façades and roof. <sup>2</sup> Instead of the exterior thermal insulation composite system (ETICS), the hypothetical non-insulated concrete wall (0 cm) has a base plaster finish.

A hypothetical non-insulated scenario (0 cm), which does not meet the legal thermal requirements, was considered with the sole purpose of better showing how operational and embodied impacts vary with the insulation level (i.e., allowing us to draw in results figures which are representative of the polynomial trend-line from 0 cm through the following insulation thicknesses). Nevertheless, in the analysis, a focus is given to insulated alternatives (3–12 cm).

#### *2.2. Building Operational Conditions*

Operational energy consumption is directly affected by the building characteristics and by the construction options studied. The operational phase included the impact of heating and cooling the house with a 10 kW air-water heat pump system (2.8 COPheating and 2.0 EERcooling). Table 4 summarizes the energy building simulation settings used to assess the house with alternative building construction alternatives in DesignBuider © v3.0. A window-shutter schedule, presented in Table 5, was assumed to account for typical use of the window-shutters to benefit from solar gains during the heating season and avoid them during cooling season.



A continuous operational pattern (OP100) that reflects continuous interior comfort conditions and occupation (identified in Table 3) was initially used to thermally assess the residential building performance. However, in mild climates, users do not heat and cool continuously, nor do they heat all the rooms simultaneously. Due to this fact, the final energy results were significantly higher when compared to statistical data on energy consumption in Portuguese houses. For instance, comparing the thermal energy requirements for an equivalent existing house with identical shape/construction (based on Portuguese building stock characteristics [47]) and the average real heating energy consumption per square meter in houses in Portugal (inferred from statistical data; [49]), a continuous operational

weekends 9 h–12 h 12 h–9 h

30 June to 30 September weekdays 7 h–8.5 h 8.5 h–7 h

pattern reveals a significant gap [9,28]. Portuguese real household consumption can be 75% lower than simulated energy needs for maintaining continuous comfort conditions. This gap, called the prebound effect [30], represents the way in which user behavior can reduce expected energy consumption levels. It seems that Portuguese dwellers heat their homes partially, or at cooler temperatures, or have their heating on for less time than assumed in the simulated continuous operational pattern. This is possible because winter climate conditions are not as harsh as in North and Central European locations and the summer climate is not hot but warm. Furthermore, occupants tend to use heating more economically in houses that are thermal underperformers [30,50]. Consequently, the prebound effect percentage might change with the thermal performance of the building, decreasing the benefit of energy efficiency measures. As real operational energy consumption data are limited, in this LCA study, two alternative operational pattern scenarios were used to inform heating and cooling habits:


This study did not intend to assess the specific effect of dynamic (zoned and intermittent) operational patterns, which widely vary with the household. Stazi et al. [51] covered these aspects and the effect of thermal mass (inertia) in three super-insulated multifamily buildings both for hot and cold climates. They concluded that, in such highly insulated envelopes, thermal mass had a low influence on operational energy savings (marginal benefit). Additionally, thermal mass (masonry alternative) had a stronger effect on comfort levels (less discomfort hours for intermittent cooling) but it had 20% higher environmental life-cycle impacts (for ecoindicator'99).

#### **3. Results**

The main LCA results are presented for two operational patterns (OP25 and OP50). Firstly, the influence of alternative ventilation and insulation levels was assessed for the base case house (house with double hollow-brick walls and double-glazing windows, using heat pump system). Later, the influence of alternative exterior wall systems and insulation level were assessed. Lastly, alternative insulation materials were considered. When assessing alternative insulation levels, trend-lines (polynomial, order 4) are shown in the figures to clearly indicate the influence of varying insulation levels from a hypothetical 0 cm insulation.

#### *3.1. Influence of Ventilation Level vs. Insulation Level*

Four total ventilation levels (0.3–1.2 ac/h) and five insulation levels (0–12 cm) were considered for the base case house. Life-cycle results are presented for non-renewable primary energy (NRPE) in Section 3.1.1. and for six environmental impact categories in Section 3.1.2.

#### 3.1.1. Non-Renewable Primary Energy

The construction phase of insulated house alternatives (3–12 cm) was the most important phase (Figure 1) in terms of life-cycle NRPE, representing 63–82% in OP25 and 49–76% in OP50, whereas the operational phase represented 8–28% and 14–43% of NRPE in OP25 and OP50, respectively. Insulation thickness tipping-points, for which NRPE was reduced, were identified: these were 3–6 cm for OP25 and 6–9 cm for OP50. However, in OP50, the total life-cycle benefit of having more insulation than 6 cm was less than 1% for all ventilation scenarios. The insulation tipping-point did not change significantly with the ventilation level.

**Figure 1.** NRPE results for base case house with a heat pump for OP25 and OP50: ventilation level vs. insulation level.

In a well-insulated (6 cm) and air-tight (0.3 ac/h) house with modest energy use (OP25), maintenance had a similar impact to operational energy. When operational energy is reduced, other life-cycle phases' relative contributions are increased.

Compared with a hypothetical non-insulated house (0 cm, 1.2 ac/h), a 6 cm XPS layer reduced operational NRPE by 39–61% (from OP25 to OP50) but it only achieved a life-cycle reduction of 8–9% (OP25) or 16–20% (OP50). Lowering the overall ventilation level from 1.2 to 0.3 ac/h reduced operational NRPE by 38–68% (from OP25 to OP50) and life-cycle NRPE by 14–15% (OP25) or 20–23% (OP50). Assessing the joint effect of the measures (6 cm insulation; 0.3 ac/h ventilation), maximum NRPE reductions of 21% (OP25) and 36% (OP50) were achieved compared to the hypothetical worst scenario. The base case house (6 cm; 0.6 ac/h) yielded a 17% (OP25) and a 30% (OP50) NRPE reduction.

#### 3.1.2. Environmental Impact Assessment

LCIA results are presented for OP25 (Figure 2) and OP50 (Figure 3) to determine whether a broader environmental impact assessment results in the same conclusions as the NRPE analysis. Results show that abiotic depletion, acidification, and GWP correlate with NRPE (Figure 1). In OP25, the insulation tipping-point was between 3 and 6 cm for most categories (exceptions: eutrophication and OLD), whereas in OP50, the tipping-point varied widely: 3–6 cm for GWP and photochemical oxidation; 9–12 cm for abiotic depletion and acidification. For eutrophication, the tipping-point was above 12 cm even in OP25, since the insulation material used (XPS) had relatively low impact in this category. Regarding OLD, the impact of construction (87–99%) surpassed, by far, operational impacts in insulated alternatives. Construction materials, especially XPS insulation, had a significant contribution to OLD. The high impact of XPS is justified by the extrusion process that uses a hydrofluorocarbon (HFC-134a).

**Figure 2.** LCIA results for base case house with a heat pump for OP25: ventilation level vs. insulation level.

**Figure 3.** LCIA results for base case house with a heat pump for OP50: ventilation level vs. insulation level.

In OP25, construction was the most significant life-cycle phase for all categories in insulated house alternatives. Furthermore, in photochemical oxidation, construction had a significant impact (77–88%). In OP50, the most significant phase (construction or operation) varies with the insulation and ventilation levels. For the house with two simple passive construction measures (6 cm XPS and 0.6 ac/h), embodied impacts had a life-cycle contribution above 67%.

#### *3.2. Influence of Exterior Wall Construction Alternatives vs. Insulation Level*

In this subsection, three exterior wall alternatives—double brick, lightweight concrete, and wooden wall—were assessed jointly with different envelope insulation levels. Results are presented for the base case house with 0.6 ac/h ventilation level.

#### 3.2.1. Primary Energy

Figure 4 presents NRPE for OP25 and OP50. Results show that the operational energy of the three exterior wall house alternatives is similar and mostly dependent on the envelope insulation level. Embodied energy (NRPE) surpassed operational energy for all insulated alternatives, amounting to 62–78% in OP25 and 52–70% in OP50, whereas operation varied from 12% to 25% in OP25 and 21% to 40% in OP50.

**Figure 4.** NRPE for OP25 and OP50 for exterior wall house alternatives (brick, concrete, and wood) vs. insulation level.

The double brick wall construction had the highest embodied energy. Comparatively, the concrete wall construction alternative had 13–15% lower embodied NRPE (depending on insulation level), and the wood wall alternative had 22–18% lower embodied NRPE. In the CED method, wood is considered a renewable source of energy and has low embodied NRPE. Thus, the wood wall house had the lowest NRPE, with a reduction of 11–14% (OP25) or 7–11% (OP50) NRPE when compared with the base case brick house. The concrete wall house had a NRPE reduction of 3–7% (OP25) or 1–6% (OP50) since the embodied energy reduction was partially offset by the higher maintenance requirements. Maintenance of a concrete wall house results in a higher NRPE than the other exterior wall alternatives, mainly due to the acrylic plaster finishing of ETICS (exterior thermal insulation composite system).

The insulation tipping-point varied both with the exterior wall alternative and with the operational patterns. For OP25, tipping-points were 6 cm for concrete and wood wall houses and 3 cm for the base case brick house. For OP50, the tipping-points were around 12 cm for concrete wall, 9 cm for wood wall, and 6 cm for brick wall house.

#### 3.2.2. Environmental Impact Assessment

Figures 5 and 6 present the LCIA results for OP25 and OP50, respectively. Acidification closely correlates with NRPE. Abiotic depletion had a slightly higher operational relative contribution. Other environmental categories present some differences in the life-cycle phase contributions, insulation tipping-points, and specific insulation material impacts.

**Figure 5.** LCIA house results for OP25: exterior wall construction alternatives vs. insulation level.

**Figure 6.** LCIA house results for OP50: exterior wall construction alternatives vs. insulation level.

In GWP, photochemical oxidation, and OLD, dissimilar embodied impacts are associated with XPS (brick and wood walls) and EPS (concrete wall ETICS). XPS had a 3.8 times higher GWP impact and 2000 times higher OLD impact than EPS for the same insulation thickness, whereas EPS had a 3.5 times higher photochemical oxidation impact than XPS. The OLD impact magnitude of XPS insulation is due to HFC-134a being used during the extrusion process, as explained in Section 3.2.1.

The insulation thickness tipping-point varied with exterior wall alternative, operational patterns, and impact categories. In OP25, the tipping-points for most environmental categories were as follows: between 3 and 6 cm for the brick wall alternative and 6 cm for wood wall alternative (exceptions: eutrophication, OLD); for the concrete wall, it was above 12 cm for three categories (GWP, eutrophication, OLD), 9 cm for abiotic depletion and acidification, and 0 cm for photochemical oxidation. Results show that the acrylic plaster finishing of ETICS had a high photochemical oxidation impact that surpasses operational energy savings due to insulation. In OP50, the tipping-points for both the brick and the wood wall were as follows: nearly 6 cm for GWP and photochemical oxidation; 9–12 cm for abiotic depletion and acidification; above 12 cm for eutrophication. The tipping-points for the concrete wall alternative were above 12 cm for five categories, except for photochemical oxidation.

Comparing the three exterior wall alternatives, the double brick wall had the highest embodied and total life-cycle impacts in four categories (abiotic depletion, GWP, acidification, eutrophication). Wood wall construction was the alternative with the lowest impacts in five categories, presenting a reduction of 7–20% in contrast to the brick wall alternative. In OP25, the embodied impacts of construction held most of the life-cycle impact in all environmental categories for insulated house alternatives. In OP50, the same was valid in four categories (except in abiotic depletion and eutrophication).

Assuming a 6 cm insulation level, which is a likely insulation level for a new house, the construction phase accounts for most of the house life-cycle impacts both in OP25 and OP50. In OP25, construction amounts to 62–84% of life-cycle impacts, operation 7–33%, and maintenance accounted for 5–16% in five categories (except OLD, which is explained below). Meanwhile, in OP50, construction accounted for 48–78% of life-cycle impacts, operation 13–49%, and maintenance accounted for 3–13%. OLD is a particular category in which embodied impacts were responsible for almost all impacts (88–98%), especially in the wall alternatives that incorporated XPS insulation (96–98% of life-cycle impacts), as explained in Section 3.2.1.

#### *3.3. Influence of Insulation Material*

To assess the specific influence of the selected insulation material, Table 6 presents how the embodied impact of the construction stage of the house varies with alternative insulation materials. The insulation materials' thicknesses were defined to have an equivalent insulation level to the base case house, which means that the building envelope delivers the same U-values of the base case (with 6 cm XPS).


**Table 6.** Influence of alternative insulation materials on the life-cycle impacts of the house compared to the base case house (XPS insulation).

<sup>1</sup> The base case XPS thermal conductivity was 0.030 W/m2K; <sup>2</sup> The non-insulation scenario allows us to account for the embodied impact of base case thermal insulation.

Results clearly show that changing the insulation material from XPS to cork panels can reduce the house embodied impact in the construction stage in all categories while ensuring the same operational impact. In fact, if cork insulation is considered, comparing the embodied with the operational LC impact (presented for alternative insulation levels in previous figures), the cork thickness tipping-point for the brick house would be between 12 cm (for OP25) and 16 cm (for OP50), being able to reduce the overall life-cycle NRPE by around 5.6–7.8% (in OP25-OP50). Results show that cork insulation is preferred compared to the other materials; the only downside would be the higher space that it takes to ensure the same performance (e.g., to ensure the same envelope U-value, cork insulation must be 1.33 times thicker than base case XPS).

#### **4. Discussion**

This study's results reinforce the idea that LCA is crucial not only to avoid problem-shifting but also to identify the most significant life-cycle processes, materials, and hotspots for improvement in new houses. Additionally, they highlight that under mild South European climates (e.g., Coimbra) and in the Portuguese context, even a lightly insulated house can have higher embodied impacts than the operational (for heating and cooling), whereas a new house (base case with 6 cm and 0.6 ac/h) is likely to have more embodied impacts in all environmental impact categories.

This finding may be surprising when compared to other studies, even for South European houses (Table 7), because both user behavior and climate widely vary. For instance, Italy and Spain are South European countries but they may have higher heating requirements or higher cooling requirements than houses in temperate, warm, summer, Mediterranean climates (Csb Köppen–Geiger climate classification) depending on the specific location of the buildings studied. Furthermore, users may heat and cool their houses differently (continually or partially) and this was shown to affect the operational energy magnitude in this study. Thus, operational patterns should reflect patterns of inhabiting and acclimatizing a house (typical user behavior). Assessing operational patterns more intensively than actual consumption might result in higher embodied energy (than needed) and be counterproductive.


**Table 7.** Comparison of case study and literature life-cycle results for GWP.

Legend: OP (operational pattern): C—continuous; P—partial. <sup>1</sup> Other operational energy needs were accounted for beyond heating and cooling; <sup>2</sup> the case study is a house-like cubicle with similar construction to the base case.

Other reasons that may justify such differences are the following:


(d) The Portuguese electricity mix, which has a substantial share of renewables [43]. In the last few years (and likely in the next few years), the electric mix should continue to have an increased contribution of renewable energy, which is expected to have lower environmental impact. Thus, it is even more likely that the operation phase has lower overall significance in new houses. Therefore, it is important to assess the embodied impacts in construction materials in order to arrive at construction alternatives with lower overall environmental impacts and consider those impacts at the project level jointly with operational environmental impacts at the local scale to avoid problem shifting.

Regarding the base case house, this study showed that reducing the ventilation level to 0.3 ac/h without compromising indoor air quality reduced life-cycle impacts by 4–14%, while adopting an alternative wood wall construction instead of the brick one reduced LC impact by 7–20%. These two measures are more beneficial passive solutions than increasing XPS insulation beyond 6 cm thickness, which only marginally reduced the overall impact (NRPE). Increasing insulation levels results in gradually lower NRPE savings and can even generate higher NRPE (when insulation is above the tipping-point), since embodied energy requirements offset operational energy savings.

This should hold true for new houses with a fairly compact shape and small window-to wall ratios, such as the base case, using a heat pump system, under similar climate conditions.

Operational impact was more affected by the insulation thermal resistance and thickness than by the varying construction of the exterior wall. This can be justified because all house alternatives had high thermal inertia, due to the heavyweight core of the house (concrete structure and brick interior walls), which remained unchanged. In this case, for the same insulation level, the life-cycle differences among exterior wall alternatives were mainly due to embodied impacts and maintenance procedures typical of different construction types.

The study also identified other material issues for improvement, namely the following:


#### **5. Conclusions**

An LCA of a house located in Coimbra (in mild, warm, Mediterranean climate) was performed, considering two operational patterns (OP25 and OP50). The influence of the following alternative building envelope options were assessed: insulation thickness levels (0–12 cm); ventilation levels (0.3–1.2 ac/h); insulation materials; exterior envelope solutions (double brick, concrete, and wood walls). The results showed that combining two simple passive construction measures, a good envelope insulation level (6 cm), and an air-tight envelope (0.3 ac/h ventilation level) may lead to important LC primary energy savings of 21% (for OP25) to 36% (for OP50) when compared to a hypothetical uninsulated house (0 cm; 1.2 ac/h). Increasing the base case XPX insulation thickness has only marginal life-cycle benefits and can even increase the overall life-cycle impacts (depending on operational patterns). Thus, to avoid problem-shifting, LCA is critical to assess the balance between embodied and operational impact. Insulation tipping-points (with reduced life-cycle impact) were identified for the

various environmental categories ranging between 3 and 6 cm for OP25 and 6 and 9 cm for OP50 for the brick wall house with XPS insulation.

Regarding the base case house (brick wall; 6 cm; 0.6 ac/h), two measures were identified to have more benefit than increasing XPS thickness: (a) the replacement of brick walls by wood walls (achieved a LC reduction of 7–20%); (b) increases in envelope air-tightness and reductions in total ventilation level to 0.3 ac/h (achieved a LC reduction of 4–14%). Regarding alternative insulations, cork panels resulted in the lowest embodied impact for an equivalent U-value envelope. Furthermore, for this material, the tipping-point thickness was around 12–16 cm, and it enabled a reduction in the life-cycle NRPE impact of the base case house by around 5–8%.

This study showed that construction represents a significant share (62–81%) of the LC impacts of new houses with fairly simple construction measures, using a heat pump system to satisfy current modest Portuguese operational user demands. This is a surprising result alongside other comparable studies, especially of buildings in Mediterranean countries because LCA impacts are strongly influenced by the climate and cultural local conditions (how to build and inhabit a house) and energy mix. Embodied impacts are currently not routinely considered in building energy performance certification [54]. However, as new buildings are expected to be very low energy in operation, neglecting embodied impacts may lead to problem-shifting, having higher embodied impacts in upfront construction than the avoided impacts in operation.

Thus, the adoption of construction options with lower embodied impact is highly important. To further reduce the environmental impact of buildings under mild climates, data on the environmental impact embodied in materials should be freely available in the marketplace—for instance, through widespread environmental product declaration (EPD) or product environmental footprint (PEF) schemes. This would greatly benefit architects, engineers, and households as they take into account the environmental impacts of their decision-making. Finally, to assess the overall sustainability of a wide range of building alternatives, future research work should further examine building life-cycle costs at higher resolutions and a greater range of the associated social impacts.

**Author Contributions:** Conceptualization, H.M. and F.F.; methodology, formal analysis and investigation, visualization, writing—original draft preparation, funding acquisition: H.M.; writing—review and editing, H.M., F.F and J.E.F.; supervision, F.F., J.E.F. and H.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work partially results from the funding that H.M. received from the Portuguese Fundação para a Ciência e a Tecnologia (SFRH/BD/33736/2009) and from the Energy for Sustainability (EfS) initiative from University of Coimbra.

**Acknowledgments:** H.M. acknowledges MIT Portugal Program and MIT for providing assistance and allowing H.M. to perform part of this research at the MIT Building Technology Lab.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Article* **Energy Analyses of Serbian Buildings with Horizontal Overhangs: A Case Study**

#### **Danijela Nikolic 1,\* , Slobodan Djordjevic 1, Jasmina Skerlic <sup>2</sup> and Jasna Radulovic <sup>1</sup>**


Received: 24 July 2020; Accepted: 1 September 2020; Published: 3 September 2020

**Abstract:** It is well known that nowadays a significant part of the total energy consumption is related to buildings, so research for improving building energy efficiency is very important. This paper presents our investigations about the dimensioning of horizontal overhangs in order to determine the minimum annual consumption of building primary energy for heating, cooling and lighting. In this investigation, embodied energy for horizontal roof overhangs was taken into account. The annual simulation was carried out for a residential building located in the city of Belgrade (Serbia). Horizontal overhangs (roof and balcony) are positioned to provide shading of all exterior of the building. The building is simulated in the EnergyPlus software environment. The optimization of the overhang size was performed by using the Hooke Jeeves algorithm and plug-in GenOpt program. The objective function minimizes the annual consumption of primary energy for heating, cooling and lighting of the building and energy spent to build overhangs. The simulation results show that the building with optimally sized roof and balcony overhangs consumed 7.12% lessprimary energy for heating, cooling and lighting, compared to the house without overhangs. A 44.15% reduction in cooling energy consumption is also achieved.

**Keywords:** building; overhangs; energy consumption; optimization; GenOpt; EnergyPlus

#### **1. Introduction**

Reduction in energy consumption is globally of great importance as the combustion of fossil fuels emits significant amounts of greenhouse gases, primarily carbon dioxide. Fossil fuels are also a limited resource which isdecreasing in Nature and should be very cautiously used. In order to reduce the primary energy consumption, it is essential to focus on reducing energy consumption in buildings. Building energy consumption is related to the exploitation conditions, where the largest consumers are the heating, cooling and domestic hot water systems, appliances, etc. Reducing energy consumption can be achieved by the construction of energy efficient buildings which have lower total energy consumption and lower greenhouse gas emissions. It is very important to apply as many measures to design energy efficient buildings as possible, primarily in the passive design of buildings. An application of passive energy elements on buildings, which include elements of shading by horizontal roof overhangs improves thermal indoor comfort, reduces primary energy consumption and hence reduces greenhouse gas emissions. Sometimes, a building design strives to insulate the building from outside influences, and thus to reduce energy exchange. At the other hand, it is necessary to utilize energy from the environment in the best way in order to achieve even better results. The implementation of these principles at the building design stage is the most effective way to achieve good results in the reduction of the energy required for heating, cooling, and lighting.

Many studies were carried out to analyze the impact of shading elements to energy consumption and most authors found a reduction in energy consumption for cooling due to shadowing. Cooling load due to solar gain represents about half of the total cooling load of residential buildings [1]. Solar radiation through the building windows can be decreased with different shading devices installed on the exterior side of building windows [2]. Skias and Kolokotsa analyzed the office building energy consumption for cooling in Athens (Greece) and ways of reducing it during the summer period by placing shadowing elements [3]. Their investigation was carried out in TRNSYS 16, and the application of the horizontal roof overhangs on the south side of the building yielded building energy savingsthat ranged from 7.2% to 17.5%. Kim et al. studied energy saving for cooling with the IES\_VE program for a shaded building located in South Korea [4]. They found that by building with horizontal overhangs on the south façade, it is possible to achieve energy savings for cooling of 11%. Raeissi and Taheri investigated the energy consumption for heating and cooling in a family home with horizontal roof overhangs located in Shiraz (Iran), at an altitude of 1491 m [5]. Analyses were performed for cooling and heating periods. The optimization of the primary energy consumption for building cooling and heating achieved a reduction in energy consumption for cooling by 12.7% and increased the energy consumption for heating by 0.63%. Bojic at al. in their paper [1] analyzed the primary energy consumption in residential building with overhangs during the summer season. The obtained results showed that in the case of a house without optimized overhangs, there is an increase in primary energy consumptionby 3.36% and in that case, the operative energy consumption is lower. Imessad at al. investigated a building with horizontal overhangs, located in Algiers, where there is a temperate Mediterranean climate [6]. This analysis was carried out in TRNSYS software, and results showed that horizontal overhangs in combination with natural ventilation can and improve thermal comfort and reduce cooling energy demand in the summer periodby 35%. Datta in his study [7] analyzed building with external fixed shading device for windows, in different cities in Italy (north to south). With a simulation model in the TRNSYS program, he optimized shading device size with the aim to minimize annual primary energy consumption in buildings. The results showed that with optimum shading a 70% solar gain can be avoided in Milan during the summer season. An air-conditioned office in England with fixed external overhang was investigated with simulations in the DOE-2 modeling program [8]. The obtained results showed that energy savings depend on latitude, so in Scotland it was between 1% and 9%. With moveable external shading the highest energy savings can be achieved. Yao [9] simulated a high residential building in Ningbo, China, which has movable solar shading devices in south-facing rooms, in the EnergyPlus software. The simulation results showed that movable solar shading devices can reduce building energy consumption by 30.87% and improve visual comfort for about 20%. Atzeri et al. [10] in their paper investigated an open-space office located in Rome (Italy). They used the EnergyPlus software, and compared the influence of indoor and outdoor shading devices on primary energy consumption, thermal and visual comfort. The main conclusion was that external shading devices can reduce cooling needs and increased heating load. Florides at al. [11] modeled and simulated a modern building with the aim to reduce its thermal load. They recommended a window overhang length of 1.5 m, with which it is possible to save 7% of annual cooling energy consumption for a building with single walls and without roof insulation, and 19% of annual cooling energy consumption for the buildings with walls and roof with 50 mm insulation [11]. Liu et al. [12] investigated shading devices on opaque facades of public buildings in Hong Kong and the possibilities for energy savings with them. They varied the length, the number and the tilt angle of the different configurations of shading devices and found optimal values for west-oriented overhangs, with an energy saving potential of up to 8%.

Aldawoud simulated the energy behavior of an office building with external shading devices and electrochromic glazed windows, located in Phoenix, a city in Arizona, USA, which has a very hot and dry climate. Simulations were carried out in the Design Builder software. Among the other energy performance factors, great attention was paid to the energy consumption for heating, cooling and interior lighting. The simulation results showed that electrochromic glazing provided the

greatest reduction of solar heat gains during hot summer days. Also, well-designed overhangs allow a significant reduction in cooling load [13].

Mandalaki et al. [14] analyzed the energy needed for heating, cooling and lighting for office rooms with shading devices, located in the cities of Athens and Chania (Greece). The aim of the analyses was to determine the optimal size of shading devices with integrated south-facing PV panels, which generate electricity for lighting. The results showed that shading devices decrease the building energy consumption. Stamatakis et al. applied multi-criteria analyses of monocrystalline PV panels mounted on south-facing shading devices on office buildings in the Mediterranean region [15]. A novel design of energy-efficient shading devices with amorphous panels was investigated by parametric modeling [16]. Objective functions were the minimal value of total energy consumption and useful daylight illuminance. The achieved savings in total building energy consumption was 14%, with a daylight level above 50%.

Bellia et al. have provided an overview of lighting analysis, energy analysis, HVAC system energy requirements and comprehensive analyses of thermal, visual and energetic aspects for buildings with fixed, movable and others shading systems [17]. Also, a review of simulation modeling for different type of shading devices which are implemented in modern buildings today was given by Kirmtat et al. [18]. The effects of horizontal and vertical louver shading devices, applied to different building façades at different locations, on building energy consumption are analyzed in [19] using the TRNSYS software. Obtained results showed significant energy savings in comparison to a building without shading devices. Valladares-Rendón et al. investigated solar protection and building energy saving in buildings with balanced daylighting and visibility and optimal orientation for façade shading systems [20]. The investigated buildings were in the subtropical zone, at 59 different locations. The results showed that passive strategies can reduce energy consumption by 4.64% to 76.57%. Numerical simulations showed that 58.62% of the locations should apply east oriented, 24.13% northeast oriented, 12.06% west oriented and 5.17%southeast oriented optimal designs. Al-Masarni and Al-Obaidi theoretically and experimentally analyzed current applications and trends of dynamic shading systems [21]. Their outcomes give a classification of shading models and analysis of their performance, with some recommendations for improving dynamic shading systems' performance, which can be very useful for architects. Tabadkani et al. reviewed studies with automatic shading control methods for balancing comfort and energy savings in buildings [22]. They concluded that existing studies investigated only automatic shading controls such as roller shades or venetian blinds, which can contribute to the reduction of energy consumption.

Serbia is among the countries that has the lowest level of energy efficiency in Europe and is therefore located at the bottom of the list of energy-efficient countries. This information is fully illustrated by the fact that in Serbia there are an estimated 300,000–400,000 energy-inefficient residential buildings (single family houses) which have no thermal insulation and with an annual final energy consumption of 220 kWh/m<sup>2</sup> [23], while the European annualenergy consumption ranges from 55 kWh/m<sup>2</sup> in Malta and 70 kWh/m<sup>2</sup> in Portugal, to 300 kWh/m2 in Romania [24].

Energy consumption in buildings at the global level is 20–40% of total energy consumption, while in Serbia it is at the 35%level [25]. This energy consumption is related to the exploitation conditions of buildings. In the structures of total energy consumption of Serbian building, about 60% of the energy consumption is related to the space heating [23], or approximately 65 million MWh per annum [26]. About 76% of this consumption pertains to single family houses and 24% to multifamily houses [26].

Residential buildings represent the biggest part of national building stock of Serbia, and more than 90% of them are single family houses. Most of these residential buildings (58.78%) are older buildings that were built in the 1960s, 1970s and 1980s, and are characterized by excessive energy consumption, due to the absence or poor thermal insulation, whether due to inefficient doors and windows, etc. In accordance with the national residential buildings typology in Serbia, these buildings belong to the groups D1, E1 and F1 [26], and they are usually two-storey, free standing, single family buildings. In the last 20 years, some basic energy-saving measures have been implemented in these buildings in order to improve their energy efficiency—application of thermal insulation on the external walls, roof and floors, replacement the old inefficient windows, doors, etc. In that way, a certain energy savings is achieved, but, it is also necessary to implement some other measures and find other ways for minimizing building energy consumption, especially for heating and cooling.

In the literature, there is almost no investigation of how the installation of overhangs influences the common consumption of energy for heating, cooling and lighting in Serbian buildings. This paper reports numerical investigations about how shading by horizontal roof and balcony overhangs influences the primary and final energy consumption for heating, cooling and lighting of residential buildings in Serbia through the year. Analyzed buildings are modeled in accordance with the national residential buildings typology in Serbia, and they represent typical buildings which were built in the period from 1960 until 1990, with thermal insulation on the external walls and energy-efficient windows. In this paper, optimal size of the horizontal roof overhangs, which are placed over east, west, north, and south wall, are obtained by simultaneous operation of the two programs EnergyPlus and GenOpt. The optimization is performed to minimize the primary energy consumption for heating, cooling and lighting. In these processes, the embodied energy of concrete horizontal roof overhangs was taken into account [27].

The primary energy saving and cooling energy consumption results obtained with numerical simulations and optimizations are within the frame of research results of the other authors who have conducted similar studies, but in some other regions of Europe. Serbia lies in the central part of the Balkan Peninsula, and has a moderate continental climate, characterized by cold winters, warm summers, and well-distributed rainfall, like in the other northern and central parts of the Balkans. The results of this study are not merely useful for the study of the methods for improving building energy efficiency aimed at optimizing overhangs and minimizing of Serbian building energy consumption, but above all, they could represent useful information for similar studies conducted in other parts of Europe that share the same or similar characteristics in terms of climate and topography.

#### **2. Materials and Methods**

#### *2.1. Description of Modeled Buildings*

In this research, the energy consumption is investigated for three buildings shown in Figure 1 as models in the EnergyPlus software [28]. These buildings are detached with two-floors. They have almost the same characteristics.

They only differ in their overhang characteristics such as type and dimensions. They were: (1) a basic building, (2) a building with optimized roof overhangs (ORO building), and (3) a building with optimized balcony overhangs (OBO building). Two types of overhangs were studied—roof and balcony. The roof overhangs were parts of roof that acted as overhangs for the second floor apartments. The balcony overhangs acted as overhangs for the first floor apartments and as the balconies for the second floor apartments. The basic building had roof overhangs with depths of 0.2 m. The ORO building had optimized roof overhangs. The OBO building had optimized balcony overhangs. The balcony overhangs were balconies of the second floor apartments that acted as overhangs for the first floor apartments. This building had also roof overhangs with the same depths as in the ORO building. All overhangs are thermally insulated with polystyrene (0.05 m) to avoid or minimize the appearance of thermal bridges. The cross-section of the building in Figure 1d shows the distribution of rooms on the first and second floor. Each floor has four rooms of identical size of 23 m2: kitchen, living room, bedroom 1 and bedroom 2. Each of them was air-conditioned and illuminated by an average brightness of 500 lux. Additionally, there were a toilet and corridor.

**Figure 1.** The house geometry: (**a**) basic building, (**b**) ORO building, (**c**) OBO building, and (**d**) the cross section of the first story of these buildings.

To study the impact of shadowing more in details, the influence of tenant activities in buildings is excluded, although in practice this is not situation. The overhang geometry is shown in Figure 2 for the ORO and OBO building.

**Figure 2.** Overhang geometry: (**a**) basic building, (**b**) ORO building and (**c**) OBO building.

Each investigated building had a total floor area of 234 m2, of which F = 186 m<sup>2</sup> were cooled and heated. The constructions used in the envelope of each house are shown in Table 1. These building materials and constructions are usual in Serbian buildings and correspond to typical Serbian construction materials. The windows were double glazed with the air gap of 15 mm, and the U-value of 2.72 W/(m2K). Inward opening side-hung windows are implemented in modeled buildings. The ratio of the areas of glass surface to that of the external wall surface was 13.96%. Then, the total area of the exterior walls was 224 m2 (with the roof of 358 m2) and that of the windows was 32 m2.


**Table 1.** Materials used in the envelope of the buildings.

The installed windows and doors on the building envelope provide the infiltration of 0.5 ach. The infiltration parameter has been adopted for load calculations to ensure minimum outdoor fresh air for building zones without any forced ventilation. It was assumed that these rooms would have almost the same occupancy, lighting, and small power schedule (see Figure 3).

The heating and cooling are assumed to operate according to the schedules, during the entire year, to meet the temperature heating and cooling setpoints given in Table 2.



**Figure 3.** Schedules of the people presence, the use of lighting devices and the use of electric equipment (**a**) in a living room and (**b**) in a bathroom.

#### *2.2. Location and Climate*

The investigated residential buildings were located in the city of Belgrade (Republic of Serbia). Its average height above sea level is about 117 m, its latitude is 44◦82 N and longitude 20◦28 E. The time zone for Belgrade is GMT+1 h. Belgrade has a moderate continental climate with four defined seasons (winter, spring, summer, autumn). In the city of Belgrade summers are very warm and humid, while the winters are cool and snowy. The EnergyPlus uses weather data from its own database file, which contains a large variety of parameters: dry bulb temperatures (minimum and maximum), relative humidity, air pressure etc. Figure 4 represents monthly averages weather data for Belgrade—maximum and minimum air temperature and relative air humidity [28].

The EnergyPlus software also calculates solar radiation for every day in the year. Daily average solar radiation for Serbia is different in different parts of country: it is about 1.1 kWh/m<sup>2</sup> at the north and 1.7 kWh/m<sup>2</sup> at the south in January; in July it is about 5.9 kWh/m<sup>2</sup> at the north and 6.6 kWh/m<sup>2</sup> at the south of Serbia. Annually average solar radiation in Serbia is from 1200 kWh/m<sup>2</sup> for north-west to 1800 kWh/m<sup>2</sup> at the south of Serbia [29]. Solar radiation is dependent on the time of day and the sun's angle toward Earth. This angle varies by latitude and longitude, and season. Also, atmospheric conditions can affect radiation levels—clouds, air pollution and the hole in the ozone layer. These factors cause typical radiation levels to differ. Figure 5 presents average monthly values of solar radiation (direct, diffuse and global) for Belgrade, obtained from EnergyPlus' own weather file [29]. In accordance with the EnergyPlus software, direct solar radiation is measured as beam normal solar

irradiance, while global and diffuse solar radiation are measured at a horizontal plane. That's the reason why global solar radiation is not equal to the sum of direct and diffuse solar radiation.

**Figure 4.** Monthly weather data for Belgrade, from EnergyPlus weather file.

**Figure 5.** Solar radiation for Belgrade, from EnergyPlus weather file.

#### *2.3. Software–Simulation and Optimization*

In this research, two software packages were used: EnergyPlus [28] and GenOpt [30]. With these packages, energy research was performed for three buildings: the basic building, the ORO building and the OBO building, shown in Figure 1. For the basic building, energy simulations were done by using EnergyPlus. By using the EnergyPlus and Genopt software, energy optimizations were performed for the ORO building and the OBO building.

The basic house shown in Figure 1a and the ORO house shown in Figure 1b were modeled by using idf files of EnergyPlus. Then, an idf model template which was made included four variables: the depth of south roof overhang (sr), the depth of north roof overhang (nr), the depth of west roof

overhang (wr), and the depth of roof east overhang (er). These variables are simultaneously varied between their minimum (0.2 m) and maximum values (3 m). For variation, the GPS HookeJeeves algorithm was used. The objective function (the minimum of Etot) was programmed by Genopt code.

The OBO house shown in Figure 1c was modeled also by using an idf file of EnergyPlus, which included also four variables: the depth of south balcony overhang (sb), the depth of north balcony overhang (nb), the depth of west balcony overhang (wb), and the depth of east balcony overhang (eb). The objective function (the minimum of Etot) was also programmed by using Genopt with HookeJeeves algorithm. At this idf model template, the values of (sr), (nr), (wr), and (er) were put constant with the optimal values obtained for the ORO house. The simulation resultsare obtainedin the output file: theoptimumhorizontal roof overhangssize, power consumptionof heating, air conditioning, lighting and total primaryenergy consumption.

To simulate the energy performance of a building, the EnergyPlus software was used, in which the architecture and all system parameters that correspond to its physical condition are set. To ensure adequate thermal comfort in winter, electric heaters are used. This is not a typical heating system used in Serbian buildings. The most significant advantageous of using electric heaters are that they have the ability to fine-tune the temperature in the room and maintain a minimum temperature (as protection against freezing). In the space that is used periodically, it can be used for heating one room or the whole house.

Heating thermostats are set to the appropriate temperature during winter. In summer to maintain proper thermal comfort in rooms air conditioners are used with the appropriate thermostats. The room air conditioners are operated by electricity. To maintain an appropriate light level, the combined impact of daylight and electric lighting is investigated by entering the appropriate parameters in a given time interval (using the DayLightingControls function implemented in EnergyPlus) [28].

Finding the optimal size of the horizontal roof overhangs was done with the Hooke Jeeves optimization method [31] with the help of GenOpt [30]. The objective function minimizes the consumption of primary energy for heating, cooling and lighting of the building and energy spent to build a horizontal roof overhangs. The program GenOpt operates with fixed parameters, and with variable parameters in which the optimization is performed. Its Ini file defines the objective function and all necessary parameters and variables that are required for optimization. The command file is given as the pattern of the traits that are necessary for the execution of the optimization algorithm.

#### *2.4. Energy Analyses of Modeled Buildings*

In these investigations, energy analyses were performed with the aim to minimize primary energy consumption of modeled buildings with optimized size of overhangs. Also there were calculated some environmental performances of the buildings, like energy payback time and greenhouse substitution time.

#### 2.4.1. Primary Operating Energy Consumption

The annual primary operating energy consumption of a house was calculated by equation:

$$\mathbf{E\_{P}} = (\mathbf{E\_{a^c}} + \mathbf{E\_{e^h}} + \mathbf{E\_{e^c}} + \mathbf{E\_{e^l}}) \mathbf{K\_{cc}/F} \tag{1}$$

Here, Eac is annual electricity consumption by the air conditioners, Eeh is annual electricity consumption by the electric heaters, Eeq is annual electricity consumption for the electric equipment, Eel is annual electricity consumption for lighting, Kec is primary energy factor and F is total conditioned floor area. The Kec is defined as the ratio of the total primary energy consumption by energy sources and the total supplied electricity, and for Serbia Kec = 3.04 [32].

#### 2.4.2. Annualized Embodied Energy

The annualized embodied energy (AEE) for horizontal roof overhangsdepends on the overhang size (width, depth, and thickness) and material.

For the geometry of roof and its overhangs (see Figure 6), AEEr (annualized embodied energy for ORO building) is calculated as

$$\text{AEE}\_{\text{r}}\left(f\_{\text{n}}\,\text{F}\right) = \rho\_{\text{c}}\,\delta\_{\text{c}}\,\text{s}\_{\text{tc}}\,\text{A}\_{\text{c}} + \rho\_{\text{ct}}\,\delta\_{\text{ct}}\,\text{s}\_{\text{ct}}\,\left(\text{A}\_{\text{ct}} - \text{A}\_{\text{0}}\right),\tag{2}$$

**Figure 6.** Sketch of roof construction: (**a**) 3d view, (**b**) view from the top of the ORO building and (**c**) view from the top of the OBO building.

Here, ρ stands for the material density for roof overhangs (concrete, ρ<sup>c</sup> = 2150 kg/m<sup>3</sup> [1,2]; clay tile, ρct = 1900 kg/m<sup>3</sup> [1,2]), δ<sup>c</sup> = 0.18 m stands for the thickness of the roof overhangs, δct = 0.014 m stands for the thickness of the roof clay tile, se stands for the roof overhangs specific embodied energy (concrete, sec = 1.924 MJ/kg; clay tile, set = 6.5 MJ/kg) [33], and *f* <sup>n</sup> stands for the roof overhangs lifecycle(20 years, [1,34]). Variable Ac stands for the area of the roof overhangs made by using concrete.

From Figure 6, this surface is obtained when the rectangle area (CNW CNE CSE CSW CNW) is subtracted from square area (BNW BNE BSW BSE BNW). Variable A0 stands for the area of the tile roof surface without overhangs. This surface represents a sum of four roofs triangular surfaces of the same size A ("O CNW CNE O", "O CNE, CSE O", "O CSC, CSW O", "O CSW, CNW O"). Variable Act stands for the areas of the tile roof surface with overhangs (optimization). This surface represents a sum of four roofs triangular surfaces An "O BNW BNE O", Ae "O BNE, BSE O", As "O BSC, BSW O", Aw "O BSW, BNW O".

When (Act − A0) is multiplied by δct, the volume of tiles is obtained because of increase in the roof area with established overhangs. The area of the roof overhangs made by using concrete (see Figure 6b) is given as:

$$\mathbf{A\_{c}} = \mathbf{L} \left( \mathbf{e\_{r}} + \mathbf{w\_{r}} + \mathbf{n\_{r}} + \mathbf{s\_{l}} \right) + \mathbf{e\_{r}}\mathbf{n\_{r}} + \mathbf{e\_{r}}\mathbf{s\_{l}} + \mathbf{w\_{l}s\_{l}} + \mathbf{w\_{l}n\_{l}} \tag{3}$$

In Figure 6b, er stands for the depth of roof overhang at the east building side, wr stands for the depth of roof overhang at the west building side, nr stands for the depth of roof overhang at the north building side and sr stands for the depth of roof overhang at the south building side, L = 10.8 m stands for length of the buildings wall.

The difference between the areas of the tile roof surface with and without overhangs is given as:

$$\mathbf{(A\_{ct} - A\_0)} = \left[ (\mathbf{A\_{e,r} - A}) + (\mathbf{A\_{w,r} - A}) + (\mathbf{A\_{n,r} - A}) + (\mathbf{A\_{s,r} - A}) \right] \tag{4}$$

When the equations from (3) and (4) are substituted into (2), the following equation which describes the annualized embodied energy for ORO building is obtained:

$$\begin{aligned} \text{AFE}\_{\mathsf{f}}(\mathsf{f}\_{\mathsf{h}}\mathsf{F}) &= \langle \rho\_{\mathsf{f}}\delta\_{\mathsf{C}}\mathsf{s}\_{\mathsf{SC}} [\mathsf{L}\{\mathsf{e}\_{\mathsf{T}} + \mathsf{W}\_{\mathsf{T}} + \mathsf{n}\_{\mathsf{T}} + \mathsf{s}\_{\mathsf{T}}\} + \mathsf{e}\_{\mathsf{T}}\mathsf{n}\_{\mathsf{F}} + \mathsf{w}\_{\mathsf{T}}\mathsf{s}\_{\mathsf{T}} + \mathsf{w}\_{\mathsf{T}}\mathsf{s}\_{\mathsf{T}}] + \rho\_{\mathsf{c}\mathsf{T}}\delta\_{\mathsf{C}}\mathsf{s}\_{\mathsf{SC}} [(\mathsf{A}\_{\mathsf{e},\mathsf{r}} - \mathsf{A}) + \mathsf{e}\_{\mathsf{T}}\mathsf{s}\_{\mathsf{F}}]] \\ &\qquad (\mathsf{A}\_{\mathsf{w},\mathsf{r}} - \mathsf{A}) + (\mathsf{A}\_{\mathsf{n},\mathsf{r}} - \mathsf{A}) + (\mathsf{A}\_{\mathsf{s},\mathsf{r}} - \mathsf{A})] )\_{\mathsf{s}} \end{aligned} \tag{5}$$

The variable A (area) is calculated as:

$$\mathbf{A} = \mathbf{L} \mathbf{h} / 2, \qquad \mathbf{h} = [(\mathbf{L}/2)^2 + \mathbf{H}^2]^{1/2}, \qquad \mathbf{A} = \mathbf{L} \left[ (\mathbf{L}/2)^2 + \mathbf{H}^2 \right]^{1/2} / 2,\tag{6}$$

Here, hr = ((L/2)<sup>2</sup> + H2) <sup>1</sup>/<sup>2</sup> = 5.99 m = const., H = 2.6 m stands for the height of the roofand the values of xe,r, xw,rxn,rxs,r in Ai,r, Aw,r, An,r, and As,r are the following:

$$\begin{aligned} \mathbf{x\_{c,r}} &= [(\mathbf{L}/2 + \mathbf{e\_r})^2 + \mathbf{H}^2]^{1/2}, \mathbf{x\_{w,r}} = [(\mathbf{L}/2 + \mathbf{w\_r})^2 + \mathbf{H}^2]^{1/2} \\ \mathbf{x\_{n,r}} &= [(\mathbf{L}/2 + \mathbf{n\_r})^2 + \mathbf{H}^2]^{1/2}, \mathbf{x\_{s,r}} = [(\mathbf{L}/2 + \mathbf{s\_r})^2 + \mathbf{H}^2]^{1/2}, \end{aligned} \tag{7}$$

Finally, the areas Ae,r, Aw,r, A,r, and As,r are the following:

$$\begin{aligned} \mathbf{A\_{e,r}} &= \mathbf{x\_{e,r}} \left(\mathbf{L} + \mathbf{s\_{r}} + \mathbf{n\_{r}}\right)/2 = \left(\mathbf{L} + \mathbf{s\_{r}} + \mathbf{n\_{r}}\right) \left[\left(\mathbf{L}/2 + \mathbf{e\_{r}}\right)^{2} + \mathbf{H}^{2}\right]^{1/2}/2 \\ \mathbf{A\_{w,r}} &= \mathbf{x\_{w,r}} \left(\mathbf{L} + \mathbf{s\_{r}} + \mathbf{n\_{r}}\right)/2 = \left(\mathbf{L} + \mathbf{s\_{r}} + \mathbf{n\_{r}}\right) \left[\left(\mathbf{L}/2 + \mathbf{w\_{r}}\right)^{2} + \mathbf{H}^{2}\right]^{1/2}/2 \\ \mathbf{A\_{n,r}} &= \mathbf{x\_{n,r}} \left(\mathbf{L} + \mathbf{e\_{r}} + \mathbf{w\_{r}}\right)/2 = \left(\mathbf{L} + \mathbf{e\_{r}} + \mathbf{w\_{r}}\right) \left[\left(\mathbf{L}/2 + \mathbf{n\_{r}}\right)^{2} + \mathbf{H}^{2}\right]^{1/2}/2 \\ \mathbf{A\_{s,r}} &= \mathbf{x\_{s,r}} \left(\mathbf{L} + \mathbf{e\_{r}} + \mathbf{w\_{r}}\right)/2 = \left(\mathbf{L} + \mathbf{e\_{r}} + \mathbf{w\_{r}}\right) \left[\left(\mathbf{L}/2 + \mathbf{s\_{r}}\right)^{2} + \mathbf{H}^{2}\right]^{1/2}/2, \end{aligned} \tag{8}$$

When these values are substituted in (5) then:

AEEr (*f* nF) = {ρcδcssc[L (er + wr + nr + sr) + ernr + ersr + wrsr + wrnr] + ρctδctssct 1/2((L + sr+nr) [(L/2 + er) <sup>2</sup> + H2] <sup>1</sup>/<sup>2</sup> +(L + sr + nr) [(L/2 + wr) <sup>2</sup> + H2] <sup>1</sup>/<sup>2</sup> +(L + er + wr) [(L/2 + nr) <sup>2</sup>+ H2] <sup>1</sup>/<sup>2</sup> +(L + er + wr) [(L/2 + sr) <sup>2</sup> + H2] <sup>1</sup>/2−4L [(L/2)<sup>2</sup> <sup>+</sup> <sup>H</sup>2] <sup>1</sup>/2}, (9)

The annualized embodied energy (AEEb) for horizontal balcony overhangs (Figure 6c) depends on overhang size (width, depth, and thickness) and material. AEEb is calculated as:

$$\text{AEE}\_{\text{b}}\left(f\_{\text{b}}\,\text{F}\right) = \rho\_{\text{c}}\,\delta\_{\text{b}}\,\text{A}\_{\text{b}}\,\text{s}\_{\text{bc}\prime}\tag{10}$$

where δ<sup>b</sup> = 0.18 m stands for the thickness of the balcony overhangs, sec = 1.924 MJ/kg stands for the balcony overhangs embodied energy [8], fb stands for the roof overhangs lifecycle (20 years). L = 10.8 m stands for the length of the buildings wall, hb stands for the depth of the balcony overhangs.

The area of the roof overhangs made by using concrete (see Figure 6b) is given as:

$$A\_{\rm b} = \mathcal{L}\left(\mathbf{e\_{b}} + \mathbf{w\_{b}} + \mathbf{r\_{b}} + \mathbf{s\_{b}}\right) + \mathbf{e\_{b}}\mathbf{r\_{b}} + \mathbf{e\_{b}}\mathbf{s\_{b}} + \mathbf{w\_{b}}\mathbf{s\_{b}} + \mathbf{w\_{b}}\mathbf{r\_{b}}\tag{11}$$

In Figure 6c, eb stands for the depth of balcony overhang at the east building side, wb stands for the depth of balcony overhang at the west building side, nb stands for the depth of balcony overhang at the north building side and sb stands for the depth of balcony overhang at the south building side, L = 10.8 m stands for the length of the buildings wall.

#### 2.4.3. Partial Annualized Primary Energy Consumption

The partial annualized primary energy consumption is equal to the sum of the primary operating energy consumption Ep and annualized embodied energy (AEEr, AEEb):

$$\rm E\_{tot} = E\_p + AEE\_r + AEE\_{br} \tag{12}$$

This equation is the objective function for the optimization routine. For the ORO building, the optimization is performed in respect to the four depths of the roof overhangs e, w, n, and s (when AEEb = 0). For the OBO building, the optimization is performed in respect to the four depths of the balcony overhangs eb, wb, nb, and sb (when AEEr = 0).

#### 2.4.4. Primary Operating Energy Savings

When the optimized overhangs are installed, the achieved primary operating energy savings for heating, cooling and lighting in buildings (in percents) is given as:

$$\mathbf{e}\_{\text{psw}} = 100 \,\mathrm{(E\_{p,0} - E\_{p,0\text{pt}})/E\_{p,0\text{\textdegree}}} \tag{13}$$

Here, Ep,opt stands for primary operating energy consumption after installation of optimized overhangs, Ep,0 stands for primary operating energy consumption without roof overhangs.

#### 2.4.5. Energy Payback Time

Energy payback time (EPBT) is time, in years, required to primary energy savings disannul the primary energy spent to overhangs building, and it is given in next equation [35]:

$$\text{EPBT} = \text{(AEE } (f\_n)) / (\text{E}\_{\text{p},0} - \text{E}\_{\text{p,opt}}), \tag{14}$$

The energy recovery (ER) is defined as number of time cycles due to primary energy saving (generated during whole lifecycle) is more than the primary energy which is needed for overhangs building. The energy recovery (ER) is given by:

$$\text{ER} = (\text{E}\_{\text{P}}, -\text{E}\_{\text{P}, \text{opt}}) / \text{AEE}\_{\text{'}} \tag{15}$$

#### 2.4.6. Greenhouse Substitution Time for Horizontal Overhangs

Greenhouse substitution time for horizontal overhangs (GHGST) is defined as the time period (in years) required for substituting the entire amount of CO2 emitted during the construction of horizontal overhangs due to the effect of emission reductions from the operation of the same horizontal overhangs. The amount of CO2 emitted in a process of production, transportation, building and installation of a horizontal roof overhangs (in t CO2) [1,2] is:

$$\text{G}\_{\text{CO2}} = \text{AEE}\_{\text{r}}(f\_{\text{n}}) \, \text{GHG}\_{\text{C}} \, \tag{16}$$

where GHGc stands for CO2 emissions intensity of the production of concrete in tCO2/t concrete. The annual decrease of the emission of CO2 due to the application of horizontal roof overhangs is:

$$\mathbf{S}\_{\rm CO2} = (\mathbf{E}\_{\rm p,0} - \mathbf{E}\_{\rm p,opt}) \,\mathrm{k}\_{\rm CO2,cc} \,\prime \tag{17}$$

where kCO2,ec stands for the equivalent to CO2 emissions for an energy mix for electricity production. Then, CO2 substitution time is given as:

$$\text{GIGGST} = \rho \,\delta \, l \, (\text{h}\_{\text{E}} + \text{h}\_{\text{S}} + \text{h}\_{\text{W}} + \text{h}\_{\text{N}}) \, \text{GIGG}\_{\text{G}} \langle \left[ (\text{E}\_{\text{p},0} - \text{E}\_{\text{p,opt}}) \, \text{k}\_{\text{CO2,sc}} \right] \, \text{} \tag{18}$$

#### **3. Results and Discussion**

Obtained optimal depths of the roof overhangs are listed in Table 3. A0 represents the results for the basic building, A1 represent the results for the ORO building (with the optimized roof overhangs) and A2 represents the results for the OBO building (with optimized depths of balconies used as overhangs, the depths of roof overhangs are the same as that for the ORO building).


**Table 3.** Results with implemented cooling, heating and lighting control.

Basic building A0 has no overhangs, so the values of 0.2 m represent only roof protrusions on the second floor. For ORO building, with optimization routine it was obtained the maximum value of east roof overhang (2.1 m) and west roof overhang (1.9 m), while the south roof overhang was 0.95 m. For the OBO building (with optimal values for roof overhangs), a maximum value of the east balcony overhang of 2.6 m and a west balcony overhang of 2.4 m were obtained, while the south balcony overhang was 0.7 m and north balcony overhang was 0.4 m. These values can be explained by the small angle of incidence of the Sun in the morning and in the afternoon during the summer period, so solar gains during that period can be significant. With implementation of overhangs overheating can be avoided through the summer months, with a reduction of solar gains.

Values of solar radiation through the windows and its reduction, in the basic and OBO buildings (monthly) are presented in Figure 7. It is not difficult to conclude that solar gains through the windows at the OBO building are significantly lower than the solar gains through the windows of the basic building. Optimized horizontal overhangs provide great protection from the solar radiation (especially during the March–October period), preventing overheating and thus reducing cooling energy consumption, i.e., the total building energy consumption.

**Figure 7.** Solar radiation through windows and its reduction in OBO house.

The total value of the annual solar radiation through the windows in the basic building is 13,404.7 kWh (57.29 kWh/m2), and in the OBO building with optimized overhangs it is 8451.11 kWh (36.12 kWh/m2). The annual difference in solar radiation of these two analyzed buildings is 4953.59 kWh (21.17 kWh/m2) and solar radiation is reduced by 36.95% (annually). The monthly reduction of solar radiation through the windows is lower in winter months, while it has a greater value during spring, summer and autumn months.

The specific final energy consumption for lighting, electric equipment, heating and air-conditioners for cooling (in kWh/m2) for all analyzed buildings is presented in Figure 8. With the implementation of overhangs, a small increase in annual lighting energy was observed for the ORO and OBO buildings compared to the basic building (0.23 kWh/m2 and 0.53 kWh/m2, respectively). Due to overhangs, a smaller amount of daylight enters these buildings, so electric lighting is used more during some time intervals (according to the software's DayLightingControls function). In the winter period there are small solar gains through the windows of the ORO and OBO buildings, so the specific final heating energy consumption increases; for ORO building the increase of heating energy consumption is 2.63 kWh/m2; for OBO building the increase of heating energy consumption is 4.98 kWh/m2. During the summer period, overhangs prevent building overheating, so the amount of cooling energy is significantly reduced. For the ORO building the decrease of annual cooling energy consumption is 6.18 kWh/m2; for the OBO building the decrease of annual cooling energy consumption is 11.9 kWh/m2. The specific final energy consumption for electric equipment is the same for all the buildings, 21.98 kWh/m2.

**Figure 8.** Specific final energy consumption for lighting, electric equipment, heating and cooling in the analyzed buildings.

The total final and primary energy consumption for the analyzed buildings are shown in Table 4. The highest annual energy consumption corresponds to the basic building, case A0, (90.02 kWh/m2 of final energy and 273.7 kWh/m<sup>2</sup> of primary energy), then the ORO building, case A1, (86.07 kWh/m2 of final energy and 263.5 kWh/m<sup>2</sup> of primary energy), while the lowest energy consumption corresponds to the OBO building, case A2, (83.63 kWh/m<sup>2</sup> of final energy and 254.2 kWh/m<sup>2</sup> of primary energy). Annual energy savings are 3.69% for the ORO building, and 7.12% for the OBO building, compared to the basic building without horizontal overhangs.

**Table 4.** Final and primary energy consumption in analyzed buildings.


The percentages of primary energy reduction and specific final energy reduction for the ORO and OBO buildings, compared to a basic house, are shown in Figure 9. Total primary energy consumption in the ORO building with the optimized roof overhangs (A1), was 3.69% lower, compared to the basic building without overhangs. Cooling energy consumption was 22.95% lower, while heating energy increased by 8.62%, in the form of energy consumption for lighting (2.1%). Significantly greater energy saving is achieved in OBO buildings with the optimized roof overhangs and optimized depths of balconies used as overhangs (A2)—total primary energy consumption in the OBO building is 7.12% lower, compared to the basic building. A greater cooling energy saving is also obtained (44.15%), while the primary heating energy was increased by 16.33%, mainly as energy consumption for lighting (4.98%). These values represent very significant energy savings.

**Figure 9.** Primary and specific final energy savings in analyzed buildings (in percents, compared to basic building without overhangs).

The duration required for any primary energy savings to compensate for the primary energy needed to build overhangs, in accordance with Equation (14) is EPBT = 6.44 years for the ORO house, and EPBT = 6.60 years for the OBO house. The number of time primary energy savings (Equation (14)) are generated during the lifecycle by using the optimal overhangs more than the primary energy needed to build overhangs is ER= 3.11 for the ORO, and 3.03 for the OBO house (Table 5).


The CO2 substitution time (GHGST) is the time required to substitute the entire amount of CO2 emitted during the construction of a technical system due to the effect of emission reductions resulting from operation of the system. The amount of CO2 emitted during the construction of concrete horizontal roof overhangs is GCO2 = 3.03 t CO2. Then, the CO2 emissions intensity of the concrete production is taken as GHGc = 0.13 t CO2/t concrete from [35,36]. The CO2 emission reductions resulting from the application of horizontal roof overhangs is annually SCO2 = 2.07 t CO2 where equivalent CO2 emissions for EPS kCO2,ec = 3.1, taken from [32]. Finally, the CO2 substitution time is GHGST = 1.47 years.

#### *Validation of the Results*

The average annual specific primary energy consumption for the buildings which belong to the groups D1, E1 and F1, according to the national residential buildings typology in Serbia [26] (with applied thermal insulation, replaced old inefficient windows and no overhangs), is 281 kWh/m2 [37]. This value is near the annual specific primary energy consumption for the basic building without overhangs, analyzed in these investigations (273.7 kWh/m2). The investigated basic building, like the ORO and OBO buildings, represents typical buildings from the D1, E1 and F1 groups, with thermal insulation on external walls and energy efficient windows. Having in mind these facts, it can be said that the data obtained by simulations and optimizations are valid.

#### **4. Sensitivity to the Accuracy of the Input Data**

In these investigations, optimization is performed for the horizontal roof overhangs. The overhangs are made by using concrete of the specific embedded energy (due its production process of materials, construction process, manufacture and installation) sec = 1.924 MJ/kg. As there are different conditions of concrete production and construction, specific embedded energy of concrete as input data may be different. An analysis, which presents how these changes influence to the output simulation results, for ORO and OBO building, is given below (Tables 6 and 7):

$$\mathbf{e}\_{\mathbb{P}^{\%}} = 100 \left( \mathbf{E}\_{\text{prim } +/-20\%} - \mathbf{E}\_{\text{primref}} \right) \mathbf{E}\_{\text{primref}} \tag{19}$$


**Table 6.** Sensitivity to the accuracy of the specific embodied energy for ORO house.


**Table 7.** Sensitivity to the accuracy of the specific embodied energy for OBO house.


During optimization for ORO building, if the specific embodied energy sec increases by 20%, the depth of east and west roof overhangs decreases, increasing the primary energy consumption by 0.08%. In this case cooling energy increases, while heating energy decreases. If the specific embodied energy sec decreases by 20%, depth of west and north roof overhang increases, with a decreasing of the primary energy consumption by 0.27%. The amount of cooling energy decreases in this case, while heating energy increases.

During optimization for the OBO building, if the specific embodied energy sec increases by 20%, the depth of east, west and north roof overhangs decreases, increasing the primary energy consumption by 0.16%. In these simulations, cooling energy has a small increase, while heating energy has

a small decrease. If the specific embodied energy sec decreases by 20%, depth of north roof overhang increases, decreasing the primary energy consumption by 0.03%. In this case, the amount of cooling energy has a small decrease, while amount of heating energy has a very small increase. The obtained results show a very small deviation (in the range of −0.03–0.16%) from the main results obtained by simulations and optimization process.

#### **5. Conclusions**

This paper represents a numerical investigation about how shading by horizontal roofs and balcony overhangs influences the annual primary and final energy consumption for heating, cooling and lighting in residential building in Serbia. Energy consumption is investigated for three buildings modeled in the EnergyPlus software in accordance with national residential buildings typology in Serbia. These buildings were detached with two-floors and they had almost the same characteristics. They only differed in their overhang characteristics such as type and dimensions. They were: (1) a basic building, (2) a building with optimized roof overhangs (ORO building), and (3) a building with optimized balcony overhangs (OBO building). The basic building had roof overhangs with depths of 0.2 m, the ORO building had optimized roof overhangs and the OBO building had optimized balcony overhangs (besides the roof overhangs with the depths as in the ORO building).

The optimal sizes of the horizontal overhangs, which are placed over the east, west, north, and south walls, are obtained by simultaneous operation of the two programs EnergyPlus and GenOpt. The aim of the optimization was to minimize the primary energy consumption for heating, cooling and lighting (embodied energy of concrete horizontal roof overhangs was taken into account).

Simulation results performed in this paper showed that horizontal overhangs on the analyzed buildings can reduce annual solar radiation through the windows by 36.95%. The reduction of the solar radiation through windows is less through the winter months than in summer months, so there is the possibility of optimizing the size of overhangs separately, for each side. This optimization process can achieve even greater energy savings and reduced primary energy consumption.

For the ORO building, the optimal dimensions of roof overhang depths are 2.1 m facing east, 0.95 m facing south, 1.9 m facing west and 0.2 m facing north. The reduction of heat gains due to solar radiation decreases the energy consumption for cooling by 22.95%, while the energy consumption for heating and lighting increases by 8.62 and 2.1%, respectively. Total primary energy consumption is reduced by 3.69%.

For the OBO building, the optimal dimensions of the depths of balconies used as overhangs, (the depths of roof overhangs are the same as that for the ORO house) are 2.6 m facing east, 0.7 m facing south, 2.4 m facing west and 0.4 m facing north. The reduction of heat gains due to solar radiation decreases the energy consumption for cooling by 44.15%, while the energy consumption for heating, and lighting are increased by 16.33% and 4.98%, respectively. The total primary energy consumption is reduced by 7.12%.

The time needed for the primary energy savings to compensate for the primary energy needed to build overhangs is 6.44 years for the ORO house and 6.60 years for the OBO house. The number of time cycles of primary energy savings generated during the lifecycle by using the optimal overhangs more than the primary energy needed to build overhangs is ER = 3.11 for the ORO and 3.03 for the OBO building. The CO2 substitution time GHGST is 1.47 years.

In the sensitivity analyses, we investigated how changes of the specific embedded energy of concrete, as input data, influence to the output simulation results for the ORO and OBO buildings. The obtained results show a very small deviation from the main results obtained by simulations and optimization process (in the range of −0.03–0.16%). Energy Plus is a software package which is intensively validated and has been tested using the IEA HVAC BESTEST E100–E200 series of tests [25,32]. Regardless of the high accuracy of the obtained results, all software tools, no matter how good and powerful they may be, can give a certain deviation in terms of the accuracy of the results.

Implementation of the roof overhangs on the existing building involves nailing rafter extensions onto existing rafters. When the rafter extensions are installed, then sheathing and roofing is started. If the roof is near the end of its life cycle, it is a good time to reroof the entire house. In that case, the overhangs will be built together with the roof. The case of implementation of balcony overhangs on an existing building can be very difficult job. First, concrete pillars must be added at the building construction, after that connection elements between the building and pillars have to be installed, and finally the balconies can be built. This process is more complex than implementation of roof overhangs.

Our future research may deal with analyses of different types of roof (flat and sloped), different roof construction types used in Serbia and different kinds of shading elements. Then, the optimization results may be compared if the roof is built using concrete, steal, laminated wood, or classic wood. In addition, the economics should be analyzed to show how these solutions are acceptable in practice and what any eventual energy penalty of this acceptance is.

**Author Contributions:** Conceptualization, D.N. and S.D.; methodology, D.N. and J.S.; software, S.D. and J.S.; validation, J.S., S.D. and J.R.; formal analysis, D.N., S.D. and J.R.; investigation, D.N., S.D., J.S.; resources, D.N., J.S. and J.R.; data curation S.D. and J.R.; writing—original draft preparation, D.N., S.D., J.S. and J.R.; writing—review and editing, D.N. and J.S.; visualization, D.N. and J.R.; supervision, D.N. and J.R.; project administration, J.S. and S.D.; funding acquisition, D.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** This paper presents results obtained within realization of two projects TR 33015 and III 42006, funded by Ministry of Education, Science and Technological Development of the Republic of Serbia.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


37. Novikova, A.; Csoknyai, T.; Jovanovic-Popovic, M.; Stankovic, B.; Zivkovic, B.; Ignjatovic, D.; Sretenovic, A.; Szalay, Z. *The Typology of the Residential Building Stock in Serbia and Modelling Its Low-Carbon Transformation*, 1st ed.; The Regional Environmental Center for Central and Eastern Europe (REC): Szentendre, Hungart, 2015; pp. 16–45.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

#### *Article*
