Do “Cool Roofs” contribute to global warming?

One of the mainstays of the green building movement is the “cool roof”.  The concept behind these roofs is fairly simple; buildings that reflect the sun’s heat back into the atmosphere rather than absorbing it are easier and cheaper to cool.

www.Coolroofs.org explains a cool roof as:

“A cool roof is one that reflects the sun’s heat and emits absorbed radiation back into the atmosphere. The roof literally stays cooler and reduces the amount of heat transferred to the building below, keeping the building a cooler and more constant temperature. Imagine wearing a white or a black T-shirt on a hot day. By wearing the white T-shirt you will remain cooler than if you wore a black T-shirt because it reflects more sunlight and absorbs less heat. Cool roofs like a white T-shirt, keep the internal temperature of the building cooler.”

US Energy Secretary Steven Chu widely promoted the use of cool roofs in 2010 and stated that their use could help in the fight against global warming. Secretary Chu stated, “Cool roofs are one of the quickest and lowest-cost ways we can reduce our global carbon emissions and begin the hard work of slowing climate change.”

The US Green Building Council (USGBC) also supports the adoption of cool roof technology.  Facilities installing cool roofs can earn credits toward USGBC LEED certification. (LEED Sustainable Site Credit 7.2)

Some states, such as Arizona, have even mandated cool roofs for new commercial and industrial construction.

Data centers are terrific candidates for cool roofs because they generally have large, low slope roofs with relatively few features.  As a result, you would be hard pressed to find a data center constructed in the last 5 years that does not feature the trademark white roof.

I looked a retrofit project last spring on a 10 year old Atlanta data center that had a black rubber roof.  At first glance replacing that roof with a white cool roof seemed like a “no brainer.”  However, running   and climate particulars through the DOE cool roof calculator indicated that the cool roof would save, at most, $0.15 per square foot per year.  When I compared the savings to the cost of a new white roof I advised my customer, if you need a new roof because the old one has reached the end of its serviceable life, spend a few extra bucks and make
it a cool roof.  But don’t replace a good roof just to get a cool roof.
  I stand by that analysis.

Now that cool roof technology has been nearly universally adopted and fully endorsed at every level of government, a new study published in the Journal of Climate claims that, “painting rooftops white only minimally reduces local cooling and actually causes a slight increase in overall global warming.”

It turns out that white roofs have almost no effect on global warming or Urban Heat Island (UHI) effect because “Lower local temperature means less water evaporates and rises up to eventually form clouds” says lead author and Stanford University researcher Mark Jacobson. “The decrease in clouds allows more sunlight to reach the Earth’s surface, leading to higher temperatures
overall.”

So, if your roof is failing and you want to save a few pennies on your cooling bill, a white roof maybe a good choice.  (Particularly if you live in temperate climates.)  But, if you think that your white roof indicates good corporate citizenship or that you are doing your little bit to save the planet, you may want to think again.

One study is never definitive when it comes to climate change.  However, it looks like the cool roof might end up in the dustbin with other faulty ideas (ethanol subsidies, carbon offsets, etc…) promoted by the green movement before the science is fully baked.

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