Academic journal article The Geographical Bulletin

An Examination of the Effect of Building Compactness and Green Roofs on Indoor Temperature through the Use of Physical Models

Academic journal article The Geographical Bulletin

An Examination of the Effect of Building Compactness and Green Roofs on Indoor Temperature through the Use of Physical Models

Article excerpt

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INTRODUCTION

Green roofs are vegetated systems that provide energy savings and environmental benefits to urban areas They can be installed during the construction of new buildings or retrofitted onto existing conventional roofs (Gedge and Kadas 2005; Getter and Rowe 2006; Youngman 2011) As a precursor to modern green roofs, roof gardens first appeared thousands of years ago in the Hanging Gardens of Babylon (Dunnett and Kingsbury 2008) In more recent times, Germany has been a leader in green roof implementation, first for aesthetic purposes beginning in the early twentieth century and later for urban ecological value in the 1950s (Dunnett and Kingsbury 2008) By the 1980s, Germany realized the benefits of green roofs for the reduction of energy use and stormwater runoff (Gedge and Kadas 2005) Green roofs have increasingly been installed in many metropolitan areas across the United States since the 1990s Between 2004 and 2013, the total green roof area in the United States increased approximately 20,000,000 ft2 (GRHC 2014)

For individual buildings, green roofs reduce energy consumption and increase the life span of the roof For urban areas in general, green roofs reduce stormwater runoff, greenhouse gas emissions, and the urban heat island effect (Alexander 2004; Blackhurst et al 2010; Dunnett and Kingsbury 2008) Energy savings are one of the main reasons to install green roofs The City of Chicago has installed green roofs since the early 2000s with City Hall as the most prominent site City Hall saves approximately 10,000 kWh and $3,600 per year due to its green roof (EPA 2008; Getter and Rowe 2006)

Green roofs reduce heat loss from buildings in winter and keep buildings cooler in summer (Youngman 2011) due to increased insulation and lower solar absorbance (Dunnett and Kingsbury 2008; Saiz et al 2006) Green roofs are more effective in reducing heat gain in the summer than reducing heat loss in the winter and thus the energy savings for green roofs are greater in the summer than in the winter (Getter and Rowe 2006; Liu and Baskaran 2005) This is primarily due to the summer heat flux (rate of heat transfer) through green roofs being considerably reduced by low thermal diffusivity due to soil moisture and low solar radiation transmission due to shadowing from leaves (Del Barrio 1998)

The Florida Solar Energy Center assessed energy savings for cooling by installing 1,000 m2 of green roofs They found that green roofs reduced 45% of the average heat flux of conventional roofs and saved 489 kWhr per year (Cummings et al 2007) Sonne (2006) found that a conventional light-colored roof had a solar reflectance of 58% compared to a green roof reflectance of 12% In the summer, the average maximum temperature of the conventional roof reached 54°C, whereas the average maximum temperature of a green roof was 33°C The average heat flux of the green roof were 18 3% less than the average heat flux of the conventional roof These differences in heat flux allowed green roofs to reduce energy consumption 700 Watt-hours per day on a roof of 307 m2

When considering the heating and cooling loads of buildings and their energy demand, building shape is critical (Gratia and Herde 2003) Buildings that have the same volume but different shapes have different heating and cooling load rates The building compactness (C, (m3/m2) is defined as

...(1)

where V is the inner volume and S is surface area (including the roof) of the building (Gratia and Herde 2003; Straube 2012) Gratia and Herde (2003) found that buildings of higher compactness have lower annual heating loads, the amount of heat needed to maintain the temperature A building with a compactness of 1 24 had 7% lower heating loads than a building with a compactness of 1, whereas a building with a compactness of 0 84 had 17 9% higher heating loads

Buildings usually gain heat from roof or wall heat transfer from the building exterior, solar radiation through windows, building internal heating from people, equipment, and artificial lighting, and the ventilation of air (ASHAE 2001; Byrne and Ritschard 1985) Exterior walls are significant components of the building thermal mass (the building's resistance to temperature change) and make a considerable impact on the heating and cooling rate of buildings (Byrne and Ritschard 1985) Heat transfer (Q) through walls and roofs is calculated by

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