The Best Colour To Paint Your Roof
Can we save the world with a tin of paint? If a report by the National Centre for Atmospheric Research is to be believed, the answer is yes. The research suggests that white or reflective roof coverings are the answer to stabilizing climate change.
The principle is simple enough. Dark coloured surfaces absorb more heat than reflective lighter coloured ones. Try wearing a black T-shirt on a sunny day. Roofs with black surfaces attract more heat.
The NCAR study used the world’s first computer module to determine the effects of climate change on urban areas. The computer module simulates how much solar energy is absorbed or reflected by roof coverings and road surfaces. It is important to determine how climate change will affect vulnerable city areas.
Urban Heat Island Effect
Highly populated urban areas are higher in temperature than the surrounding rural areas. This is due to the urban heat island effect. Man made constructions such as rooftops and roads have dark surfaces. 50-65% of urban areas are covered by roads or roofing. These artificial surfaces absorb more heat than lighter coloured surfaces. The heat from the sun is absorbed, and then released back into the air. This is particularly noticeable at night. As an example, London is 3 degrees warmer at night than the surrounding rural areas.
Cool or white roofs reflect more ultra violet rays, resulting in a cooler roof. The suns rays are directed back up into the atmosphere. This helps mitigate the urban heat island effect. There are further financial savings as well. Cool roofs lead to cooler buildings inside. This puts less drain on air conditioning units and saves on fuel bills. There is also the added benefit of reduced carbon emissions. In buildings with no air conditioning, cool roofs can reduce the temperature by three degrees.
Equivalent to removing every car from the road
The NCAR research points to some dramatic benefits if we were to change the colour of man made constructions. It suggests if we were to globally replace constructions with reflective surfaces, it would reduce green house gasses by 44 billion tonnes of CO2.
That is equivalent to removing every car in the world from the road, for eleven years. Cities the size of New York would be two degrees cooler on a summer’s afternoon.
There are further financial benefits to white roofs. As well as the money saved on air conditioning costs, the life of the roof will be extended. UV damage is one of the biggest reasons for early roof replacement. The suns rays’ damage bitumen and flat roofing surfaces. Reflective roof coverings are less susceptible to damage. The roof can last 50% longer before it needs to be replaced.
How much do cool roofs cost?
A reflective or white roof has no extra installation costs over that of darker coloured roofs. If white roofs are replaced as part of the natural re-roofing cycle, there is no additional cost.
Why don’t more buildings have cool roofs?
The initiative is growing. Backed by United States energy secretary Steven Chu, cities such as Seattle and Washington are now testing the theory. Further, the United States and Mexico have signed up to the cool roof working group. Secretary Chu said “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”.
Governmental departments that have already initiated white roofing have reported huge energy savings. The NNSA (an agency of the United States department of energy) have constructed over 2 million square feet of cool roofing. The net result is an energy saving of $500,000. This is projected to increase to a saving of Ten million dollars over 15 years.
Cool Roofs are not perfect
Of course cool roofs may not be right for every situation. Location and roof type are the determining factors. There is little point reducing the emissions of one roof only to have the heat deflect into the neighbouring building. Location within the urban area is important. A pitched roof structure surrounded by high rise buildings could actually divert UV rays into the adjacent properties. Moreover, the energy costs could actually increase in cooler climates. The cooling effect of the roof would require an increase in heating bills.
The future for cool roofing
Cool roof technology is still in its infancy. Is it possible to have a roof that changes colour according to energy needs? A black roof that absorbs free energy in cold weather, then magically changes white when things heat up would be the optimum. The truth is, we are nearly there.
Thermo Chromic paint is a relatively new development which could have a big impact on cool roofing. The paint uses liquid crystals to reversibly change colour at different temperatures.
As the temperature of the paint rises, the molecular structure changes. This allows it to emit light at different wavelengths according to the temperature. So a roof in a low temperature would remain black and absorb heat. As the sun heats up the paint would turn white reflecting heat. The paint is expensive and will need significant development before it is suitable for roofing. So for now, paint your roof white!