CHAPTER 9: LIFE CYCLE ASSESSMENT/COST-BENEFIT ANALYSIS
9.2 Theoretical
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CHAPTER 9: LIFE CYCLE ASSESSMENT/COST-BENEFIT
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Figure 9.1.1: Tanyard Branch Urban Watershed (after Carter & Keeler, 2008) The researchers had determined that the green roofs would differ based on structural and locality considerations. It is for this reason that an average cost of R1310.265/m2 (based upon costs obtained from manufacturers) per green roof system was utilised.
The sensitivity analysis of the study utilised a 4% discount rate applied over a 40-year period (the design life of a green roof system) and upon quantification of the discounted costs and associated benefits, the total cost of installation associated with an extensive green roof system within the entire watershed study area amounted to R226 472.02 (see Table 9.1.1). However, the total cost of utilising a traditional roof system instead, amounted to R177 588.20 (see Table 9.1.1). This shows an approximate 28% increase in costs when utilising a green roof system. However, the researchers assumed an equal stormwater distribution across the entire watershed study area and with that, determined that the associated social (public) benefits would amount to R27 088 779.05 with a social (public) net present value of R199 383 236.30. The benefits of utilising green roof systems in the entire watershed was determined per respective categories, presented in Table 9.1.2.
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Table 9.1.1: Green roof VS conventional roof NPV (after Carter & Keeler, 2008) Private Roof (R) Total Public roofs in
study (R) Conservative Average Conservative Average Green roof costs 1 191 120,15 894 911,57 226 472 017,53 170 152 862,80
Green roof benefits 79 483,64 157 081,98 27 088 779,05 41 889 338,54
Green roof NPV 1 111 636,35 737 829,59 199 383 238,48 128 263 524,26
Conventional roof NPV 935 161,84 935 161,84 177 805 700,33 177 805 700,33
Green/conventional roof cost ratio
1,19 0,79 1,12 0,72
Table 9.1.2: Green roof benefits from a social (public) perspective (after Carter &
Keeler, 2008)
Green roof benefit Unit benefit (R/m2)
Avoided BMP cost 74.75
Energy 3.05
Air Quality 0.91
Total social benefits 78.71
Conservatively, this proved to be approximately 12% greater than that of conventional roof systems. However, on average it was found that due to the benefits of green roof systems, these roof systems proved to be cheaper in the long term than that of conventional roof systems.
In addition to the study performed at a public scale, the researchers carried out an analysis to determine a cost-benefit analysis to building owners. Utilising the same methodology as the previous case, the results of the analysis showed that the nett present value of a green roof system would be much higher to a building owner that in comparison to a conventional roof system. The researchers utilised a 929 m2 roof area as the test model for the analysis and had found that the total cost of constructing a green roof amounted to R1191.94/m2 whilst, the cost of a conventional roof amounted to 935.16/m2 for the same roof area. However, the associated green roof benefits to the building owner Table 9.1.3 amounts to 18.87% more in comparison to the conventional roof system.
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Table 9.1.3: Green roof benefits from a private perspective (after Carter & Keeler, 2008)
Green roof benefit Unit benefit (R/m2)
Stormwater utility credit 0.33
Energy 3.05
Air Quality 0.91
Total private benefits 4.29
The study has shown that although green roofs have a higher construction cost attached to them initially, the benefits associated with green roof systems that conventional roofing do not offer, make the systems more cost efficient in the long term. In addition, the study shows that from a private aspect, the associated benefits are much higher.
9.2.2 Life cycle assessment of extensive green roofs in Lisbon
The experimental findings of the research carried out by (Alves, 2015) suggest that extensive green roof systems present no advantage over the conventional roof system utilised in the study at an individual scale. This is based on the data demonstrating that the green roof system consumes a higher energy load throughout its life cycle. However, an analysis of the utilisation of extensive green roof systems at an urban scale shows that these systems consume less energy over their lifespan, as a collective, therefore, proving to be advantageous over the conventional roof system. This was partially attributed to the indirect reduction in the urban heat island effect, consequently reducing the impacts on the environment. These findings have allowed the researchers to go on and state that green roof systems can be utilised as a potential avenue for mitigation interventions for issues experienced in the urban areas. The researchers utilised a German software package that allowed for the modelling of the life cycle assessment in accordance with the ISO 14040 standard.
The study found that there was an approximate 4.2 kg.CO2.eq./m2 positive impact at an individual scale. On an urban scale the equivalent carbon emission savings were estimated to be more than the emissions emitted by a new light motor vehicle that has travelled over 300 million kilometres. An additional factor that was found to play a large part in the impact of life cycle is the indirect temperature decrease effect through the heat island effect.
On a long-term scale, extensive green roof systems prove to be sustainable. However, with the progression of time, it was found that the constituent materials may need to be
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replaced with materials that are prove to be more sustainable and environmentally friendly.
9.2.3 Life-cycle cost-benefit analysis of green roofing systems: Installation on Atlanta public schools
Research conducted during a study carried out Whatley (2011), had found that studies conducted in Oregon in the united states of America, comparing the life cycle costs of a single conventional roof and that of a single green roof over a period of 60 years, resulted in the green roof system amounting to approximately 7% more than the conventional roof over the period. The analysis took into account factors such as extension of roof life, savings from energy and stormwater reduction.
In another study investigated by the researcher, it was found that when considering low installation costs of a green roof and associated high environmental benefits of a single green roof project conducted in the state of Michigan, the return on investment was determined to be 11 years.
When comparing green roofs in the context of sustainability, utilising metrics as opposed to measures of a monetary basis, the findings of these studies show that relative to the life cycle and embodied energy of various materials green roof systems have a significantly higher environmental benefit in comparison to conventional roof systems.