Carbon dioxide emissions of green roofing case study in ... · –case study in southern Brazil Giane de Campos Grigoletti, DrEng Marcos Fabrício Benedetti Pereira, MEng [Universidade

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.

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] [Universidade Luterana do Brasil]

[email protected]

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

30th INTERNATIONAL PLEA CONFERENCE16-18 December 2014, CEPT University, Ahmedabad

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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.

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

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Figure 1 (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 distanceprodsale + emissionCO2/km x distancesalejobplace (2)

grass

organic soil sand

crushed rock asphalt fabric

concrete slab

grass

organic soil water proofing pebble crushing water proofing

concrete slab


3

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 distancehomeworkjob x worked days x emissionCO2/km

where

WTCO2 is the total emission worker transport, kg CO2;

EE is the embodied energy, MJ/kg;

distancehomeworkjob 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

substrate for plant growth is considered equal to 0.945 kgCO2 / (m2.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

due to transport for the green roof located at the big city (green roof 1) with 28,41m2 of surface.

Table 3. Carbon dioxide emissions due to material production for green roof 1.

layer area or volume

density (kg/m³)

mass (kg)

relative embodied

energy (MJ/kg)

total embodied energy (MJ)

CO2 emissions (kgCO2)

asphalt fabric 4mm

28.41m2 1.125 127.84 51.00 6,519.84 342.62

crushed rock

0.28m3 1.400 397.74 0.15 59.66 4.21

sand 0.57m3 1.470 835.25 0.05 41.76 3.31

organic soil

0.075m3 1.600 120.00 0.00 0.00 -

soil 0.075m3 1.400 105.00 0.00 0.00 -

garden grass 1 (60%)

16.93m2 1.500 1,523.70 0.00 0.00 -

garden grass 2 (40%)

11.48m2 1.500 1,033.20 0.00 0.00 -

total - - 4,142.73 - 6,621.26 350.14

total per m2

233.06MJ/m2 12.32kgCO2/m2


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Table 4. Carbon dioxide emissions due to material transport for green roof 1.

layer

transport

modeprodsal

e

distanceprodsal

e (km)

emission

CO2prodsal

e (CO2 kg)

transport mode

salejobplace

distance

salejobplac

e (km)

emission CO2

salejobpla

ce (CO2 kg)

CO2 total layer

emission (KgCO2)

asphalt

fabric

4mm

road 1.185 1,060.58 car 21.90 2.32 1,062.90

crushe

d rock road 52 46.54

wheelbarro

w 0.24 0 46.54

sand road 52 46.54 wheelbarro

w 0.24 0 46.54

organic

soil in situ 0 0.00 in situ 0.00 0 0.00

soil road 77.2 69.09 car 10.90 1.16 70.25

garden

grass 1

(60%)

road 68 60.86 car 10.90 1.16 62.02

garden

grass 2

(40%)

in situ 0 0.00 car 13.40 1.42 1.42

- - 1,434.2 1,283.61 - - 57.58 1,289.66

total

per m2 45.39kgCO2/m

2

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

transported

workers (kg)

distancehomeworkjob

(km)

worked days transport

mode

embodied

energy

(MJ)

CO2

emission

(kg CO2)

owner 140 Kg - 3 - - -

worker 140 Kg 15 1 car 10.815 1.59

total 10.815 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.

layer area or volume

density (kg/m³)

mass (kg)

relative embodied

energy (MJ/kg)

total embodied

energy (MJ)

CO2 emission (kgCO2)

waterproofing 45.28 litres 1.3 (kg/l) 58.86 65.00 3,826.16 201.06

pebble crushing 2.3 m³ 1000 2,300.00 0.00 0.00 0,00

waterproofing

coating 56.6 m² 0.12 6.79 51.00 346.29 25.50

soil 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

- - - - - 4,172.45 226.56

total per m2 73.72MJ/m

2 4.00kgCO2/m

2


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Table 7. Carbon dioxide emissions due to material transport for green roof 2.

layer

transport mode

prodsale

distance

prodsale (km)

emission CO2

prodsale (CO2 kg)

transport mode

sale

jobplace

distance

sale

jobplace (km)

emission

CO2 sale

jobplace (CO2 kg)

CO2 total layer

emission (CO2 Kg)

waterproof. road 1265 1,132.18 car 6.90 0.73 1,132.91

pebble

crushing

in situ 0 0.00 - 0.00 0.00 0.00

waterproof.

coating

road 1,431 1,280.75 car 6.90 0.73 1,281.48

soil 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

total per m2

42.66

kgCO2/m2

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

transported

workers (kg)

distancehomeworkjob

(km)

worked days transport

mode

embodied

energy

(MJ)

CO2

emission

(kg CO2)

owner 280 Kg - 3 - - -

worker 280 Kg 11.4 1 car 8.2194 1.21

total 8.2194 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, asbestos-

cement, 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 auto-

construction process, reforcing the use local workforce and techniques.


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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.

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Carbon dioxide emissions of green roofing case study in ... · –case study in southern Brazil Giane de Campos Grigoletti, DrEng Marcos Fabrício Benedetti Pereira, MEng [Universidade