History of Cool Roofs

A cool roof is a roof surface designed to reflect more sunlight and emit more absorbed heat than a conventional dark roof, so it stays cooler in the sun. Light-coloured and whitewashed roofs have long been used in hot climates, but the modern cool roof movement, with measured reflectance and policy targets, dates from the 1990s.

Traditional practice

Whitening roofs to reflect heat is an old practice in hot regions. A modern example is the greenhouse district of south-eastern Spain, where farmers whitewash roofs in summer; a 2008 study linked the expansion of these whitened greenhouses to a regional cooling of about 1.6 °C (2.9 °F) over 20 years.

Research foundations

The Heat Island Group at Lawrence Berkeley National Laboratory (LBNL) in California, including Arthur Rosenfeld and Hashem Akbari, studied how dark roofs and pavements heat cities and raise air-conditioning demand. A 1997 study by Bretz and Akbari examined the long-term performance of high-albedo roof coatings. A 2001 LBNL-led study of a Texas retail building retrofitted with a white vinyl roof measured a large drop in surface temperature and about 11% less air-conditioning energy.

Rating and policy

• 1998: the Cool Roof Rating Council (CRRC) was established to develop methods for measuring and labelling the radiative properties of roofing products. It added wall products in 2019 and pavement products in 2024.
• 2005: California's energy efficiency standards encouraged reflective roofs.
• 2009: New York City launched NYC CoolRoofs under PlaNYC; it coated its 500th building in 2013. Akbari, Menon and Rosenfeld published "Global Cooling", arguing reflective roofs could offset carbon emissions.
• 2010: the US Department of Energy announced cool roofs for its facilities.
• 2011: a Columbia University study found white roofs in New York averaged 43 °F (about 24 °C) cooler than black roofs.

Debate

Not all assessments are positive. A 2009 Royal Society report ranked white roofs among the less effective and more expensive climate engineering options, and studies such as Georgescu et al. (2014) found regional trade-offs, including possible effects on rainfall.

New materials

Research continues into pigments that stay cool in colour, including LBNL work on fluorescent ruby red (2016) and Egyptian blue (2018).