The Open Atmospheric Science Journal




ISSN: 1874-2823 ― Volume 13, 2019

Green Roofs in the Tropics Conserve Energy



Greg Wiecko*
Western Pacific Tropical Research Center, University of Guam, UOG Station; Mangilao, GU 96923, USA

Abstract

Background:

Concrete buildings on Guam are exceptionally strong but also accumulate large amounts of heat. In the tropical environment of Guam, where 24 h average temperature ranges from 28 to 29°C year round, air conditioning is used every day and continuously. Concrete roofs are often painted light colors, which make them more reflective and accumulate less heat. They are also suitable for establishment of vegetation, which results in a large decrease in roof temperature and therefore decreases the need for cooling.

Objective:

The objective was to determine the magnitude of temperature reductions resulting from light color and from vegetation covering roof tops and to use this information to estimate energy savings.

Method:

Temperature was measured on the undersides of concrete model roofs in both sunny and rainy weather.

Results:

The temperatures on the undersides of light-colored concrete model roofs rose up to 3°C less in the course of the day than did those of dark-colored ones. The temperatures of "green" (vegetation-covered) model roofs rose up to 12°C less than did those of either of the bare concrete models.

Conclusion:

The differences were so large that use of green roofs on the tropical island of Guam, where most buildings are concrete and air-conditioning is needed year round, could cut a typical household's electric consumption in half.

Keywords: Green roof, energy conservation, roof vegetation.


Article Information


Identifiers and Pagination:

Year: 2016
Volume: 10
First Page: 1
Last Page: 5
Publisher Id: TOASCJ-10-1
DOI: 10.2174/1874282301610010001

Article History:

Received Date: 21/8/2015
Revision Received Date: 14/12/2015
Acceptance Date: 14/12/2015
Electronic publication date: 24/2/2016
Collection year: 2016

Article Metrics:

CrossRef Citations:
0

Total Statistics:

Full-Text HTML Views: 1656
Abstract HTML Views: 855
PDF Downloads: 210
ePub Downloads: 146
Total Views/Downloads: 2867

Unique Statistics:

Full-Text HTML Views: 853
Abstract HTML Views: 488
PDF Downloads: 169
ePub Downloads: 121
Total Views/Downloads: 1631
Geographical View

© Greg Wiecko; Licensee Bentham Open.

open-access license: This is an open access article licensed under the terms of the Creative Commons Attribution-Non-Commercial 4.0 International Public License (CC BY-NC 4.0) (https://creativecommons.org/licenses/by-nc/4.0/legalcode), which permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided the work is properly cited.


* Address correspondence to this author at the Western Pacific Tropical Research Center, University of Guam, WPTRC 303 University Drive Mangilao, GU 96923, USA, Tel: (671) 735-2132 E-mail: gwiecko@uguam.uog.edu; gwiecko44@gmail.com




INTRODUCTION

Roof tops are usually unattractive places where heating and cooling equipment, telecommunication towers, and satellite dishes are installed [1Peck SW. Award winning green roof designs: Green roofs for healthy cities. Atglen, Pennsylvania, USA: Schiffer Pub 2008.]. Several decades ago in Europe, and then in other countries, ecologists, anthropologists, sociologists, and environmentalists stressed the need for more harmony between lifeless city buildings and their occupants [2Brenneisen S. Green roofs. How nature returns to the city. Acta Hortic 2004; 643: 289-93.
[http://dx.doi.org/10.17660/ActaHortic.2004.643.37]
]. As a result, thousand of hectares of roof-top area in large cities around the world were converted into gardens, as places of recreation or simply esthetically pleasing components of city skylines [3Greater London Authority. Living roofs and walls Technical Report Greater London Authority, City Hall, The Queens Walk, London SE12AA, England ISBN 978 1 84781 132 5 2008.]. These "green roofs" also provide many functional benefits. The most important are energy conservation [4Liu K. Engineering performance of rooftop gardens through field evaluation In: Proc 18th International Convention of the Rooftop Consultants Institute; March 13-16, 2003; Tampa, FL. -8Wong NH, Chen Y, Ong CL, Sia A. Investigation of thermal benefits of rooftop garden in the tropical environment. Build Environ 2003; 38: 261-70.
[http://dx.doi.org/10.1016/S0360-1323(02)00066-5]
], cooling effects [9Peng LH, Jim CY. Green-roof effects on neighborhood microclimate and human thermal sensation. Energies 2013; 6: 598-618.
[http://dx.doi.org/10.3390/en6020598]
, 10Williams NS, Rayner JP, Raynor KJ. Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia. Urban For Urban Green 2010; 9: 245-51.
[http://dx.doi.org/10.1016/j.ufug.2010.01.005]
], reduction of city noise [11Dunnett N, Kingsbury N. Planting green roofs and living walls. Portland, Oregon, USA: Timber Press 2004., 12Peck SW, Kuhn ME. Design guidelines for green roofs. Canada Mortgage and Housing Corporation, Ottawa, Canada. Available at: http://www.cmhc-schl.gc.ca. [Accessed on: 15 Sep 2015]], mitigation of air pollution [13Getter KL, Rowe DB. The role of extensive green roofs in sustainable development. HortScience 2006; 41: 1276-85.-15Tan PY, Sia A. A pilot green roof research project in Singapore In: Proc 3rd North American Green Roof Conference: Greening rooftops for sustainable communities; May 4-6 2005; Washington DC. Toronto, Canada: The Cardinal Group 2005.], delayed storm-water runoff, and improvement of runoff quality [16Dwight RH, Baker DB, Semenza JC, Olson BH. Health effects associated with recreational coastal water use: urban versus rural California. Am J Public Health 2004; 94(4): 565-7.
[http://dx.doi.org/10.2105/AJPH.94.4.565] [PMID: 15054006]
-18Scholz-Barth K. Green roofs. Storm water management from the top down. Environ Des Construct 2001; 4: 63-70.].

Guam, a tropical island in the western Pacific populated by almost 180 thousand people [19US Census Bureau. Statistical Abstract of the United States. 131st ed. Washington, D.C. 2012.], serves as major year-round destination for 1.5 million Asian tourists annually [20Guam Visitor Bureau. Available at: http://www.guamvisitorsbureau.com [Accessed on: 15 Sep 2015]]. Because of frequent typhoons and occasional earthquakes since the 1950s, nearly all walls and roofs of residential and commercial buildings are constructed from solid concrete. Concrete buildings are exceptionally strong and safe but also accumulate large amount of heat. The heat is currently removed by air-conditioning units (AC), but the need for AC could be reduced if roofs were shielded from hot sun. Because concrete structures assure exceptional weight safety, roofs on Guam are suitable for establishment of vegetation. Abundant tropical rainfall and selection of appropriate plant species could eliminate or greatly reduce the need for irrigation, making roof vegetation relatively easy to grow and maintain.

The objective of this study was to determine the magnitude of temperature reductions resulting from vegetation covering roof tops and to use this information to estimate the impact of roof-top vegetation on the amount of energy needed to cool a typical residential house on Guam.

MATERIALS AND METHODS

This study was conducted on Guam (13°N latitude). Eight 3.1m by 0.9m concrete plates were placed on concrete blocks 0.4m above the ground. The plates were 0.13m thick, were reinforced with steel bars, and resembled those used for an ordinary residential roof (Fig. 2). Each plate was divided into two 1.5m by 0.9m plots. Steel wire mesh was affixed to the surface of each plate, and a 10cm layer of turf substrate (Scotts Lawn Supersoil®) mixed with sand was spread over four plates on eight of the plots. The other eight plots were left without soil substrate. Four soilless plots were similar in color to a well-maintained concrete surface, and the other four were darkened to resembled a poorly maintained (and unfortunately quite typical) concrete surface covered by black fungi and algae.

Zoysia tenuifolia turf was established on four soil-covered plots from plugs, and full coverage was achieved within a year ("turf roof"). A mixture of indigenous weeds (monocots and dicots) was permitted to establish itself on the remaining four soil-covered plots and established full coverage within several months ("weed roof"). Turf was not mowed or fertilized after establishment but was lightly top-dressed with sand after a year. Weeds thrived and were trimmed occasionally. Turf roofs and weed roofs were irrigated only occasionally and during sunny periods suffered frequent water stress. Before data collection began, turf and weeds were thriving but were not irrigated. Data were collected during four 3-day periods. Two of the periods, January 19-21 and April 1-3, were without precipitation (sunny). The other two, Mar. 20-22 and September 27-29, encompassed multiple heavy rain events and substantial water runoff (rainy).

During sunny and rainy periods, average minimum air temperature ranged from 24 to 26°C and maximum temperature from 30 to 32°C. Day length ranged from 11.5 h in January to approximately 12 h in the remaining three periods. Average humidity for the sunny periods was 75% and for the rainy periods was around 90%.

Temperature was measured on each plate’s bottom surface in five locations per plot with an infrared thermometer (Ryobi, Model E49IR01), and the resulting readings averaged. Temperature readings began around sunrise and were repeated every 3 h until sunset. Accuracy of infrared thermometer readings was evaluated by comparison to a direct method of 20 random readings taken with Traceable® probe. The differences were less than 0.5°C.

All data were analyzed with SAS 9.1.3 software (SAS Institute Inc., Cary, NC). Experimental design was an unbalanced split-plot design with repeated measures. The SAS-MIXED procedure was used to produce a table of Type III tests of fixed effects. Means were separated by means of Fisher’s LSD procedure at P < 0.05.

RESULTS

Temperature differences between bare concrete plates and these covered by vegetation were significant by 3 h after sunrise and reached a maximum within 9 h. On sunny days, plates covered by vegetation were up to 12°C cooler, and on rainy days about 9°C cooler, than bare concrete plates (Fig. 1). Temperature fluctuations in concrete under vegetation were small on both sunny and rainy days. On sunny days, light-colored concrete was about 3°C cooler than dark-colored concrete; on rainy days, temperature differences were smaller. Temperature of plates covered by turf was similar to that of roof covered by weeds. During sunny days, plates under vegetation cooled down more slowly than did bare concrete and remained at least 2°C warmer at the next sunrise. During rainy days, all plates reached the same temperature by the next sunrise.

DISCUSSION

In tropical locations, sunlight falls more nearly vertically than in other regions, so the roofs of buildings receive most of the solar energy. Because most residential houses on Guam are all concrete and without attics, the underside of the roof radiates heat directly into the living area. Powerful AC is therefore needed to offset heat generated by the roof inside the house. In the tropical environment of Guam, where 24 h average temperature ranges from 28 to 29°C year round, AC is used every day and continuously.

Fig. (1)

Temperature of model concrete roofs, in °C (6-d average.), showing that, in both sunny and rainy weather, the temperatures on the undersides of light-colored concrete model roofs rose less than did those of dark-colored ones and that temperatures of "green" (vegetation-covered) model roofs rose less than did either of the others.



Collected data indicated that maintaining a concrete model roof surface free of algae and light in color resulted in a meaningful decrease in roof temperature and therefore would decrease the need for AC. Installation of vegetation on the model roof increased this desired effect even more. Santamouris [21Santamouris M. Cooling the cities: a review of reflective and green roof mitigation technologies to fight heat island and improve comfort in urban environments. Sol Energy 2014; 103: 682-703.
[http://dx.doi.org/10.1016/j.solener.2012.07.003]
], in his comprehensive review, evaluated results from numerous studies involving green roofs and reflective roofs and concluded that green roofs are advantageous in conserving energy in cold climates, whereas reflective roofs are better in warm and sunny climates. Results from my experiment indicate just an opposite. The clear advantage of the green roof over the reflective roof on Guam results from the location's uniform year-round hot climate. Most of the studies cited by Santamouris were conducted under conditions where green roofs often served as insulation, retaining the structure's internal heat rather than preventing warming. On Guam, concrete roofs are the warmest part of the house structure, and heat is transmitted always toward the inside of the building. In addition, reflective roofs in extremely humid climates are impractical. The rapid growth of dark algae on the roof surface significantly reduces its reflectance within several months after painting, making them expensive to maintain. In future studies, I plan to conduct similar experiments on structures that more realistically simulate house construction on Guam, for example structures at typical roof height and with side walls in place. Liu [22Liu K. Energy efficiency and environmental benefits of rooftop gardens. Green roofs. Construction, Canada 2002; 44(17): 20-3.] reported that roof vegetation reduced energy consumption by 75% during summer months in Ottawa, Canada. Dunnett and Kingsbury [11Dunnett N, Kingsbury N. Planting green roofs and living walls. Portland, Oregon, USA: Timber Press 2004.] reported that every decrease in internal building temperature of 1°C can reduce electricity use by AC up to 16%. Peck et al. [7Peck SW, Callaghan C, Kuhn ME, Bass B. Greenbacks from green roofs: Forging a new industry in Canada. Ottawa, Canada: Canada Mortgage and Housing Corporation 1999.] reported that roof vegetation in southern Canada reduced indoor temperatures by about 4°C when outdoor temperatures were between 25 and 30°C. In this experiment, indoor air temperature was not measured, but observations (not shown) indicate that, during daytime on Guam, heat from a concrete roof can increase indoor temperature by 10°C or more. Using the conservative number of 4°C reported by Peck et al. [7Peck SW, Callaghan C, Kuhn ME, Bass B. Greenbacks from green roofs: Forging a new industry in Canada. Ottawa, Canada: Canada Mortgage and Housing Corporation 1999.] rather than the higher numbers from Guam or Ottawa, calculations recommended by Dunnett and Kingsbury [11Dunnett N, Kingsbury N. Planting green roofs and living walls. Portland, Oregon, USA: Timber Press 2004.] indicate that roof vegetation can reduce AC energy consumption by 64%. On Guam, typical energy consumption by an air-conditioned residential house maintained at 24°C oscillates around 2000 kWh per month [23Guam Power Authority. Available at: http://guampowerauthority.com/gpa_authority/environmental/gpa_planning_reg.php [Accessed on: 15 Sep 2015]]. Because the Guam Power Authority estimates that 75% of all such energy is used by AC, a reduction of 64% could cut total energy use in half. Because the price of electricity on Guam is twice the U.S. average [24US Energy Information Administration. Analysis and projections. Available at: http://www.eia.gov/electricity [Accessed on: 15 Sep 2015]], the benefit of roof vegetation could be immense.

Fig. (2)

Concrete plates used in the experimental structures were 0.13 m thick and resembled those used for ordinary residential roofs. Vegetation was established on half of the plates, and half were left without vegetation.



Temperature data indicated little difference between turf and a mixture of weeds, but zoysia turf is considered more attractive than weeds and, unlike weeds, requires infrequent trimming, every 2-3 years. In addition, the root system of zoysia turf is exceptionally dense, and should therefore attach itself more firmly to roofs and hold soil particles better than weeds. High root density would also make turf more resilient to occasional typhoons accompanied by extremely heavy tropical rainfall.

CONFLICT OF INTEREST

The author confirms that this article content has no conflict of interest.

ACKNOWLEDGEMENTS

Declared none.

References

[1] Peck SW. Award winning green roof designs: Green roofs for healthy cities. Atglen, Pennsylvania, USA: Schiffer Pub 2008.
[2] Brenneisen S. Green roofs. How nature returns to the city. Acta Hortic 2004; 643: 289-93.
[http://dx.doi.org/10.17660/ActaHortic.2004.643.37]
[3] Greater London Authority. Living roofs and walls Technical Report Greater London Authority, City Hall, The Queens Walk, London SE12AA, England ISBN 978 1 84781 132 5 2008.
[4] Liu K. Engineering performance of rooftop gardens through field evaluation In: Proc 18th International Convention of the Rooftop Consultants Institute; March 13-16, 2003; Tampa, FL.
[5] Liu K, Baskaran B. Thermal performance of green roofs through field evaluation. In: Proc 1st North American Green Roof Conference: 2003 Greening rooftops for sustainable communities; May 29-30 2003; Chicago. Toronto, Canada: The Cardinal Group 2003.
[6] Liu KS, Hsueh SL, Wu WC, Chen YL. A DFuzzy-DAHP decision-making model for evaluating energy-saving design strategies for residential buildings. Energies 2012; 5: 4462-80.
[http://dx.doi.org/10.3390/en5114462]
[7] Peck SW, Callaghan C, Kuhn ME, Bass B. Greenbacks from green roofs: Forging a new industry in Canada. Ottawa, Canada: Canada Mortgage and Housing Corporation 1999.
[8] Wong NH, Chen Y, Ong CL, Sia A. Investigation of thermal benefits of rooftop garden in the tropical environment. Build Environ 2003; 38: 261-70.
[http://dx.doi.org/10.1016/S0360-1323(02)00066-5]
[9] Peng LH, Jim CY. Green-roof effects on neighborhood microclimate and human thermal sensation. Energies 2013; 6: 598-618.
[http://dx.doi.org/10.3390/en6020598]
[10] Williams NS, Rayner JP, Raynor KJ. Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia. Urban For Urban Green 2010; 9: 245-51.
[http://dx.doi.org/10.1016/j.ufug.2010.01.005]
[11] Dunnett N, Kingsbury N. Planting green roofs and living walls. Portland, Oregon, USA: Timber Press 2004.
[12] Peck SW, Kuhn ME. Design guidelines for green roofs. Canada Mortgage and Housing Corporation, Ottawa, Canada. Available at: http://www.cmhc-schl.gc.ca. [Accessed on: 15 Sep 2015]
[13] Getter KL, Rowe DB. The role of extensive green roofs in sustainable development. HortScience 2006; 41: 1276-85.
[14] Liesecke HJ, Borgwardt H. Abbau von Luftschadstoffen durch extensive Dachbegrunungen (Degradation of air pollutants by extensive green roofs). Stadt und Grun 1997; 33: 245-51.
[15] Tan PY, Sia A. A pilot green roof research project in Singapore In: Proc 3rd North American Green Roof Conference: Greening rooftops for sustainable communities; May 4-6 2005; Washington DC. Toronto, Canada: The Cardinal Group 2005.
[16] Dwight RH, Baker DB, Semenza JC, Olson BH. Health effects associated with recreational coastal water use: urban versus rural California. Am J Public Health 2004; 94(4): 565-7.
[http://dx.doi.org/10.2105/AJPH.94.4.565] [PMID: 15054006]
[17] Grant G, Engelbeck L, Nicholson B. Green roofs: their existing status and potential for conserving biodiversity in urban areas. English Nature Research Report 498 English Nature, Peterborough, UK. 2005.
[18] Scholz-Barth K. Green roofs. Storm water management from the top down. Environ Des Construct 2001; 4: 63-70.
[19] US Census Bureau. Statistical Abstract of the United States. 131st ed. Washington, D.C. 2012.
[20] Guam Visitor Bureau. Available at: http://www.guamvisitorsbureau.com [Accessed on: 15 Sep 2015]
[21] Santamouris M. Cooling the cities: a review of reflective and green roof mitigation technologies to fight heat island and improve comfort in urban environments. Sol Energy 2014; 103: 682-703.
[http://dx.doi.org/10.1016/j.solener.2012.07.003]
[22] Liu K. Energy efficiency and environmental benefits of rooftop gardens. Green roofs. Construction, Canada 2002; 44(17): 20-3.
[23] Guam Power Authority. Available at: http://guampowerauthority.com/gpa_authority/environmental/gpa_planning_reg.php [Accessed on: 15 Sep 2015]
[24] US Energy Information Administration. Analysis and projections. Available at: http://www.eia.gov/electricity [Accessed on: 15 Sep 2015]

Endorsements



"Open access will revolutionize 21st century knowledge work and accelerate the diffusion of ideas and evidence that support just in time learning and the evolution of thinking in a number of disciplines."


Daniel Pesut
(Indiana University School of Nursing, USA)

"It is important that students and researchers from all over the world can have easy access to relevant, high-standard and timely scientific information. This is exactly what Open Access Journals provide and this is the reason why I support this endeavor."


Jacques Descotes
(Centre Antipoison-Centre de Pharmacovigilance, France)

"Publishing research articles is the key for future scientific progress. Open Access publishing is therefore of utmost importance for wider dissemination of information, and will help serving the best interest of the scientific community."


Patrice Talaga
(UCB S.A., Belgium)

"Open access journals are a novel concept in the medical literature. They offer accessible information to a wide variety of individuals, including physicians, medical students, clinical investigators, and the general public. They are an outstanding source of medical and scientific information."


Jeffrey M. Weinberg
(St. Luke's-Roosevelt Hospital Center, USA)

"Open access journals are extremely useful for graduate students, investigators and all other interested persons to read important scientific articles and subscribe scientific journals. Indeed, the research articles span a wide range of area and of high quality. This is specially a must for researchers belonging to institutions with limited library facility and funding to subscribe scientific journals."


Debomoy K. Lahiri
(Indiana University School of Medicine, USA)

"Open access journals represent a major break-through in publishing. They provide easy access to the latest research on a wide variety of issues. Relevant and timely articles are made available in a fraction of the time taken by more conventional publishers. Articles are of uniformly high quality and written by the world's leading authorities."


Robert Looney
(Naval Postgraduate School, USA)

"Open access journals have transformed the way scientific data is published and disseminated: particularly, whilst ensuring a high quality standard and transparency in the editorial process, they have increased the access to the scientific literature by those researchers that have limited library support or that are working on small budgets."


Richard Reithinger
(Westat, USA)

"Not only do open access journals greatly improve the access to high quality information for scientists in the developing world, it also provides extra exposure for our papers."


J. Ferwerda
(University of Oxford, UK)

"Open Access 'Chemistry' Journals allow the dissemination of knowledge at your finger tips without paying for the scientific content."


Sean L. Kitson
(Almac Sciences, Northern Ireland)

"In principle, all scientific journals should have open access, as should be science itself. Open access journals are very helpful for students, researchers and the general public including people from institutions which do not have library or cannot afford to subscribe scientific journals. The articles are high standard and cover a wide area."


Hubert Wolterbeek
(Delft University of Technology, The Netherlands)

"The widest possible diffusion of information is critical for the advancement of science. In this perspective, open access journals are instrumental in fostering researches and achievements."


Alessandro Laviano
(Sapienza - University of Rome, Italy)

"Open access journals are very useful for all scientists as they can have quick information in the different fields of science."


Philippe Hernigou
(Paris University, France)

"There are many scientists who can not afford the rather expensive subscriptions to scientific journals. Open access journals offer a good alternative for free access to good quality scientific information."


Fidel Toldrá
(Instituto de Agroquimica y Tecnologia de Alimentos, Spain)

"Open access journals have become a fundamental tool for students, researchers, patients and the general public. Many people from institutions which do not have library or cannot afford to subscribe scientific journals benefit of them on a daily basis. The articles are among the best and cover most scientific areas."


M. Bendandi
(University Clinic of Navarre, Spain)

"These journals provide researchers with a platform for rapid, open access scientific communication. The articles are of high quality and broad scope."


Peter Chiba
(University of Vienna, Austria)

"Open access journals are probably one of the most important contributions to promote and diffuse science worldwide."


Jaime Sampaio
(University of Trás-os-Montes e Alto Douro, Portugal)

"Open access journals make up a new and rather revolutionary way to scientific publication. This option opens several quite interesting possibilities to disseminate openly and freely new knowledge and even to facilitate interpersonal communication among scientists."


Eduardo A. Castro
(INIFTA, Argentina)

"Open access journals are freely available online throughout the world, for you to read, download, copy, distribute, and use. The articles published in the open access journals are high quality and cover a wide range of fields."


Kenji Hashimoto
(Chiba University, Japan)

"Open Access journals offer an innovative and efficient way of publication for academics and professionals in a wide range of disciplines. The papers published are of high quality after rigorous peer review and they are Indexed in: major international databases. I read Open Access journals to keep abreast of the recent development in my field of study."


Daniel Shek
(Chinese University of Hong Kong, Hong Kong)

"It is a modern trend for publishers to establish open access journals. Researchers, faculty members, and students will be greatly benefited by the new journals of Bentham Science Publishers Ltd. in this category."


Jih Ru Hwu
(National Central University, Taiwan)


Browse Contents



Webmaster Contact: info@benthamopen.net
Copyright © 2019 Bentham Open