Editing Energy-efficient landscaping (section)

=== Green roofs ===
{{Main|Green roof}}
Often, landscape design and architecture refers to the design in ground surface; in many contexts, specifically, the design guidance and topics are for a typical residential landscape in suburban housing, where there is a yard (garden), a driveway, and a house. In the crowded urban area, however, there is not abundant ground surface for landscape design. [[Green roof]]s, then, become an appealing option to add some aesthetics and green to the crowded cities. Not limited to the cities, green roofs can be applied to wherever it will fit. Most of times, actually, the decision to build Green roofs is based on local climate and policy. It is because other than its aesthetics, green roofs are used often for their ability to conserve energy, such as increasing insulation of the building roof, retaining and infiltrating rainwater, and potentially reducing urban heat island effect when it was installed to a certain scale. In Germany, for example, partly because of EU's regulation, 17% of the new roof construction are green roofs. In Washington DC, green roofs are used as an alternative storm-water retention technique.<ref>{{Cite book|last1=Celik|first1=Serdar|last2=Morgan|first2=Susan|last3=Retzlaff|first3=William A.|title=2010 IEEE Green Technologies Conference |chapter=Energy Conservation Analysis of Various Green Roof Systems |date=April 2010|chapter-url=https://ieeexplore.ieee.org/document/5453802/;jsessionid=6F3BXmjr1H6YrefP9isxV7onuenq3QRp-4AO1RNIZmYtDIQVhc8_!-222243947|pages=1–4|doi=10.1109/GREEN.2010.5453802|isbn=978-1-4244-5274-3|s2cid=23090814}}</ref>
[[File:20080708_Chicago_City_Hall_Green_Roof.JPG|thumb|Chicago City Hall Green Roof]]

==== Benefits ====
Reducing building energy consumption by increasing the roof insulation: In total energy consumption reduction, green roof would have the best performance relative to a bare roof in a colder climate, which require nighttime heating. The reduction in heating load of the building increase as the soil depth of the green roof increase, though an increased soil depth would mean heavier roof. On the other hand, if a building is cooling-dominated, [[leaf area index]] is more important. In peak energy consumption reduction, green roof also has a notable effect, and the leaf area index and soil depth are both positively related to its performance.<ref>{{Cite journal|last1=Sailor|first1=David J.|last2=Elley|first2=Timothy B.|last3=Gibson|first3=Max|date=2011-09-13|title=Exploring the building energy impacts of green roof design decisions – a modeling study of buildings in four distinct climates|url=http://dx.doi.org/10.1177/1744259111420076|journal=Journal of Building Physics|volume=35|issue=4|pages=372–391|doi=10.1177/1744259111420076|s2cid=108512300|issn=1744-2591}}</ref>

Rainwater retention and [[evapotranspiration]]: 3-4 inches of soil can retain about 1 inch of rainwater. That is about 75% of [[precipitation]] in most areas in United States.<ref>{{Cite web|last=Johnson|first=Peter|date=Sep 2008|title=Green Roof Performance Measures|url=https://doee.dc.gov/sites/default/files/dc/sites/ddoe/service_content/attachments/D.%20Green_Roof_Performance-05-04-2009.pdf}}</ref> By retaining the rainwater in soil, the water would not become runoff, instead they would result in evapotranspiration.

==== Controversies ====
Water runoff quality: When green roof is not able to hold the amount of the precipitation, the excessive rainwater will become runoff. In a field experiment where contaminated water is dripped into a green roof section to mimic rainfall in the green roof, the exfiltrate water was studied and analyzed. It was found that since the average level of suspended solid, nitrogen, and phosphorus concentrations in Green roof water outflow is significantly higher than that in conventional roof outflow, extensive green roofs will become a source of nutrient contamination in urban water environment.<ref>{{Cite journal|last1=Liu|first1=Wen|last2=Wei|first2=Wei|last3=Chen|first3=Weiping|last4=Deo|first4=Ravinesh C.|last5=Si|first5=Jianhua|last6=Xi|first6=Haiyang|last7=Li|first7=Baofeng|last8=Feng|first8=Qi|date=September 2019|title=The impacts of substrate and vegetation on stormwater runoff quality from extensive green roofs|url=http://dx.doi.org/10.1016/j.jhydrol.2019.06.061|journal=Journal of Hydrology|volume=576|pages=575–582|doi=10.1016/j.jhydrol.2019.06.061|bibcode=2019JHyd..576..575L|s2cid=197576248|issn=0022-1694}}</ref>

Fire Hazard: Green roofs can be more easily ignited than conventional roofs; it is a concern that when the green roof caught fire, the high temperature would damage the roof structure itself. Not only the idea of damaging the roof is contradictory to energy conservation and sustainability, the fire and the roof damage could cause safety issue to the residents. It remains a matter of debate as to whether a green roof will exacerbate or mitigate the effects of a fire. Some argue that, because vegetation is about 95% water, the green roof actually reduces chances of a fire. On the other hand, some argue that during autumn and winter, when the vegetation is dry, fire hazard is increased. A recent study has found, through mathematical modelling, that <ref>{{Cite journal|last1=Gerzhova|last2=Blanchet|last3=Dagenais|last4=Côté|last5=Ménard|date=2019-09-19|title=Heat Transfer Behavior of Green Roof Systems Under Fire Condition: A Numerical Study|journal=Buildings|volume=9|issue=9|pages=206|doi=10.3390/buildings9090206|issn=2075-5309|doi-access=free|hdl=20.500.11794/66393|hdl-access=free}}</ref> when the vegetation itself caught fire, heat does penetrate downward (rather slowly as the thermal conductivity of soil is low),   eventually damaging the roof itself. Thus the key to whether ignited vegetation will damage the roof or not depends on the thickness of the soil. The study also found that by installing a gypsum layer beneath the soil layer, the possibility of damaging the roof can be greatly reduced.

Additional structural load: Most old buildings were not designed for the extra roof dead load of the green roofs. If more energy is consumed in building the additional load bearing structure for the green roofs than the energy saved through insulation enhancement and water retention, it would be contradictory to the idea of energy conservation. By study, common green roofs types in the market would increase the load on the rood by 1.2 to 2.43 kilo-newton per square meter.<ref>{{Cite journal|last1=Cascone|first1=Stefano|last2=Catania|first2=Federico|last3=Gagliano|first3=Antonio|last4=Sciuto|first4=Gaetano|date=May 2018|title=A comprehensive study on green roof performance for retrofitting existing buildings|url=http://dx.doi.org/10.1016/j.buildenv.2018.03.052|journal=Building and Environment|volume=136|pages=227–239|doi=10.1016/j.buildenv.2018.03.052|bibcode=2018BuEnv.136..227C |issn=0360-1323}}</ref>