How Much CO2 is Captured by a Green Roof?

by Dr. Anna Zakrisson on Friday, January 15, 2021 updated Friday, June 11, 2021

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What's in a green roof CO2 budget?

Green roofs, or vegetated roofs capture CO2. But how much CO2 can they capture? What numbers do we land on when we do a carbon budget that considers the whole manufacturing and transport chain? This article aims at giving some estimates and present the currently available literature on the topic.

A green roof carbon budget – an easy matter?

It is not uncommon to hear and read statements such as "green roofs consists of vegetation; hence they are super-efficient at capturing CO2, thus making vegetated roofs excellent options to combat anthropogenic climate change".

While there is absolutely no doubt that green roofs capture carbon (in the form of carbon dioxide), we need to recognize that the capture level strongly varies between the different roof types and vegetation used. Add to this equation variations in materials, transportation, longevity, and climate, and you will quickly find significant variations between roofs when it comes to the total carbon budget.

Let's try to unpack this complexity.

Plants "breathe" carbon dioxide

Contrary to us, humans, plants photosynthesize, which means that plants can produce high-energy carbohydrate compounds using only solar radiation, water, and carbon dioxide. The byproduct of this reaction is oxygen. This oxygen is used in plant respiration, but the bulk is released back into the air through little openings in the leaves called stomata. Thus, it is not too far-fetched to claim that plants "breathe" in carbon dioxide and breathe out oxygen. This is precisely the opposite of what humans do.


As green roofs include plants, they will absorb carbon dioxide and release oxygen, and this is where some people stop. But there is much more to this story.

Learn more about the photosynthetic reactions in this video:

Carbon capture by different species

For a plant to efficiently store carbon long-term, it must produce biomass that is not broken down by microorganisms. One way to achieve this is to produce large, wooden stems; carbon goes into the system, is stored, and not released until the plant dies many years later.

This type of carbon storage might work on an intensive green roof, or if screen systems are used that enable woody-stemmed plants to climb the outside of a house. But, it is very unlikely that we will find things like trees on extensive roofs. The substrate is too shallow for such plants to thrive.

Instead, plants like Sedums or grasses are used that survive in such inhospitable environments. These plants have a vastly different growth pattern; they grow and die off, grow, and die off. Microorganisms break down dead plant material, and carbon is released into the atmosphere through the process of respiration.

It should also be known that microbial respiration increase with temperature, reaching a peak value at somewhere around 45°C [based on research done on agricultural soils]1. Exactly how this effect the urban carbon cycle is yet not extensively researched.

Nonetheless, data shows that there still appears to be an accumulation of carbon matter on researched extensive roofs2,3, and, spoiler alert: the positive effects the vegetated roof has on energy performance and urban cooling result in even extensive roofs being carbon sinks.

Green roof carbon payback time

We need not forget that a lot of carbon dioxide is released during the manufacturing process, installation, and maintenance. Also, the transportation of materials and people must be added to the budget. Thus, every type of roof will have a slightly different carbon budget.

For extensive roofs, Kurunoma et al. 2018 provided a carbon budget proposal using various standard extensive green roof species such as different Sedum species and three grass species, namely Cynodon dactylon Pers., Festuca arundinacea Schreb. and Zoysia matrella L.

They used the following equation to calculate the carbon dioxide payback time:

CO2 payback time = CO2 e-p / (CO2 r-s + CO2 r-e - CO2 e-m), where
CO2 e-p equals the amount of CO2 emitted during the green roof production.
CO2 r-s is the reduction in CO2 due to plant sequestration.
CO2 r-e is the reduction in CO2 due to energy savings attributed to the green roof.
CO2 e-m is CO2 maintenance emissions.

This study concludes that an extensive green roof has an approximate carbon payback time of 6.4-15.9 years depending on species used and if irrigation was required. This shows that extensive green roofs contribute to carbon dioxide reduction within their lifespan, estimated to 40-50 years3. It should be noted that the substrate and the aluminum parts were the most CO2-costly per m2 of all profile components2.

Green roofs, urban heat island effects, and air quality parameters

Green roofs can improve urban air quality in many ways. The evaporation- and transpiration processes counteract the urban heat island effect (UHI). Further, this temperature reduction also reduces the formation of hazardous compounds, such as ground ozone and many nitrogen-based free radicals4.

Several studies have shown that vegetated roof coverings can directly capture fine particle pollution and other air pollutants such as SO2 4, 5. The table below shows pollution capture by a 1000m2 green roof extrapolated from data published by Yang, J. et al. 20086.


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So, what has this to do with the green roof carbon budget?

Firstly, it should be noted that the urban heat island effect results in a significantly higher peak energy demand, as well as higher overall electricity demand during hot spells.
During hot days, the city's air conditioning units running high, and while this cools the inside of the buildings, the aircon units increase the heating of the outside further. The lack of evaporative cooling from urban areas devoid of greenery exacerbates the problem7; for every 1°C temperature increase, the energy demand from air conditioning increased by 1-9%8. Green infrastructure such as green roof coverings can counteract this heating effect and indirectly reduce the city's carbon footprint.

Despite this strong effect of green roofs on cooling, R (thermal resistance per unit area) and U-values (overall heat transfer coefficient) of vegetated roofs tend to be moderately estimated. The green roof's positive impact on building energy consumption is more complex and indirect than what can be described by a simple R or U-value, but not necessarily less effective9. R and U-values of a green roof fluctuate significantly depending on the profile's moisture content.

Green roofs help cool a building through four main processes9:

  1. Cooling by evapotranspiration
  2. Creating a thermal buffer against daily fluctuations [this also helps preserve the roof membrane compared with bare roof coverings, but more about that in another article10].
  3. Shading
  4. Advection [the transport of energy by bulk motion].

The effect of the above-mentioned parameters depends on many factors such as climate and building geometry; a high-rise with a small roof will be less affected by the cooling capacity of a green roof than a low warehouse with a massive roof area. Also, the extent of building insulation affects how much the green roof cools, as does the position of HVAC equipment. However, many buildings can benefit significantly from a vegetated roof due to energy savings and reduce CO2 emissions.

Carbon budgets are complicated due to a multitude of indirect effects.

Carbon savings by replacing carbon-heavy concrete stormwater structures with a detention green roof

Green roofs differ in thickness and species palette, and they can also have a range of different functions.

Read more in my article on green roof functionality here: A Guide to Green Roof Functionality

The functionality of a friction-based detention green roof (Purple-Roof compliant roof) is analogous to that of a bioretention cell. It allows for significant peak flow reduction of, e.g., 100-year design storms, irrespective of if the profile is soaked or dry. Hence, these new generation detention green roofs can remove, or significantly reduce, below-grade concrete stormwater solutions such as cisterns, or tanks. Keep in mind that the concrete industry is responsible for approximately 7% of global carbon emissions (95% of which is due to the manufacturing process)11.

Further, we should not forget that the roof membrane's longevity significantly increases when a green roof is installed – at least by a factor of two, and probably even longer12. This means that instead of changing the full roof every 15-25 years, you might not have to change it at all. Apart from being a massive financial benefit, this also saves carbon otherwise spend in the production of new material, the avoided waste volume that would go to an incinerator or landfill, and the transport of new products and waste product.

Summary and discussion

Based on the published data available, traditional extensive green roofs effectively reduce the carbon footprint of a building and have a carbon payback time of 6.4-15.9 years. A green roof can extend the life span of the roof membrane by 10-50 years (this range depends on the quality of the membrane and quality of installation) which leads to a positive effect on the carbon footprint when installing a green roof on a building.

Further, detention-based green roof concepts such as Blue-Green and Purple-Roof-compliant systems can potentially improve carbon payback time further as they can replace large below-grade concrete stormwater management structures. These types of roofs reduce stormwater peak outflow in the same manner as below-grade gray stormwater infrastructure and green our urban environments. Of course, the carbon payback-time varies from project to project.

Don't hesitate to contact us if you have any questions!
Info[at]purple-roof.com

Bibliography

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  2. Kuronuma T, Watanabe H, Ishihara T, et al. CO2 Payoffof extensive green roofs with different vegetation species. Sustain. 2018;10(7):1-12. doi:10.3390/su10072256
  3. Getter K, Rowe D, Robertson G, Cregg B, Andresen J. Carbon sequestration potential of extensive green roofs. Environ Sci Technol. 2009;43(19):7564-7570. http://www.ncbi.nlm.nih.gov/pubmed/19848177.
  4. Yang J, Yu Q, Gong P. Quantifying air pollution removal by green roofs in Chicago. Atmos Environ. 2008. doi:10.1016/j.atmosenv.2008.07.003
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  7. Agency U. EP. Heat Island Impacts | Heat Island Effect | U.S. EPA. 2013. https://www.epa.gov/heat-islands/heat-island-impacts#2%0Ahttp://www.epa.gov/heatislands/impacts/index.htm.
  8. Santamouris M. Recent progress on urban overheating and heat island research. Integrated assessment of the energy, environmental, vulnerability and health impact. Synergies with the global climate change. Energy Build. 2020;207:109482.
  9. Revisiting the Insulating Effects of Green Roofs _ 2019-06-12 _ Building Enclosure.
  10. Soprema Inc. Living Architecture Monitor - Spring 2020. 2020.
  11. Ali MB, Saidur R, Hossain MS. A review on emission analysis in cement industries. Renew Sustain Energy Rev. 2011;15(5):2252-2261.
  12. Porsche U, Kohler M. Life Cycle Costs of Green Roofs: A Comparison of Germany, USA, and Brazil. In: Rio 3 - World Climate & Energy Event. ; 2003:461-467. doi:10.1007/s00158-009-0416-y