Paint Roofs White to Save Money and Delay Global Warming

The Concept

Dark surfaces warm the air over urban areas leading to the creation of urban “heat islands.” Summer heat islands increase system wide air conditioning loads and smog production because of higher urban air temperatures. Use of solar reflective (cool) urban surfaces is an inexpensive measure (similar to planting trees) that can reduce summertime temperatures. One of the simplest ways of creating a solar reflective surface is to white wash the surface, which is a commonly used practice for roofs in many parts of India and the world.

This measure reduces their solar heat gain, lowers their temperatures, and avoids transferring heat back into the atmosphere. The process of negative radiative forcing counters global warming and improves outdoor air quality. The lower surface temperatures of such cool roofs also reduce heat conduction into the building, thus improving comfort and in air conditioned buildings, reducing cooling load.
Evidence that this concept works

Installing a white roof coating reduced the average summer time daily maximum roof surface temperature of a two-storey office building in Hyderabad from 52^oC to 32^oC. The total air conditioning energy use was reduced by 30 kWh per day (see http://cbs.iiit.ac.in/Cool%20Roof/results.htm). Another study conducted by USAID in Hyderabad shows a maximum temperature reduction of up to 3.5^oC owing to heat-island reduction measures. Other regions report outdoor temperature reduction near buildings of around 2^oC owing to cool roofs. Cool roofs are a part of the Bureau of Energy Efficiency’s Energy Conservation Building Code (ECBC) and California’s Title 24. Electric utilities in California provide discounts to consumers on cool roofs as it reduces their peak air conditioning load and avoids expensive power purchase.

What are the potential savings?

Applying white plaster or paint, or whitewashing 100m² of rooftops on average offsets ten tons of carbon dioxide  equivalent and costs only around Rs 1,000. An average car emits around 4 tons of carbon dioxide per year. So painting a small roof white would offset more than a year’s worth of CO2 emissions from a car! If white washing is not aesthetically acceptable, using cool pigments that absorb less near infra-red radiation than normal colored coatings offsets five tons. Retrofitting pavements offsets four tons as well.

Cool-roofing would provide 10-20% electricity savings in air conditioning load in buildings. For people who use a one-ton air conditioner in summer months for around 6 hours a day, this would translate to almost Rs 150 per month in savings. The investment in whitewashing a roof would thus payoff within one year’s time.

What can the government do?

1. Initiate pilot projects to further test the benefits of cool roofs and test other local materials for converting to cool roofs
2. Initiate an awareness campaign and provide financial incentives to accelerate the adoption of this concept
3. Consider white washing/painting roofs of state government buildings
4. Set a goal of 50% of the roofs to be cool roofs in Delhi State in next three years
5. Mandate in Delhi what has been suggested by BEE regrading cool roofs at a national level for all new buildings as part of the Energy Conservation Building Code
6. Explore the option of painting all government vehicles and bus-tops white

What can you do?

1. Make sure that the roof-tops of your own houses and workplaces are white
2. Convince your friends, family, and co-workers that white roofs are actually ‘cool’
3. ….

Further Reading:

coolcolors.lbl.gov

http://www.energy.ca.gov/publications/searchReports.php?div_num=999

www.coolroofs.org