The Thermal and Rainwater Runoff Performance of an Extensive ...

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6 Green Roof ResearchPrevious research concerning green roofs has primarily focused on two key areas: thermalperformance and rainwater runoff performance. The quantitative analysis of these two areashas gained increasing importance in recent years as green roofs become more established.6.1 Thermal Performance of Green RoofsA number of studies on the thermal performance of green roofs have been carried out as thisis one of the most attractive benefits [24]. Green roofs can dissipate the heat absorbed fromthe sun in several effective methods, thereby reducing the heat transferred into the building.Wong et al. [25], Theodosio [26], Barrio [27], Takakura et al. [28], Onmura et al. [29] andSailor [1] have all studied the energy balance of green roofs and concluded that the dominantway for green roofs to dissipate the heat absorbed is through evapotranspiration. However,some earlier studies have suggested that thermal radiation from leaves was the dominant formof heat dissipation [30, 31].6.11 Reduction of Heat Flux and Solar ReflectivityTypical dark roof materials can reach temperatures as high as 80 o C during summer months –nearly 3 times as high as an equivalent green roof [32]. As mentioned, green roofs dissipateheat absorbed through evapotranspiration and also through improved reflectivity of incidentsolar radiation. The ratio of total reflected to incident electromagnetic radiation is defined asalbedo. The albedo of green roofs is equivalent to that of a bright white roof; with an anequivalent albedo of 0.7 - 0.85 compared with 0.1 - 0.2 for a bitumen/tar/gravel roof [33].A study by Wong et al. [25] found that, during summer, heat absorbed by conventional darkroofs during the day continued to enter the building during the night. This effect was greatlyreduced for a green roof thus reducing the cooling load for rooms below. A similar study byLui and Minor [34] found that heat gain through an extensive green roof was reduced by anaverage of between 70 – 90 % during the summer and heat loss by 10 – 30 % in winter(compared with an insulated steel deck roof).Niall Carroll Page 38The Thermal and Rainwater Runoff Performance of an Extensive Green Roof System
Figure 16: Summer Temperatures of Reference Roof and Green Roof [34]Figure 16 shows the comparison between the insulated steel deck roof and the green roofwith thermocouples located under the roof (E0), waterproof membrane (E3), subsrate (E4), inthe middle of the substrate (E5) and outside measuring the ambient air temperature (OUT).As can be seen, the peak temperature is reduced by around half and the internal temperaturepeak is slightly delayed (although the thermal insulation of the reference roof reduced thetransfer of any heat flux).6.12 Thermal MassAs well as reducing diurnal temperature fluctuations, the thermal mass of a green roof canalso improve the insulation properties of a building. The UK building regulations require thatthe roof U-value be 0.25 W/m 2 or less for all types of buildings [35]. Prior to 1965, therewere no insulation requirements in building standards and since then the U-value hasgradually decreased from 1.42 W/m 2 . This means there is a lot of old building stock whichwill benefit from the insulating properties of green roofs.Although green roofs are predominantly seen as a passive cooling technique (in terms ofthermal properties), there is evidence from a study by Nichaou et al. [36] suggesting thatwinter heating savings are larger than summer cooling savings – a result especially beneficialfor the UK.Niall Carroll Page 39The Thermal and Rainwater Runoff Performance of an Extensive Green Roof System
Page 1 and 2: Department of Mechanical Engineerin
Page 3 and 4: AbstractThis project addresses the
Page 5 and 6: Table of Contents1 Introduction…
Page 7 and 8: 6.2.1 Rainfall Runoff Relationship
Page 9 and 10: List of FiguresFigure 1: Traditiona
Page 11 and 12: Figure 32: Humidity Sensor Located
Page 13 and 14: 1 Introduction1.1 Green Roof Defini
Page 15 and 16: incorporate green roofs into their
Page 17 and 18: 2.3 ExtensiveFigure 5: Extensive Gr
Page 19 and 20: 2.33 Green/Brown Roofs for Biodiver
Page 21 and 22: 3 Construction of Green RoofsThe co
Page 23 and 24: 3.2 Inverted Warm Green Roof Constr
Page 25 and 26: Although less versatile than intens
Page 27 and 28: There are four main methods of esta
Page 29 and 30: • extension of park system• pro
Page 31 and 32: In summer, green roofs can retain 7
Page 33 and 34: The urban heat island effect descri
Page 35 and 36: 4.6 Building FabricWhen vegetation
Page 37: 5 UK Policy on Green RoofsThere are
Page 41 and 42: However, the previous study mention
Page 43 and 44: Similarly, Simmons et al. [41] foun
Page 45 and 46: Figure 20: Green Roof Test Rig for
Page 47 and 48: The mineral content of the substrat
Page 49 and 50: Figure 25: NI USB-9211 Thermocouple
Page 51 and 52: It was used to acquire the relative
Page 53 and 54: Figure 33: NI USB-6008 Terminal Ass
Page 55 and 56: Figure 35: Labview Signal Express T
Page 57 and 58: 7.2 Green Roof Energy BalanceThe gr
Page 59 and 60: (4)Where,α s = Shortwave Absorptiv
Page 61 and 62: A green roof test rig was set up to
Page 63 and 64: Figure 40: Green Roof Test Rig for
Page 65 and 66: second day of the experiment. The l
Page 67 and 68: with the artificial rain event. Whe
Page 69 and 70: References1. Sailor D.J. ‘A Green
Page 71 and 72: 31. Mellor, R.S. Salisbury, F.B. Ra
Page 73 and 74: Appendix 1Niall Carroll Page 73The

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