Effectiveness Analysis of Insulation and Roof Covering Material in Office Flat Roof

Wasteful consumption of electricity in high-rise building in Indonesia is the primary trigger for the increase in carbon emissions. Predominantly, the excessive use of electrical appliances and air conditioning systems is the principal factor contributing to the use of energy within these structures. To effectively address this issue, the importance lies in adopting suitable building envelope material. Therefore, this study aimed to analyze the effectiveness of different insulation types and roof covering in improving the energy-saving performance of office building. There were six proposed renovation for the flat roof of building, consisting of two approaches, namely (1) the incorporation of insulation and (2) the replacement of roof covering layers. Scenarios A1, A2, and A3 have incorporated 100 mm of polyurethane (PU) foam, polyisocyanurate (PI) foam, and fiberglass batt into the design. Meanwhile, Scenarios B1, B2, and B3 opted for ceramic material, Ethylene Propylene Diene Terpolymer (EPDM) membrane, and grass vegetation as roofing material for the existing roof. The results showed that the addition of insulation to roof through Insight 360 and the replacement of roof covering layers produced an energy reduction of 19.5% to 20.2% and 19.5% and 23.5%, respectively. The implementation of green roof in scenario B3 was selected as the most promising renovation option, achieving a remarkable 23.5% reduction in energy consumption.

Covering Layers; Energy Usage; Flat Roof; Insulation Layers

High-rise building in Indonesia is contributing to an increase in carbon gas emissions. The increase in carbon gas emissions is caused by the extensive operations of conventional power plants, required to meet the high electricity demands of building. Among the various types of high-rise building, office is object with the highest energy consumption. In 2020, Energy Use Intensity (EUI) of 213.27 kWh/m2/year was produced (B2TKE-BPPT, 2020). The major factor behind the electricity demand is the excessive use of air conditioning to maintain indoor thermal comfort, regardless of the weather (Hakim et al., 2021). Therefore, providing warm and cool rooms during rainy and dry seasons can reduce excessive reliance on air conditioning and energy use. 

The presence of a fit building envelope can prevent rapid heat transfer, effectively regulating indoor thermal comfort. Building envelope includes the outermost layer of architectural elements, namely walls, roof, basement, doors, windows, and other external element that withstands the surrounding environment (Bachrun, Ming, and Cinthya, 2019). Roof is one of the most essential envelope components for maintaining the quality of indoor thermal comfort and reducing the energy consumption of building. The study by (Feng et al., 2020) showed that the installation of insulation material on roof reduced energy consumption by up to 78%. Furthermore, (Ibrahim et al., 2018) proved that a roof design with ventilation effectively mitigated overheating. Building designers commonly use various roof insulation material, including polyurethane (PU) foam, polyisocyanurate (PI) foam, and fiberglass batts. Even with the comparable attributes, PU foam is distinguished by its low thermal conductivity and density (Gama, Ferreira, and Timmons, 2018), while PI shows reduced thermal conductivity under low-temperature conditions (Makaveckas, Bliudzius, and Burlingis, 2021). Meanwhile, fiberglass batts are good in insulation performance due to the higher R-value.

The type of covering material used for roof affects its ability to absorb or reflect heat. The study by (Algarni, Almutairi, and Alqahtani, 2022) showed that green roof with shallow-rooted grass lowered energy usage by 3.6% in six months (Shaharuddin, Khalil, and Saleh, 2019). Apart from the use of green roof, ceramic tiles and Ethylene Propylene Diene Terpolymer (EPDM) membranes can also serve as roofing material. (Sedaghat et al., 2023) showed that these tiles were capable of reflecting solar radiation by up to 50%, resulting in energy savings for building. EPDM membranes also show commendable reflective qualities, particularly when white in color. Therefore, the incorporation of suitable insulation material and roof covering can help anticipate an increase in electricity consumption due to rapid heat transfer.

Insulating and using covering material must be adapted to the geometric shape of an office building. In this context, flat roof represents the most prevalent geometric configuration found in office building, satisfying the functional requirements of HVAC system components. Prior study substantiated the effectiveness of using insulation and covering material on flat roof. The comparison between roof insulation usage and the addition of covering layers has not been addressed. Therefore, this study aims to analyze the effectiveness of different insulation types and roof covering in improving the energy-saving performance of office building. Building designers should be assisted in selecting the most effective flat roof renovation options to minimize energy usage. 

    In this section, a description of the study process is provided and the flow of each stage is shown in Figure 1. This study used a design model in which the dimensions and location of the selected design were determined based on the latest government data on building energy consumption (B2TKE-BPPT, 2020). From this data, the subject was configured with dimensions of 30 meters and 31.5 meters in length and width, under the office typology observed in government data. Furthermore, the model was assumed to be situated in Jakarta, which was a metropolitan city with numerous office building. This allowed the results to better represent office building in Indonesia. To define building components, information was obtained by referring to (Fitriani et al., 2022) and (Berawi et al., 2022) for material selection. Meanwhile, the material thickness was adjusted based on the availability in Jakarta building supply stores, as shown in Table 1.

Figure 1 Study methods workflow

Table 1 Summary of material types and thickness for initial design

Table 1 Summary of material and thickness for initial design [Cont.]

The initial building design was created using Autodesk Revit and default settings. The default settings are inherently consistent with ASHRAE standards, obviating the necessity for system adjustments, as explained in Table 2. Following the completion of the initial building design, the initial EUI analysis was conducted using Insight 360. The energy analysis used an equation created based on the DOE 2.2 engine program, where the initial EUI calculations were compared to the six renovation scenarios.

Table 2 Summary of space type distribution for each room

The two categories of renovation implemented in this study are 1) the incorporation of 100 mm-thick roof insulation and 2) the adoption of novel roofing material. The first type of renovation introduced PU foam, PI foam, and fiberglass batt materials onto roof, denoted as Scenarios A1, A2, and A3, respectively. The selection of these three materials was informed by the existence of prior studies discussing the characteristics, as shown in the Introduction section. Furthermore, the second renovation category comprised the use of glossy ceramic, EPDM membrane, and vegetation layers, identified as Scenarios B1, B2, and B3. The vegetation layer included a waterproof to prevent seepage into the concrete, soil serving as a growth medium, and a layer of grass and rocks as the topmost cover. These three roofing materials were selected for the capacity to create a cool roof environment by reflecting or absorbing sunlight, which cools the space beneath. Finally, the most preferable renovation scenario for diminishing EUI value is shown in the conclusion.

    This section shows the outcomes derived from the calculations conducted through Insight 360. Subsequently, a comprehensive discussion will be provided, delving into the comparative analysis among the various scenarios. Building design model is shown in Figure 2, where the colors do not influence the energy analysis results. In Figure 2, the basement and roof floors are black and gray, respectively.

Figure 2 Building design model using Autodesk Revit: basement (black) and roof (gray)

 Energy analysis conducted using Insight 360 showed that the initial EUI value for the designed building was 272 kWh/m²/year. According to the government data (B2TKE-BPPT, 2020), the value was settled in the third quartile range from 161.75 kWh/m²/year to 297.13 kWh/m²/year. Therefore, the proposed design was appropriate, allowing the continuation of the renovation scenario. Based on the calculations conducted using Insight 360, EUI for each renovation scenario is listed in Table 3. The percentage reductions were obtained by calculating the difference between EUI values obtained from the renovation scenario and the initial. The installation of insulation in Scenarios A and B resulted in an average percentage reduction of 19.98% and 20.96%. Therefore, the approach adopted in Scenario B, which included the installation of roof covering, showed a more significant impact on reducing EUI value of building.

Table 3 EUI calculation results for each renovation scenario

Previous studies showed that EUI calculation was influenced by the heat transfer capability of building envelope material. The capability can be measured using thermal coefficient commonly referred to as U-value (W/m². K). Based on the basic formula, U-value is affected by thermal conductivity () and material thickness. Table 4 presents  values of each material used in Scenarios A and B, with the corresponding U-value and roof thicknesses after the renovation. Roof thickness in Scenario A remains consistent using the same insulation thickness, specifically 100 mm. Meanwhile, Scenario B shows variations in roof thickness due to adjustments made to accommodate the dimensions of each type of roof covering.

Table 4 Summary of U-value for each material used

As presented in Table 4, Scenario A reported lower U-value compared to Scenario B. U-value increases with higher , even though Scenario A had the same roof thickness. The results were consistent with the theory that an increase in U-value was directly proportional to but inversely related to material thickness. Despite a lower value and thicker layer compared to green roof, the calculation showed that green roof U-value was lower than EPDM membrane. This contradicts existing theories, suggesting the presence of other factors influencing U-value of material.

The drainage and substrate layers act as heat absorbers while simultaneously providing a cooling effect on roof area of building. This phenomenon occurs because the substrate and drainage layers consist of organic material with high emissivity values, facilitating an extremely efficient heat transfer circulation. According to (Yildirim, Ozburak, and Ozden, 2023) and (Wang , Huang, and Li, 2021), green roof possesses the capability to provide a cooling effect on building roof through effective heat transfer circulation. A reduction in U-value becomes possible through this mechanism and green roof was intentionally designed with densely packed grass vegetation. This was conducted to ensure that incoming heat could be effectively absorbed by the high grass density, reducing heat convection into building. The observation is consistent with (Arabi, 2018), where the thermal performance of tropical plants depends on factors such as canopy density and coverage area percentage. Therefore, U-value can also be influenced by the effectiveness of heat transfer circulation depending on the type of roof covering used.

The examination of the relationship between U-value and EUI shows interesting trends. Scenario A, comprising A1, A2, and A3, shows that smaller U-value corresponds to lower EUI value, particularly evident with fiberglass batts. However, Scenarios B1 and B2 reports EUI value that can match or be lower than A1, despite having U-value five times higher. This contradicts the results of (Carvalho et al., 2021), where smaller U-value leads to lower EUI value. This phenomenon is attributed to the low emissivity values of glossy ceramic and EPDM membranes in B1 and B2, which effectively block excessive solar heat radiation, showing a reflective effect. This study also explores the influence of thermal conductivity and thickness on EUI, as presented in Figure 3. A linear relationship was observed between higher thermal conductivity and larger EUI value, as well as a negative correlation with thickness. However, the very low R² value is due to the lack of data samples, resulting in a non-normal distribution. The need for additional data samples becomes apparent to ensure and strengthen the statistical validity of the results. However, the trendline shows a relationship consistent with the results in this study.

Figure 3 The trendline of thermal conductivity and thickness with EUI value

The results complement previous studies on energy assessment in building roof renovation, as shown in Table 5 with five previous examples. The study by (Borras et al., 2022) reported the installation of three types of building roof renovation including 2 green roofs in 6 different climate zones in Spain. Therefore, a more comprehensive change in building envelope led to a higher reduction in EUI. According to (He et al., 2020), cool roof was more effective in reducing a building's EUI due to the excellent reflectivity compared to green roof. However, (Jovanovic , Zivkovic, and Stevanovic, 2018) stated the advantages of green roof in improving indoor and outdoor building quality by generating additional oxygen and absorbing air pollutants. (Cai et al., 2019) also provided evidence that green roof was effective in reducing EUI and room temperature.

Table 5 Five examples of previous study outcomes

From Table 5, this study complements information regarding building energy analysis concerning the addition of insulation and replacement of covering layers in building roof renovation in a single discourse. This study also developed six different scenarios including three types of insulation material and covering layers. The energy calculation results showed that the use of green roof in Scenario B3 obtained the highest percentage reduction in EUI. Meanwhile, the addition of insulation in Scenarios A1-A3 did not have a significant impact on reducing EUI despite the better heat transfer capability. Furthermore, the study conducted by (Francke and Gerylo, 2018) reported the limitations of insulation material quality in response to environmental temperature changes. Based on EUI calculation, the use of green roof is selected as the best solution for application in office building roof in Jakarta.

In conclusion, the optimization of flat roof was reported to prevent excessive building energy consumption. The implementation of several envelope renovation showed that the addition of insulation or alterations to roof covering effectively reduced EUI value. The additional insulation layer restricted the solar heat radiation rate due to extremely low conductivity, acting as a heat-absorbing material. Covering material had a cooling effect on roof area. Through Insight 360 analysis, six scenarios were investigated, resulting in energy consumption reductions ranging from 19.49% to 20.22% for scenarios A1–A3. Similarly, Scenarios B1, B2, and B3 reduced energy consumption by approximately 19.49%, 19.5%, and 23.53%, respectively. The final energy analysis and previous results showed that green roof system in Scenario B3 yielded the most significant energy savings. However, this study necessitated the incorporation of additional data regarding insulation thickness and the varieties of covering material. This inclusion aimed to enhance the data distribution efficacy, obtaining more accurate results.

    This study was funded by the Directorate of Research and Development, Universitas Indonesia, under Hibah PUTI Q2 (Grant No. NKB-821/UN2.RST/HKP.05.00/2023).

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