BRE IP13/10 PDF

BACKGROUND

As global climates continue to warm, managing the amount of radiant heat entering buildings is becoming increasingly important. Excessive gains can cause thermal discomfort for building occupants and can conflict with the building's use (for instance, as chilled storage space). Although designers and users typically have turned to mechanical cooling systems to counteract overheating, the significant energy demands of air-conditioning systems has caused many to explore alternative low-carbon, passive cooling strategies, including cool roofs.

Cool roofs are not a new concept. BRE first undertook research into solar reflective roof finishes in the 1930s and white-washed buildings have been built throughout the Mediterranean and tropical climates worldwide for centuries (Figure 1).

The drive to reduce CO₂ emissions by improving the energy efficiency of buildings has led to a resurgence of interest in cool roofs in recent years. The centre for this revival has been California where Lawrence Berkley National Laboratory (LBNL) scientists and roofing contractors have been migrating the concept from traditional vernaculars to new and existing contemporary buildings since the 1980s. For some buildings in the State of California, cool roofs have been shown to generate up to a 40% reduction in energy use which corresponds to annual savings of $10–100 per m² of roof area.

Research has shown that, on a global scale, cool roofs can be an effective form of ‘geoengineering', ie a method of quickly combating climate change by manipulating the earth's environment. Experts at LBNL have calculated that, by permanently retrofitting urban roofs and pavements in the tropical and temperate regions of the world with solar-reflective materials, 44 billion tonnes of emitted CO₂ would be offset. Proposals to expand cool roofing mandates across the US have been made on the basis that such policies have the potential to save $2 trillion annually in air-conditioning bills. Often costing no more than their conventional roofing counterparts, cool roofs offer excellent whole life value with short payback periods and are now considered an essential tool for decarbonising the UK's building stock.

Although further research is needed to discover whether cool roofs can deliver the same energy efficiency savings to buildings in the UK, predictions made by the United Kingdom's Climate Impacts Programme (UKCIP) under a general scenario for longer hotter summers, suggest that cool roofs and other forms of passive cooling must be explored if mass market uptake of mechanical cooling is to be avoided.