White roofs for cooler homes

In developing countries an unpainted tin roof keeps the rain and sun off of hundreds of millions of people. But as it turns out, simply painting the roof white could vastly improve the ability of the roof to reduce heat in the home. In many places like India, Africa, and Latin America temperatures soar into the 110s but many people do not have air conditioning, so this is a serious health issue.

Choosing roofing materials to reduce heat involves both albedo and emissivity.

Albedo = reflectivity of a surface. Measured on a scale from 1 to 100%
High albedo = a mirror or white surface
Low albedo = a dark asphalt street

Emissivity = ability of material to release/emit stored heat back into the atmosphere thus cooling the building down. Measured on a scale from 0 to 1 with higher number meaning heat released faster. Emissivity really comes into play more in sunny, warmer climates.

So of the total incoming solar energy
a. on one hand albedo prevents some heat from ever being absorbed into the material in the first place while
b. of the heat that is absorbed, emissivity is how fast that heat is then released.

So for a good roofing material ideally both albedo and emissivity would be high.
This site also mentions that white metal roofing is the best material for warmer climates. It has a fairly high albedo of 66% due to the color but also a super high emissivity of .83.
http://home.howstuffworks.com/home-improvement/construction/green/10-best-roofing-materials.htm#page=10

By stark contrast, an unpainted tin roofs like the ones found all over the developing world drops down to an emissivity of .08.
http://www.coolmetalroofing.org/content/faq/faqs.cfm?FAQCatId=1

So just painting a tin roof white both completely reverses the emissivity and also increases the albedo, cooling houses down.

There is even an organization called the White Roof Project.
http://www.whiteroofproject.org/faq
http://enduse.lbl.gov/Projects/ESRoof-Tab4.pdf

What if these were all painted white? Tin roofs in San Jose, Costa Rica.



In New York City the CoolRoofs initiative has painted 6,000,400 square feet already at the time of writing.
http://www.nyc.gov/html/coolroofs/html/home/home.shtml


Green roofs in big cities bring relief from above - NY times
http://opinionator.blogs.nytimes.com/2012/05/23/in-urban-jungles-green-roofs-bring-relief-from-above/?_r=0