Carbon dioxide emissions of green roofing – case study ... - PLEA2014 [PDF]

Carbon dioxide emissions of green roofing – case study in southern Brazil

Giane de Campos Grigoletti, DrEng

Marcos Fabrício Benedetti Pereira, MEng

[Universidade Federal de Santa Maria] [email protected]

[Universidade Luterana do Brasil]

ABSTRACT

Nowadays there are several efforts in define carbon dioxide emissions of buildings components and materials. This data shall be in accordance with local building technology or methods of construction. Therefore studies of local alternatives are important. This work presents results of carbon dioxide emissions for two solutions of usual green roofing in southern Brazil. The method considers production of main inputs, road transport from point of sale to the site of construction, workmanship transport. The two green roofs are compared with ceramic and asbestos-cement tiles solution. The data were obtained by surveys, interviews with owners and scientific literature. The building materials, distances of transport were quantified. The results demonstrate that the carbon dioxide emissions are larger than the emissions of the conventional roofing and the main contribution is due to the road transport of components and materials from the point of sale to the site of construction. However, we must consider that the green roofing has a high potential for the carbon sequestration, promotes thermal resistance, humidify and filter the air, reduce the urban surface temperatures. INTRODUCTION

Civil construction is responsible for 40% of energy demand and 38% of air emissions that contribute for global warming. However there is 30% to 50% of potential for reduction of energy consumption and 35% for reduction of air emissions [1]. In Brazil the civil construction has substantial participation on greenhouse gases. Excluding the carbon dioxide (CO2) emitted by burnoffs, the building construction represents a quarter of significant air emissions, either by chemical reactions of industrial processes of materials or by the energy sources involved in these industrial processes [2]. Further, the materials transportation, mainly by roads with fossil fuel, contributes significantly for CO2 emissions [3]. Table 1 illustrates the CO2 emissions coefficient (Kg CO2 eq) for the mainly fuels used in Brazil [2]. Table 1. CO2 emissions due to some fuel sources fuel source emissões de CO2 (kg CO2/GJ) diesel oil 79,8 natural gas 50,6 petroleum coke 72,6 other sources derived of petroleum 0,0 electrical energy 18,1 fuel wood 81,6 Table 2 ilustrates the embodied energy in some construction materials expressed in percentage according to [2]. The use of energy in industrial processes also significantly contributes for CO2 Author A is a professor in the Department of Architecture and Urbanism, Santa Maria University, Santa Maria, Brazil. Author B is a professor in Brazilian Luterane University, Santa Maria, Brazil.

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emissions; therefore the consideration of production emissions is important in the life cycle of construction materials. Table 2. Percentage of embodied energy due to source for some materials construction material/source diesel oil natural gas coke other sources electrical energy wood sand 99 1 mortar 86 4 ceramic 4 2 85 cement 3 61 12 asbestos 84 14 waterproofing substances 10 30 34 26 polymer 10 30 34 26 The choice of the best environmentaly sound building technologies promotes the envinronmental impacts reduction, such as energy consumption and toxical gases emissions [4]. Technologies must be in accordance with local and regional traditional technology and disponibility of natural resources and industrialized local materials. Therefore the study of local solutions is important to achieve the building environmental performance. In this study green roofs are understood as vegetal layer intentionally incorporate on top of buildings. They have been pointed as alternatives more sustainable if compared with conventional roofs, such as tile and asbestos-cement roofs. There are many vantages associated to green roofing, such as natural top ground, life cycle extended, better thermal performance and consequently building occupants comfort more acceptable, reduction of urban heat-island effect, carbon sequestration, among others [5]; [6]. A negative factor associated to green roofs regards to the water comsuption. This aspect is not studied in this research, but some authors pointed that there are benefits to manage stormwater in order to restore the capacity of water retation lost by excessive paving of soil in cities [7]. It is possible to reduce about 60% of runoff for rain water captation. Further, the use os plant species that require little irrigation can be reduce the water comsuption, one of negative factors associated to green roofing [7]. In Brazil some studies about green roofing has been already enhanced. Through computational simulation [8] and prototypes submitted to measurement [9] the potential of green roofs for water catchment and retention was verified. Also was verified the viability of green roofs for low-cost housing [10]. A research concerning to occupants’ satisfaction indicated that the need for constant maintenance was one of the problems more mentioned [11]. However there are a few studies about the environmental impacts of green roofs mainly referring to CO2 emissions. This study aims to contribute to this issue through the quantification of carbon dioxide emissions of four roofs commonly built in Brazil, two green roofs built in two different regions, provincial medium town and industrial city, and two conventional ceramic and asbestos-cement roofing in order to compare their environmental performance due to carbon dioxide emissions. Additionaly the carbon sequestration potential was quantified in order to verify this important contribution of green roofs for sound environments. METHOD Selected green roofs

The green roofs are approximately 200km away each other (with different proximities of industries that produce the building materials involved), they are selected in accordance with the occupants permission to access the necessary data for the life cycle inventory, the construction system involves little labour and artisanal method. The ceramic tiles and asbestos-cement roof do not have the same thermal insulation, since the owners have chosen the green roofs for aesthetic and environmental sustainability, without refering their thermal performance. The Figure 1 ilustrates the green roofs studied.

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grass organic soil sand crushed asphalt fabric rock concrete slab Figure 1

grass

organic soil water proofing pebble water proofing crushing concrete slab

(a) Layers of green roof located in a big city, (b) in a mendium town in country.

Inventory

The data for the inventory were obtained from interviews with the owners of analysed houses, invoices, private diaries and reports elaborated by owners, regular direct observations, in situ measurements during building production process. Layers constituting the roofs, quantitative of materials, products’ points of sale, places of production of listed materials, distances of production, sale and jobsite area, materials modal transportation were collected. The demand of labour and the distances between jobsite, workers housing, means of transport also were measured from interviews and data registered by owners. The distances were obtained from virtual maps.

Quantification of carbon dioxide emissions

Contribution of different energy inputs was defined for constituting layers and materials. For each material, the total embodied energy CE was computed, the percentage of each significant source present in material production was also computed, obtained from [2], and is represented by P%. The individual contribution of each source was obtained by the product of total embodied energy and the individual carbon dioxide source contribution named coefCO2source obtained from [2]. The somatory of individual contributions is the total carbon dioxide emissions ECO2 represented by Equation 1.

ECO2 = [CE (MJ)  P%/100  coefCO2source] (1) where ECO2 is the carbon dioxide emission, kg CO2; CE is the contribution of different energy inputs, MJ; P% is the percentage of a kind energy in production process, %; coefCO2source is an index representative of CO2 emission of energy source, kg CO2/MJ. Since it was not possible to determine the characteristics of vehicle used as mean of transport and the kind of fuel, the indices established by [3], which studied carbon dioxide emissions for Brazilian road transport, were considered as reference. The mentioned author considers that it is possible to determine the CO2 emissions with an admissible error considering the distances and a medium factor according to type of transport. The carbon dioxide emission index for transportation from place of production to place of sale, with heavy road transport, was considered equal to 0.895 kg CO2 / km [3] since that is the conventional transport for construction materials in Brazil; for transportation from place of sale to jobplace (conventionaly transport of light load in Brazil) was considered equal to 0.106 kg CO2 / km [3] by the same previus reason. The carbon dioxide emission related to mean transport for each material was calculated using the Equation 2.

total emission material transport = emissionCO2/km x distanceprodsale + emissionCO2/km x distancesalejobplace (2)

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The Equation 3 was used for calculating the emissions due to transport of workers. The dioxide carbon emission index per day for mean transport was considered equal to 0.106 kg CO2 / km.day, embodied energy was considered equal to 0.0015 MJ / kg and the weight for worker is equal to 70kg.

WTCO2 = EE x total weight x distancehomeworkjob x worked days x emissionCO2/km where WTCO2 is the total emission worker transport, kg CO2; EE is the embodied energy, MJ/kg; distancehomeworkjob is the distance between the home and the workplace, km; total weight is the transported weight, kg; emissionCO2/km is the carbon dioxide emissions per kilometer due to worker transport, CO2/km. Carbon sequestration

Addionally the potential for carbon sequestration was calculated in order to verify one of main environmental contribution of green roofing. The larger carbon sequestration for grass with 20cm of 2 substrate for plant growth is considered equal to 0.945 kgCO2 / (m .year) [12]. The mentioned value was multiplied for the area of each green roof. Total calculated carbon dioxide emission for each green roof was divided for the index in order to obtain the number of years necessary to sequester. RESULTS

Table 3 presents the carbon dioxide emissions due to materials production and Table 4 emissions 2 due to transport for the green roof located at the big city (green roof 1) with 28,41m of surface. Table 3. Carbon dioxide emissions due to material production for green roof 1. relative area or density mass embodied total embodied CO2 emissions layer volume (kg/m³) (kg) energy energy (MJ) (kgCO2) (MJ/kg) asphalt fabric 28.41m2 1.125 127.84 51.00 6,519.84 342.62 4mm crushed 0.28m3 1.400 397.74 0.15 59.66 4.21 rock sand 0.57m3 1.470 835.25 0.05 41.76 3.31 organic 0.075m3 1.600 120.00 0.00 0.00 soil soil 0.075m3 1.400 105.00 0.00 0.00 garden grass 1 16.93m2 1.500 1,523.70 0.00 0.00 (60%) garden grass 2 11.48m2 1.500 1,033.20 0.00 0.00 (40%) total 4,142.73 6,621.26 350.14 total per 233.06MJ/m2 12.32kgCO /m2 m2

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Table 4. Carbon dioxide emissions due to material transport for green roof 1. emissio CO2 total emission transport distance n CO2 transport layer distanceprodsal mode CO2prodsal modeprodsal salejobplac salejobpla emission e (km) salejobplace e (km) ce (CO2 e (CO2 kg) e (KgCO2) kg)

layer

asphalt fabric 4mm crushe d rock

road

1.185

1,060.58

road

52

46.54

21.90

2.32

1,062.90

0.24

0

46.54

road

52

46.54

0.24

0

46.54

in situ

0

0.00

in situ

0.00

0

0.00

road

77.2

69.09

car

10.90

1.16

70.25

road

68

60.86

car

10.90

1.16

62.02

in situ

0

0.00

car

13.40

1.42

1.42

-

1,434.2

1,283.61

-

-

57.58

1,289.66

sand organic soil soil garden grass 1 (60%) garden grass 2 (40%) total per m2

car wheelbarro w wheelbarro w

45.39kgCO2/m2

The major emissions are due to asphalt fabric that is the component with industrial process more complex among the green roof layers; involves large energy inputs; with centralized production. Therefore replacement of that layer is a possibility in reducing de CO2 impact. Table 5 presents the carbon dioxide emissions due to worker transport for the green roof 1. Table 5. Carbon dioxide emissions due to workers transport for green roof 1. weight of embodied CO2 distancehomeworkjob worked days transport owner worker total

transported workers (kg) 140 Kg 140 Kg

mode

(km) 15

3 1

car

energy (MJ) 10.815 10.815

emission (kg CO2) 1.59 1.59

For the ceramic tile roof built in the big city, the main contributions are due to production of ceramic tiles (481.17 kgCO2) and due to transport of truss materials (peroba wood) (1,100.22 kgCO2). In this case, the use of wood which production is strongly centralized (with environmental license) contributes significantly to carbon dioxide emissions. For the asbestos-cement roof built in the same place, the main contributions are due to transport of truss materials (1,100,22 kgCO2), since there are local industries that produce fibercement tiles. Table 6 presents the carbon dioxide emissions due to materials production and Table 7 emissions due to transport for the green roof located at the medium town (green roof 2). Table 6. Carbon dioxide emissions due to material production for green roof 2. relative total CO2 area or density embodied embodied emission layer mass (kg) volume (kg/m³) energy energy (MJ) (kgCO2) (MJ/kg) waterproofing

45.28 litres

1.3 (kg/l)

58.86

65.00

3,826.16

201.06

pebble crushing waterproofing coating soil

2.3 m³

1000

2,300.00

0.00

0.00

0,00

56.6 m²

0.12

6.79

51.00

346.29

25.50

3.4 m³

1,400

4,760.00

0.00

0.00

0.00

garden grass

56.6 m²

1,500

5,114.44

0.00

0.00

0.00

-

-

-

-

-

total per m2

30th INTERNATIONAL PLEA CONFERENCE 16-18 December 2014, CEPT University, Ahmedabad

4,172.45

226.56

73.72MJ/m2

4.00kgCO2/m2

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Table 7. Carbon dioxide emissions due to material transport for green roof 2. transport emission transport distance emission CO2 total distance mode CO2 mode layer CO2 sale prodsale (km) sale emission prodsale prodsale sale jobplace jobplace (CO2 kg) (km) (CO2 kg) (CO2 Kg) jobplace

layer waterproof. pebble crushing waterproof. coating soil

road

1265

1,132.18

car

6.90

0.73

1,132.91

in situ

0

0.00

-

0.00

0.00

0.00

road

1,431

1,280.75

car

6.90

0.73

1,281.48

in situ

0

0.00

-

0.00

0.00

0.00

garden grass

road,

0

0.00

-

0.00

0.00

0.00

-

-

2,696

2,412.92

-

13.80

1.46

2,414.38 42.66 kgCO2/m2

total per m

2

Table 8 presents the carbon dioxide emissions due to worker transport for the green roof 2. Table 8. Carbon dioxide emissions due to workers transport for green roof 2. weight of embodied CO2 distancehomeworkjob worked days transport owner worker total

transported workers (kg) 280 Kg 280 Kg

mode

(km) 11.4

3 1

car

energy (MJ) 8.2194 8.2194

emission (kg CO2) 1.21 1.21

In the same way of the precedent green roof 1, the major emissions are due to more industrialised component that is the waterproofing layers. The use of two waterproofing layers is critical for the poor performance of this roof. For the ceramic tile roof built in the medium town, such as for the big city, the main contributions are due to production of ceramic tiles (958.12 kgCO2) and due to transport of truss materials (peroba wood) (1,178.00 kgCO2). In this case, the use of wood which production is strongly centralized (with environmental license) contributes significantly to carbon dioxide emissions. For the asbestos-cement roof built in the medium town, the main contributions are due to transport of truss materials (1,178.00 kgCO2). The incorporated cement in the asbestos tiles is responsible for 486.40 kgCO2 emissions. The Figure 3 illustrates the total emissions per square metres due to the six roofs, green, asbestoscement, ceramic. In relation to transport materials both green roofs present lower performance than ceramic and cement-asbestos conventional roofs. This result is due to presence of layers based on fossil source (asphalt fabric and water proofing layer) with centralized production. In relation to carbon dioxide emissions produced from manufacturing the green roof 1 is more unsustainable due to asphalt fabric, presenting best performance only compared with the ceramic tile roof. Production of ceramic tiles envolves large energy for burning and transport due to their weight since this type of roofing has large embodied energy and carbon dioxide emissions. The cement-asbestos tile results in the best performance for the case study in the big town because there are local industries for this material. The three roofs type 2 located in the town far of production regions present the lower contribution in CO2 emissions what is an unexpected result since is further away from production centers. This result demonstrates the importance of contextualized solutions. Green roof 2 is technically simpler; a despite of using a water proofing layer with large embodied energy and carbon dioxide emissions, it requires less amount of material to fullfil the same function comparatively with roof 1. The emissions associated to worker transport are insignificant compared to production and transport materials due to artisanal and autoconstruction process, reforcing the use local workforce and techniques.

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60,00 50,00 40,00 transport materials

30,00 20,00

production

10,00 transport workers

0,00

total CO2 emission (kgCO2)

Figure 3

Partial and total CO2 emissions due to different analised roofs per square metres.

Considering the three contributions analyzed, transport materials, production, and transport workers, there is little difference between the three roofing in the medium town, which is does not do in the case of roofing in the big city where the cement-asbestos is the best solution. It takes the green roof 1 about 61 years for carbon sequestration due to production and transport of materials and workers. For the green roof 2, it takes about 50 years. These results demonstrate that the main benefit of green roof is obtained in very long time-lag, which counters to principal benefit associated to green roofs. CONCLUSION

Through results the green roofs present large CO2 emissions due to use of layers based on polymers or fossil source materials which production involves large embodied energy and several emissions that contributes for greenhouse. It pointed to need to replace the waterproofing layer based on fossil source for another one more environmentally sound. For the case studies illustred material transport is responsible for the largest emissions for six simulated roofing. Results reinforce the importance of choosing local and regional technologies, materials, and workforce. The cement-asbestos roof has the best performance relative to carbon dioxide emissions; it flies in the face of common sense in considering the green roof necessarily an environmently good solution. Furthermore, one of benefits associated to green roofs, the carbon sequestration, is reached in a long time opposing to general idea of sustainability. Green roofing has been considered as a building system with low environmental impacts. The analyses of carbon dioxide emissions demonstrated that it has lower performance than the conventional solutions even if were regarded the potential for carbon sequestration. However the easiest solution adopted for the conventional roofing, without a thermal insulation, collaborate for the results achieved.

REFERENCES

[1] HOBALLAH, Arab. 2012. Building design and construction: forging Resource Efficiency and Sustainable Development. UNEP. [2] TAVARES, Sérgio Fernando. 2006. Metodologia de análise do ciclo de vida energético de edificações residenciais brasileiras. Doctoral Thesis. Santa Catarina Federal University (UFSC). 30th INTERNATIONAL PLEA CONFERENCE 16-18 December 2014, CEPT University, Ahmedabad

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[3] BARTHLOMEU, Daniela Bacchi. 2006. Metodologia de análise do ciclo de vida energético de edificações residenciais brasileiras. Doctoral Thesis. São Paulo University (USP). [4] BRIBIÁN, Ignacio Zabalza; CAPILLA, Antonio Vallero; USÓN, Alfonso Aranda. 2011. Life cycle assessment of building materials: Comparative analysis of energy and environmental impacts and valuation of the eco-efficiency improvement potential. Building and Environment, 46: 1133-1140. [5] HOPKINS, Graeme; GOODWIN, Christine. 2011. Living Architecture: Green Roofs and Walls. Collingwood. CSIRO. [6] SNODGRASS, Edmund C.; MCINTYRE, Linda. 2010. The Green Roof Manual: A Professional Guide to Design, Installation, and Maintenance. London. Timber. [7] TASSI, Rutnéia; et al. 2014. Green roof: a sustainable alternative for stormwater management. Ambiente Construído, Porto Alegre, 14, n. 1: 139-154. [8] SANTOS, Pedro T. da Silva; et al. 2013. Green roof: performance of the constructive system in the reduction of runoff. Ambiente Construído, Porto Alegre, 13, n. 1 : 161-174. [9] BALDESSAR, Silvia Maria Nogueira. 2012. Telhado verde e sua contribuição na redução da vazão da água pluvial escoada. Master Thesis. Parana Federal University (UFPR). [10] OLIVEIRA, Eric Watson Netto de. 2009. Telhados verdes para habitações de interesse social: retenção das águas pluviais e conforto térmico. Master Thesis. Rio de Janeiro Estadual University. [11] KIST, Rubens Sallaberry. 2011. Coberturas verdes sobre edificações: avaliação da satisfação de moradores de um condomínio horizontal na cidade de Porto Alegre. Monography. Rio Grande do Sul Federal Univesity (UFRGS). [12] SEGNINI, Aline; et. al. 2007. Sequestro de carbono em solos com gramíneas. Revista Circular Técnica. São Carlos. EMBRAPA. 41. set.

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