Albedo in the built environment - Revision history

Designing Buildings at 06:27, 15 September 2022

2022-09-15T06:27:18Z

← Older revision		Revision as of 06:27, 15 September 2022
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	NB Whilst increasing roof albedo and infrared emittance can reduce energy consumption in hot climates, it may increase heating-energy consumption in winter months or in cooler climates (Akbari, Levinson et al. 2008).		NB Whilst increasing roof albedo and infrared emittance can reduce energy consumption in hot climates, it may increase heating-energy consumption in winter months or in cooler climates (Akbari, Levinson et al. 2008).

−	= Related articles on Designing Buildings ~~Wiki~~ =	+	[https://www.ipcc.ch/sr15/ Global Warming of 1.5 ºC, Glossary], published by the Intergovernmental Panel on Climate Change (IPCC) in 2018, defines solar radiation modification (SRM) as: ‘…the intentional modification of the Earth’s shortwave radiative budget with the aim of reducing warming. Artificial injection of stratospheric aerosols, marine cloud brightening and land surface albedo modification are examples of proposed SRM methods. SRM does not fall within the definitions of mitigation and adaptation (IPCC, 2012b, p. 2). Note that in the literature SRM is also referred to as solar radiation management or albedo enhancement.’
		+
		+	= Related articles on Designing Buildings =

	* Adaptation		* Adaptation
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	* Solar gain.		* Solar gain.
	* Solar heat gain coefficient.		* Solar heat gain coefficient.
		+	* Solar radiation modification.
	* Solar reflectance index.		* Solar reflectance index.
−	* Test Reference Year (TRY).
	* Thermal comfort.		* Thermal comfort.
	* Thermal mass		* Thermal mass

Editor at 08:43, 19 July 2022

2022-07-19T08:43:33Z

← Older revision		Revision as of 08:43, 19 July 2022
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	The term 'albedo' describes the proportion of incident radiation reflected by a system. A perfect reflector would have an albedo of 1, whereas a perfect absorber would have an albedo of 0.		The term 'albedo' describes the proportion of incident radiation reflected by a system. A perfect reflector would have an albedo of 1, whereas a perfect absorber would have an albedo of 0.

−	Albedo is sometimes used to describe the proportion of solar radiation reflected by the earth back into space, and this reflectance of solar radiation is also an important property in the built environment.	+	Albedo is sometimes used to describe the proportion of solar radiation reflected by the earth back into space, and this reflectance of solar radiation is also an important property in the built environment. The albedo effect describes this phenomenon on land or indeed at sea.
		+
		+	It has in recent years gained greater attention because of cascade effect or positive feedback loop brought about by a reduction in albedo resulting from the melting of glaciers, ice sheets and ice caps. As bright white and high albedo areas of ice melt due to temperatures rise they are replaced with darker areas of land mass or surface water, these have a low albedo and thus absorb more heat from the sun, which in turn raises the temperature in the local micro-climate which in turn can speed up the melting of ice. The light-coloured surfaces such as ice return a large part of the sunrays back to the atmosphere (high albedo) whereas the dark surfaces such as the ocean absorb the rays from the sun (low albedo).

	Indirect solar gain in buildings can be a significant contributor to overheating. Solar radiation absorbed by the envelope of a building can be transmitted through the fabric of the envelope to the interior. In addition, the urban heat island effect, which refers to higher localised temperatures that are experienced in urban environments compared with surrounding green spaces, is primarily caused by the replacement of natural surfaces with hard impervious surfaces that are generally dark and absorb large amounts of solar radiation.		Indirect solar gain in buildings can be a significant contributor to overheating. Solar radiation absorbed by the envelope of a building can be transmitted through the fabric of the envelope to the interior. In addition, the urban heat island effect, which refers to higher localised temperatures that are experienced in urban environments compared with surrounding green spaces, is primarily caused by the replacement of natural surfaces with hard impervious surfaces that are generally dark and absorb large amounts of solar radiation.
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	= Related articles on Designing Buildings Wiki =		= Related articles on Designing Buildings Wiki =

		+	* Adaptation
	* Climate change science.		* Climate change science.
	* Cool paint.		* Cool paint.
	* Cool roofs.		* Cool roofs.
		+	* Design Summer Year (DSY).
	* Emissivity.		* Emissivity.
	* Green roofs.		* Green roofs.
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	* Solar heat gain coefficient.		* Solar heat gain coefficient.
	* Solar reflectance index.		* Solar reflectance index.
		+	* Test Reference Year (TRY).
	* Thermal comfort.		* Thermal comfort.
	* Thermal mass		* Thermal mass

Editor at 17:06, 12 February 2021

2021-02-12T17:06:19Z

← Older revision		Revision as of 17:06, 12 February 2021
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	* Climate change science.		* Climate change science.
		+	* Cool paint.
	* Cool roofs.		* Cool roofs.
	* Emissivity.		* Emissivity.

Editor at 17:04, 15 September 2020

2020-09-15T17:04:58Z

← Older revision		Revision as of 17:04, 15 September 2020
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	NB Whilst increasing roof albedo and infrared emittance can reduce energy consumption in hot climates, it may increase heating-energy consumption in winter months or in cooler climates (Akbari, Levinson et al. 2008).		NB Whilst increasing roof albedo and infrared emittance can reduce energy consumption in hot climates, it may increase heating-energy consumption in winter months or in cooler climates (Akbari, Levinson et al. 2008).

−	~~= Find out more =~~	+	= Related articles on Designing Buildings Wiki =
−		+
−	=== Related articles on Designing Buildings Wiki ===	+

	* Climate change science.		* Climate change science.
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	* Thermal mass		* Thermal mass
	* Thermal optical properties.		* Thermal optical properties.
		+	* Types of cool roofs.
	* Urban heat island effect.		* Urban heat island effect.

−	=== External references ===	+	= External references =

	* Akbari, H., R. Levinson, et al. (2008). "Procedure for measuring the solar reflectance of flat or curved roofing assemblies." Solar Energy 82(7): 648-655.		* Akbari, H., R. Levinson, et al. (2008). "Procedure for measuring the solar reflectance of flat or curved roofing assemblies." Solar Energy 82(7): 648-655.
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	* Yang, J., Q. Yu, et al. (2008). "Quantifying air pollution removal by green roofs in Chicago." Atmospheric Environment 42(31): 7266-7273.		* Yang, J., Q. Yu, et al. (2008). "Quantifying air pollution removal by green roofs in Chicago." Atmospheric Environment 42(31): 7266-7273.

−	[[Category:~~Standards_/_measurements~~]] [[Category:~~DCN_Standard~~]]	+	[[Category:DCN_Definition]] [[Category:DCN_Guidance]] [[Category:DCN_Research,_Development_and_Innovation]] [[Category:Standards_/_measurements]]

Editor at 11:00, 4 December 2015

2015-12-04T11:00:46Z

← Older revision		Revision as of 11:00, 4 December 2015
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−
	The term 'albedo' describes the proportion of incident radiation reflected by a system. A perfect reflector would have an albedo of 1, whereas a perfect absorber would have an albedo of 0.		The term 'albedo' describes the proportion of incident radiation reflected by a system. A perfect reflector would have an albedo of 1, whereas a perfect absorber would have an albedo of 0.

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	The word ‘albedo’ is derived from the Latin word for whiteness, and very broadly, white-coloured surfaces have a high albedo, and can be effective in minimising solar gain as they tend to be poor absorbers of solar radiation and good emitters (Al-Homoud 2005). The general idea of white washing structures to reject heat has been known since antiquity (Berdahl and Bretz 1997).		The word ‘albedo’ is derived from the Latin word for whiteness, and very broadly, white-coloured surfaces have a high albedo, and can be effective in minimising solar gain as they tend to be poor absorbers of solar radiation and good emitters (Al-Homoud 2005). The general idea of white washing structures to reject heat has been known since antiquity (Berdahl and Bretz 1997).

−	However, colour is not always a good indicator of the albedo of a surface as it is determined by reflectance of visible light, rather than other wavelengths of the spectrum (see thermal optical properties for more information). For example commonly used ‘white’ coloured roofing shingles and galvanised steel can reach ~~35<sup>o</sup>C~~ and ~~43<sup>o</sup>C~~ hotter than air temperatures on a sunny day. Conversely, surfaces painted with red or green acrylic paint may be just ~~22<sup>o</sup>C~~ hotter, even though they are not visibly bright (Rosenfeld, Akbari et al. 1995). Galvanised mild steel becomes hot, not due to its low albedo but because of its low emissivity meaning it is slow to cool by re-radiation of long wave infrared radiation (Rosenfeld, Akbari et al. 1995).	+	However, colour is not always a good indicator of the albedo of a surface as it is determined by reflectance of visible light, rather than other wavelengths of the spectrum (see thermal optical properties for more information). For example commonly used ‘white’ coloured roofing shingles and galvanised steel can reach 35oC and 43oC hotter than air temperatures on a sunny day. Conversely, surfaces painted with red or green acrylic paint may be just 22oC hotter, even though they are not visibly bright (Rosenfeld, Akbari et al. 1995). Galvanised mild steel becomes hot, not due to its low albedo but because of its low emissivity meaning it is slow to cool by re-radiation of long wave infrared radiation (Rosenfeld, Akbari et al. 1995).

	Quantitative assessment of the benefits of albedo and thermal emittance is complicated by a number of issues (Suehrcke, Peterson et al. 2008):		Quantitative assessment of the benefits of albedo and thermal emittance is complicated by a number of issues (Suehrcke, Peterson et al. 2008):
−	*Heat flow across surfaces combines with that due to air temperature differences between the outside and inside.	+
−	*Heat flows due to solar absorption and outside-to-inside air temperature differences are variable and influenced by the thermal mass of the building fabric.	+	* Heat flow across surfaces combines with that due to air temperature differences between the outside and inside.
−	*The solar absorbance of a surface will change with time due to dust and ageing.	+	* Heat flows due to solar absorption and outside-to-inside air temperature differences are variable and influenced by the thermal mass of the building fabric.
−	*If the surface is shaded, the amount of incident sunlight is reduced, which tends to reduce the potential of cool surfaces (Akbari, Menon et al. 2009).	+	* The solar absorbance of a surface will change with time due to dust and ageing.
−	*Effects such as surface roughness and small impurities in materials can lower the albedo of a surface (Berdahl and Bretz 1997).	+	* If the surface is shaded, the amount of incident sunlight is reduced, which tends to reduce the potential of cool surfaces (Akbari, Menon et al. 2009).
		+	* Effects such as surface roughness and small impurities in materials can lower the albedo of a surface (Berdahl and Bretz 1997).

	Despite this, many equations have been derived for quantitatively assessing the effect of albedo and infrared emittance on surface temperatures and internal building temperature (Levinson, Akbari et al. 2007).		Despite this, many equations have been derived for quantitatively assessing the effect of albedo and infrared emittance on surface temperatures and internal building temperature (Levinson, Akbari et al. 2007).
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	=== Related articles on Designing Buildings Wiki ===		=== Related articles on Designing Buildings Wiki ===
−	*Cool roofs.	+
−	*Emissivity.	+	* Climate change science.
−	*Green roofs.	+	* Cool roofs.
−	*G-value.	+	* Emissivity.
−	*Low-e glass.	+	* Green roofs.
−	*Part L.	+	* G-value.
−	*Passive building design.	+	* Low-e glass.
−	*Shading coefficient.	+	* Part L.
−	*Solar gain.	+	* Passive building design.
−	*Solar heat gain coefficient.	+	* Shading coefficient.
−	*Solar reflectance index.	+	* Solar gain.
−	*Thermal comfort.	+	* Solar heat gain coefficient.
−	*Thermal mass	+	* Solar reflectance index.
−	*Thermal optical properties.	+	* Thermal comfort.
−	*Urban heat island effect.	+	* Thermal mass
		+	* Thermal optical properties.
		+	* Urban heat island effect.

	=== External references ===		=== External references ===
−	*Akbari, H., R. Levinson, et al. (2008). "Procedure for measuring the solar reflectance of flat or curved roofing assemblies." Solar Energy 82(7): 648-655.
−	*Akbari, H., S. Menon, et al. (2009). "Global cooling: increasing world-wide urban albedos to offset CO2." Climatic Change 94(3): 275-286.
−	*Al-Homoud, D. M. S. (2005). "Performance characteristics and practical applications of common building thermal insulation materials." Building and Environment 40(3): 353-366.
−	*Berdahl, P. and S. E. Bretz (1997). "Preliminary survey of the solar reflectance of cool roofing materials." Energy and Buildings 25(2): 149-158.
−	*Levinson, R., H. Akbari, et al. (2007). "Cooler tile-roofed buildings with near-infrared-reflective non-white coatings." Building and Environment 42(7): 2591-2605.
−	*Rosenfeld, A. H., H. Akbari, et al. (1995). "Mitigation of urban heat islands: materials, utility programs, updates." Energy and Buildings 22(3): 255-265.
−	*Suehrcke, H., E. L. Peterson, et al. (2008). "Effect of roof solar reflectance on the building heat gain in a hot climate." Energy and Buildings 40(12): 2224-2235.
−	*Wolf, D. and J. T. Lundholm (2008). "Water uptake in green roof microcosms: Effects of plant species and water availability." Ecological Engineering 33(2): 179-186.
−	*Yang, J., Q. Yu, et al. (2008). "Quantifying air pollution removal by green roofs in Chicago." Atmospheric Environment 42(31): 7266-7273.

−	[[Category:Standards_/_measurements]]	+	* Akbari, H., R. Levinson, et al. (2008). "Procedure for measuring the solar reflectance of flat or curved roofing assemblies." Solar Energy 82(7): 648-655.
		+	* Akbari, H., S. Menon, et al. (2009). "Global cooling: increasing world-wide urban albedos to offset CO2." Climatic Change 94(3): 275-286.
		+	* Al-Homoud, D. M. S. (2005). "Performance characteristics and practical applications of common building thermal insulation materials." Building and Environment 40(3): 353-366.
		+	* Berdahl, P. and S. E. Bretz (1997). "Preliminary survey of the solar reflectance of cool roofing materials." Energy and Buildings 25(2): 149-158.
		+	* Levinson, R., H. Akbari, et al. (2007). "Cooler tile-roofed buildings with near-infrared-reflective non-white coatings." Building and Environment 42(7): 2591-2605.
		+	* Rosenfeld, A. H., H. Akbari, et al. (1995). "Mitigation of urban heat islands: materials, utility programs, updates." Energy and Buildings 22(3): 255-265.
		+	* Suehrcke, H., E. L. Peterson, et al. (2008). "Effect of roof solar reflectance on the building heat gain in a hot climate." Energy and Buildings 40(12): 2224-2235.
		+	* Wolf, D. and J. T. Lundholm (2008). "Water uptake in green roof microcosms: Effects of plant species and water availability." Ecological Engineering 33(2): 179-186.
		+	* Yang, J., Q. Yu, et al. (2008). "Quantifying air pollution removal by green roofs in Chicago." Atmospheric Environment 42(31): 7266-7273.
		+
		+	[[Category:Standards_/_measurements]] [[Category:DCN_Standard]]

Designing Buildings: moved Albedo to Albedo in the built environment

2015-03-05T10:10:29Z

moved Albedo to Albedo in the built environment

← Older revision

Revision as of 10:10, 5 March 2015

Designing Buildings at 08:55, 2 August 2014

2014-08-02T08:55:34Z

← Older revision		Revision as of 08:55, 2 August 2014
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	*Solar gain.		*Solar gain.
	*Solar heat gain coefficient.		*Solar heat gain coefficient.
		+	*Solar reflectance index.
	*Thermal comfort.		*Thermal comfort.
	*Thermal mass		*Thermal mass

Designing Buildings at 10:29, 1 August 2014

2014-08-01T10:29:44Z

← Older revision		Revision as of 10:29, 1 August 2014
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	*The solar absorbance of a surface will change with time due to dust and ageing.		*The solar absorbance of a surface will change with time due to dust and ageing.
	*If the surface is shaded, the amount of incident sunlight is reduced, which tends to reduce the potential of cool surfaces (Akbari, Menon et al. 2009).		*If the surface is shaded, the amount of incident sunlight is reduced, which tends to reduce the potential of cool surfaces (Akbari, Menon et al. 2009).
−		+	*Effects such as surface roughness and small impurities in materials can lower the albedo of a surface (Berdahl and Bretz 1997).
−	~~Whilst it is accepted that the albedo of a surface does have an effect on the heat transfer across it, measuring albedo is not straight forward.~~ Effects such as surface roughness and small impurities in materials can lower the ~~reflectance and~~ albedo of a surface (Berdahl and Bretz 1997).	+

	Despite this, many equations have been derived for quantitatively assessing the effect of albedo and infrared emittance on surface temperatures and internal building temperature (Levinson, Akbari et al. 2007).		Despite this, many equations have been derived for quantitatively assessing the effect of albedo and infrared emittance on surface temperatures and internal building temperature (Levinson, Akbari et al. 2007).

Designing Buildings at 10:27, 1 August 2014

2014-08-01T10:27:11Z

← Older revision		Revision as of 10:27, 1 August 2014
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	Despite this, many equations have been derived for quantitatively assessing the effect of albedo and infrared emittance on surface temperatures and internal building temperature (Levinson, Akbari et al. 2007).		Despite this, many equations have been derived for quantitatively assessing the effect of albedo and infrared emittance on surface temperatures and internal building temperature (Levinson, Akbari et al. 2007).

−	Calculations have also been performed to estimate large-scale energy, carbon and cost savings of the widespread implementation of increasing albedo.	+	Calculations have also been performed to estimate large-scale energy, carbon and cost savings of the widespread implementation of increasing albedo.

	It is estimated that pavements and roofs account for 60% of urban surfaces, roofs 20-25% and pavements approximately 40% (Akbari, Menon et al. 2009). Presently these surfaces have relatively low albedo values and high thermal conductivities, typically absorbing and re-radiating around 90% of the total incident solar radiation (Wolf and Lundholm 2008). This contributes to an urban heat island effect that can result in a rise in summer temperatures of 4-7oC (CIBSE 2007; Wolf and Lundholm 2008) in comparison with adjacent vegetated areas.		It is estimated that pavements and roofs account for 60% of urban surfaces, roofs 20-25% and pavements approximately 40% (Akbari, Menon et al. 2009). Presently these surfaces have relatively low albedo values and high thermal conductivities, typically absorbing and re-radiating around 90% of the total incident solar radiation (Wolf and Lundholm 2008). This contributes to an urban heat island effect that can result in a rise in summer temperatures of 4-7oC (CIBSE 2007; Wolf and Lundholm 2008) in comparison with adjacent vegetated areas.

Designing Buildings at 10:26, 1 August 2014

2014-08-01T10:26:25Z

← Older revision		Revision as of 10:26, 1 August 2014
Line 1:		Line 1:

−	The term 'albedo' describes the proportion of incident radiation reflected by a ~~specific~~ system. A perfect reflector would have an albedo of 1, whereas a perfect absorber would have an albedo of 0.	+	The term 'albedo' describes the proportion of incident radiation reflected by a system. A perfect reflector would have an albedo of 1, whereas a perfect absorber would have an albedo of 0.

−	Albedo is sometimes used to describe the proportion of solar radiation reflected by the earth back into space, and this reflectance of solar radiation is also an important property ~~for~~ the built environment.	+	Albedo is sometimes used to describe the proportion of solar radiation reflected by the earth back into space, and this reflectance of solar radiation is also an important property in the built environment.

−	Indirect solar gain in buildings can be a significant contributor to overheating. Solar radiation absorbed by the envelope of a building can be transmitted through the fabric of the envelope to the interior. In addition, the urban heat island effect, which refers to higher localised temperatures that are experienced in urban environments compared with surrounding green spaces is primarily caused by the replacement of natural surfaces with hard impervious surfaces that are generally dark and absorb large amounts of solar radiation.	+	Indirect solar gain in buildings can be a significant contributor to overheating. Solar radiation absorbed by the envelope of a building can be transmitted through the fabric of the envelope to the interior. In addition, the urban heat island effect, which refers to higher localised temperatures that are experienced in urban environments compared with surrounding green spaces, is primarily caused by the replacement of natural surfaces with hard impervious surfaces that are generally dark and absorb large amounts of solar radiation.

−	The word ‘albedo’ is derived from the Latin word for whiteness, and very broadly, white-coloured ~~surface~~ have a high albedo, and can be effective in minimising solar gain as they tend to be poor absorbers of solar radiation and good emitters (Al-Homoud 2005). The general idea of white washing structures to reject heat has been known since antiquity (Berdahl and Bretz 1997).	+	The word ‘albedo’ is derived from the Latin word for whiteness, and very broadly, white-coloured surfaces have a high albedo, and can be effective in minimising solar gain as they tend to be poor absorbers of solar radiation and good emitters (Al-Homoud 2005). The general idea of white washing structures to reject heat has been known since antiquity (Berdahl and Bretz 1997).

−	However, colour is not always a good indicator of the albedo of a surface as it is determined by reflectance of visible light, rather than other wavelengths of the ~~solar~~ spectrum (see ~~solar~~ optical properties for more information). For example commonly used ‘white’ coloured roofing shingles and galvanised steel can reach 35<sup>o</sup>C and 43<sup>o</sup>C hotter than air temperatures on a sunny day. Conversely, surfaces painted with red or green acrylic paint may be just 22<sup>o</sup>C hotter, even though they are not visibly bright (Rosenfeld, Akbari et al. 1995). Galvanised mild steel becomes hot, not due to its low albedo but because of its low emissivity meaning it is slow to cool by re-radiation of long wave infrared radiation (Rosenfeld, Akbari et al. 1995).	+	However, colour is not always a good indicator of the albedo of a surface as it is determined by reflectance of visible light, rather than other wavelengths of the spectrum (see thermal optical properties for more information). For example commonly used ‘white’ coloured roofing shingles and galvanised steel can reach 35<sup>o</sup>C and 43<sup>o</sup>C hotter than air temperatures on a sunny day. Conversely, surfaces painted with red or green acrylic paint may be just 22<sup>o</sup>C hotter, even though they are not visibly bright (Rosenfeld, Akbari et al. 1995). Galvanised mild steel becomes hot, not due to its low albedo but because of its low emissivity meaning it is slow to cool by re-radiation of long wave infrared radiation (Rosenfeld, Akbari et al. 1995).

−	~~It is estimated that pavements and roofs account for 60%~~ of ~~urban surfaces, roofs 20-25%~~ and ~~pavements approximately 40%~~ (~~Akbari~~, ~~Menon~~ et al. ~~2009~~)~~. Presently these~~ surfaces ~~have relatively low albedo values~~ and ~~high thermal conductivities, typically absorbing~~ and re-~~radiating around 90%~~ of the ~~total incident~~ solar ~~radiation (Wolf~~ and ~~Lundholm 2008)~~. ~~This contributes~~ to ~~an urban heat island effect that can result in a rise in summer temperatures~~ of ~~4-7oC~~ (~~CIBSE 2007; Wolf and Lundholm 2008~~) ~~in comparison with adjacent vegetated areas~~.	+	Quantitative assessment of the benefits of albedo and thermal emittance is complicated by a number of issues (Suehrcke, Peterson et al. 2008):
		+	*Heat flow across surfaces combines with that due to air temperature differences between the outside and inside.
		+	*Heat flows due to solar absorption and outside-to-inside air temperature differences are variable and influenced by the thermal mass of the building fabric.
		+	*The solar absorbance of a surface will change with time due to dust and ageing.
		+	*If the surface is shaded, the amount of incident sunlight is reduced, which tends to reduce the potential of cool surfaces (Akbari, Menon et al. 2009).

−	~~Vegetation reduces~~ the ~~urban heat island~~ effect ~~by changing~~ the ~~albedos (~~the ~~fraction~~ of ~~incoming radiation reflected by~~ a ~~body) of urban surfaces as well as evapotranspiration cooling. It also slows down photochemical reactions that lead to less secondary air pollutants, such as ozone~~ (~~Yang, Yu et al. 2008~~)~~. They also provide some shading, thus reduce the temperature of the roof~~.	+	Whilst it is accepted that the albedo of a surface does have an effect on the heat transfer across it, measuring albedo is not straight forward. Effects such as surface roughness and small impurities in materials can lower the reflectance and albedo of a surface (Berdahl and Bretz 1997).

−	~~Quantitative assessment of~~ the ~~benefits~~ of ~~roof~~ albedo and ~~thermal~~ emittance ~~is complicated by a number of issues (Suehrcke, Peterson et al. 2008):~~	+	Despite this, many equations have been derived for quantitatively assessing the effect of albedo and infrared emittance on surface temperatures and internal building temperature (Levinson, Akbari et al. 2007).
−	*Heat flow across the roof surface ~~combines with that due to air temperature differences between the outside~~ and ~~inside.~~	+
−	*Heat flows due to solar absorption and outside-to-inside air temperature ~~differences are variable and influenced by the thermal mass of the roof.~~	+
−	*The solar absorbance of a roof will change with time due to dust and ageing.	+
−	*If the roof is shaded the amount of incident sunlight is reduced, ~~which tends to reduce the potential of cool surfaces (~~Akbari~~, Menon~~ et al. ~~2009~~).	+

−	~~Whilst it is accepted that the albedo of a roof does~~ have ~~an effect on the heat transfer across the roof~~, ~~measuring roof albedo is not straight forward. Effects such as surface roughness~~ and ~~small impurities in~~ the ~~material can lower the reflectance and~~ albedo ~~of a surface (Berdahl and Bretz 1997)~~.	+	Calculations have also been performed to estimate large-scale energy, carbon and cost savings of the widespread implementation of increasing albedo.

−	~~Despite this, many equations have been derived~~ for ~~quantitatively assessing the effect~~ of ~~roof albedo~~ and ~~infrared emittance on roof temperature and internal building temperature~~ (~~Levinson,~~ Akbari et al. 2007).	+	It is estimated that pavements and roofs account for 60% of urban surfaces, roofs 20-25% and pavements approximately 40% (Akbari, Menon et al. 2009). Presently these surfaces have relatively low albedo values and high thermal conductivities, typically absorbing and re-radiating around 90% of the total incident solar radiation (Wolf and Lundholm 2008). This contributes to an urban heat island effect that can result in a rise in summer temperatures of 4-7oC (CIBSE 2007; Wolf and Lundholm 2008) in comparison with adjacent vegetated areas.

−	~~Calculations have also been performed to estimate large-scale energy, carbon and cost savings of the widespread implementation of increasing the albedo of roofs.~~ It is estimated that changing the albedo of roofs and paved surfaces has the potential to increase the albedo of urban areas by 10%. If this was implemented globally across all urban areas, there would be a negative radiative force equivalent to offsetting 44Gt of CO2 emissions (24Gt by roofs, 20Gt by pavements) (Akbari, Menon et al. 2009).	+	It is estimated that changing the albedo of roofs and paved surfaces has the potential to increase the albedo of urban areas by 10%. If this was implemented globally across all urban areas, there would be a negative radiative force equivalent to offsetting 44Gt of CO2 emissions (24Gt by roofs, 20Gt by pavements) (Akbari, Menon et al. 2009).

−	NB the Solar Reflectance Index (SRI), can be considered a better indicator of solar gain as it includes both solar reflectance and emissivity.	+	NB the Solar Reflectance Index (SRI), can be considered a better indicator of solar gain as it includes both solar reflectance and emissivity.

	NB Whilst increasing roof albedo and infrared emittance can reduce energy consumption in hot climates, it may increase heating-energy consumption in winter months or in cooler climates (Akbari, Levinson et al. 2008).		NB Whilst increasing roof albedo and infrared emittance can reduce energy consumption in hot climates, it may increase heating-energy consumption in winter months or in cooler climates (Akbari, Levinson et al. 2008).