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The Difference Between White Roofs and Green Roofs in Terms of Their Positive Contribution to Global Warming Reduction – by Ian Montangees

A number of people have asked about the difference between green roofs and white roofs in contributing to global warming reduction in a positive way. The short answer is that they are both good in their different ways and in any particular situation it depends on what you are wanting to achieve as both types produce positive outcomes.

Green roofs are roofs covered with vegetation planted in a bed of soil and growing media which may be 150 mm thick, and has a waterproof membrane underneath.

Green roofs help towards global warming reduction partly by increasing the insulation of a roof from the addition of a layer of soil and this reduces energy consumed by air conditioning in summer and to a lesser extent by heating in winter, which gives reductions in emissions from electricity power generation. In summer the plants can provide some additional cooling by shading and by evaporation of moisture, provided that the vegetation is thick enough and is kept alive.

In addition:

– A planted roof can be visually attractive, a way of introducing green space into an urban environment, which can be softening and visually restful. For example, in some locations green roofs are used as recreational green space at ground level on top of a building underneath. A green roof can add value to a property.

– Green roofs even out storm water runoff from roofs during heavy rainfalls because the rain takes longer to percolate through the soil, easing the load on storm water drains. In this way green roofs perform similar to storm water detention tanks that are often required on new buildings to help even out peak rainfall runoffs.

However, green roofs are expensive to install and require buildings that are strong enough to take the extra load of the green roof soil.

White roofs help global warming reduction by:

– Directly reflecting sunlight back into space and help cool the planet mimicking way that the polar icecaps do.

– Reducing electricity use by air conditioning systems in summer, and typically producing an overall annual savings in both energy and emissions (summer cooling savings minus winter heating penalty equals overall savings) compared to a dark roof.

– Zero cost, often, when the choice to use white is made at the design stage or during regular maintenance

– Can be applied onto a wide range of roofs

– White paint is typically cheaper than dark paint

– It is a win-win solution that gives an effective action plan that many people can do themselves instead of waiting for others to take action.

I have written elsewhere in Happyzine about the science and benefits and simplicity of white roofs so I won’t repeat it in this article.

A couple of studies have compared both white roofs and green roofs with traditional dark roofs, such as one  carried out at the University of Columbia’s Centre for Climate Systems Research in 2010, which was commissioned by Con Edison, one of the largest energy companies in the US. An earlier study was carried out by Walmart, the largest retailer in the US.

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The Columbia study found that both white and green roofs perform equally well in preventing the heat island effect, which is the heat concentration found in many cities resulting from replacing vegetation with man-made buildings and roads that absorb sunlight and convert it into heat. The study was carried out on adjacent roofs of Con Edison. The green roof (using 21,000 plants), the white roof and the dark roof each had embedded sensors to measure heat flows through the roof structure. It was found that both white roofs and green roofs produced significant energy savings compared to a standard dark roof, that the green roof savings were greater than the white, which is to be expected considering the amount of insulation 150 mm of soil would provide.

Note that these studies did not include for the amount of global cooling produced by the white roofs directly reflecting sunlight back into space, which green roofs do not offer, and which has been found to be a very effective mechnism for global warming reduction in other studies with that focus. The emphasis of the Columbia study was more focused on energy savings that also put money in your pocket, and did not include for global cooling of the planet.

The goal of the study was to provide the best science to help these companies make choices and cost-benefit estimates in how they might achieve better energy efficiency. Which perhaps gives understanding to why companies like Con Edison and Walmart have embraced white roofs, with Con Edison having installed 25,000 square metre of white roof at the time this report was written, with at least a further 22,000 m2 planned by the end of 2010, and Walmart has had a policy of installing white roofs on all its new stores since 1999, and by 2010 more than 75 percent of the chain’s 4,268 outlets in the United States now have them.

So in summary, both white roofs and green roofs are good choices for helping reduce global warming, and in any particular situation it depends on what one is trying to achieve as both types produce different outcomes while also helping to save the many species threatened by our man-made climate change. So yea for white roofs! and yea for green roofs! We all have our parts to play. Never before has mankind been faced with such a stark choice in future outcomes. May we learn to quickly evolve to overcome our motivation based on self interest only, that seems to have lead us to this situation with global warming. May we learn to include along with self interest a motivation to also do what is b

Ian Montanjees

est for the greater whole. May we come to know that we are all connected, part of one big family, whether we realise it or not (I can’t put petrol in my car without the emissions affecting someone’s weather in Africa), and come to know it is in our self interest to also do what is best for the greater good, for our economies and our civilisation are much more fragile than we might currently understand them to be, and their future survival depends on our present actions.

(As usual, my comments are offered to the best of my understanding at the time and are my opinion only.)

by Ian Montangees – www.whiteroofs.org.nz

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Tags: green design, green roofs, white roofs