Cool roofs — Designing Buildings Wiki

Cool roofs - Designing Buildings Wiki

Cool roofs are roofs that stay cool in the sun by minimising solar absorption and maximising thermal emittance (Akbari 2008).

“Substituting a cool roof for a non-cool roof decreases cooling-electricity use, cooling-power demand, and cooling-equipment capacity requirements, while slightly increasing heating-energy consumption. Cool roofs can also lower citywide ambient air temperature in summer, slowing ozone formation and increasing human comfort. “ (Akbari 2008)

The subject areas of cool roofs. surface solar absorption and emittance, Urban Heat Islands (UHI), urban air pollution, and building and city level cooling energy requirements are all highly interrelated. Literature on these issues is vast but much more work is required to implement findings and realise potential benefits in the real world.

The parameters of a roof’s surface can have a large influence on the surface temperature of the roof. During clear sky conditions, up to approximately 1kW/m 2 of solar radiation can be incident on a roof surface, and typically, between 20% and 95% of this radiation is absorbed (Suehrcke, Peterson et al. 2008). This large range can be explained by the influence of the surface parameters on heat gain.

The main parameters which influence maximum roof surface temperatures are solar absorptance, infrared emittance, and the convection coefficient (Berdahl and Bretz 1997). Roofs that have high solar reflectance (high ability to reflect sunlight) and high thermal emittance (high ability to radiate heat) tend to stay cool in the sun (Akbari, Levinson et al. 2008).

The colour of roofs can significantly influence the temperatures they reach. The term ‘albedo ‘ designates the total reflectance of a specific system. White can be effective in minimising heat transfer into buildings as it is a poor absorber of energy and a good emitter (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 indication of the albedo of a surface as it depends not only on visible reflectance but also reflectance of Infra Red (IR) light. For example commonly used ‘white’ coloured roofing shingles and galvanised steel run 35 o C and 43 o C hotter than air temperature on a sunny day. Conversely, surfaces painted with red or green acrylic paint run only 22 o C hotter, even though they are not visibly bright (Rosenfeld, Akbari et al. 1995). Galvanised mild steel gets hot, not due to its low albedo but because of its low emissivities meaning it is slow to cool by radiation (Rosenfeld, Akbari et al. 1995).

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).

Quantitative assessment of the benefits of roof albedo and thermal emittance is complicated by a number of issues (Suehrcke, Peterson et al. 2008):

  • 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).

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). Calculations have also been performed to estimate large-scale energy, carbon and cost savings of the widespread implementation of increasing the albedo of roofs. Tools are available for assessing the potential energy reductions such as the Energy Star Roofing Comparison Calculator (US Environmental Protection Agency 2004).

The focus of research to date has mostly been into the impacts of changing the thermal properties of a roof surface to improve building comfort or decrease building energy use. However, recently there has been interest on increasing world-wide urban albedos to offset CO2 emissions that contribute to global warming. The high albedo of roofs and paved surfaces have 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). Taking the rate that CO2 was traded at in Europe in 2009, (approximately $25/tonne), this offset would be worth $1,100 billion (Akbari, Menon et al. 2009).

Additional benefits from reducing building temperature and energy use include a reduction in peak power demand meaning that fewer power stations are required. In Los Angeles peak cooling demand increases by 3.0% for every 1 o C rise in air temperature above 18 o C, and across the US, urban heat islands raise air conditioning demand by about 10GW annually (Rosenfeld, Akbari et al. 1995).

There are also secondary advantages to increasing the albedo of urban surfaces. The resulting lower urban temperatures improve air quality and decrease smog in cities. The probability of smog increases by 6% per o C increase in temperature above a threshold of 22 o C (Rosenfeld, Akbari et al. 1995). Preliminary calculations show that a moderate change in surface albedo in Los Angeles Basin could reduce smog about around 10%, the equivalent of removing 10 million cars from Los Angeles roads (Rosenfeld, Akbari et al. 1995).

It is for these reasons that in 2009, Professor Steven Chu, President Obama’s Energy Secretary made clear his enthusiasm for the widespread implementation of cool roofs (Times Online 2009). However, implementation of heat island mitigation measures such as cool roofs was also a prominent part of President Clinton’s Climate Change Action Plan and yet only limited progress was made on the ground (Rosenfeld, Akbari et al. 1995).

Rosenfeld, Akbari et al. (1995) suggest that the costs of increasing the albedo of a city are quite low if performed during routine maintenance. Roofs are typically refinished every 10-20 years and cooler roofing material is either already available or can be developed at very little cost. Additionally, they suggest that light coloured surfaces suffer less damage from daily thermal expansion and contraction. UV damage is also reduced as this is caused by free radicals which interact more strongly the warmer the material. Rosenfeld, Akbari et al. (1995) suggest policy steps to implementing cool surfaces and shade trees, some of which have now been achieved with various levels of success since 1995.

Passive cooling involves designing buildings and selecting construction materials in a way that reduces the energy consumption of active cooling systems. There are many passive cooling techniques that can be applied to roofs (Tiwari, Upadhyay et al. 1994):

  • Modifying orientation.
  • Providing natural ventilation .
  • Selecting appropriate roof materials.
  • Spraying water to aid evaporative cooling.
  • Shading.
  • Green roofs .
  • Painting roofs with reflective colours.

Tiwari, Upadhyay et al’s 1994 paper includes mathematical modelling of these techniques and concludes that evaporative cooling is most effective at reducing incoming heat through the roof if water is easily available. Green roofs also have significant potential as passive cooling systems.

This article was created by —Buro Happold .

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