Last edited 04 Dec 2015

Albedo in the built environment

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.

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 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 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):

  • 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).
  • 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).

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 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 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).

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[edit] 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.