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Questions and Answers Questions and answers will be added here as the project progreses. If a white roof cools my building in summer, will it also cool my building in winter and require my heating to use more energy? What energy savings might be typical from a white roof in New Zealand? What is the difference between white roofs and green roofs in terms of their positive contribution to a reduction in global warming? How much cooler is a roof with white paint? The evidence for global warming is very clear If a white roof cools my building in summer, will it also cool my building in winter and require my heating to use more energy? Answer - This is discussed here firstly for commercial buildings with air conditioning and then for houses without air conditioning. For buildings with air conditioning, if you have demand for cooling in summer and heating in winter, a white roof typically pays off in energy cost savings and in emission savings. The most rigorous study ever undertaken on this issue considered commercial buildings across the USA and found that savings are greatest in the hotter parts of the country (in the south of the country) and less in cooler parts (in the north near the Canadian border) but still there are savings. The study was by the engineers at the Heat Island Group, Lawrence Berkeley National Lab, California, (see hear for a link to it on Science & Case Studies). The study models four types of commercial construction (new/old retail and office), as well as local energy prices and local electricity emission factors. It gives results across the US for overall savings (summer cooling savings minus winter heating penalty), as well as emissions reductions. The study is for buildings with electric cooling and natural gas heating. It found that savings are greatest in the hotter parts of the country and less in the cooler parts but that even in these cooler parts there are still savings. The study estimates that immediately retrofitting 80% of commercial buildings in the USA would yield an annual energy-cost saving of $735 million. It would also offer an annual CO2 reduction of 6.23 Mega tonne, offsetting the annual emission of 1.2 million cars or 25.4 peak power plants. But what about white roofs on houses that do not have air conditioning? If a house is comfortable in summer and does not need air conditioning cooling then it seems a darker roof would make only marginal difference to the winter heating bill for the following reasons. - In winter the difference in temperature between a white roof and a dark roof is much smaller because the sun is less intense and at a lower angle. IWinter solar energy is typically only one third that in summer. - In winter there are far more cloudy days than in summer, and at those times the difference in temperature between a white and a dark roof may be small, and the days are shorter and significantly fewer sunshine hours than summer. If there is no sunshine on the house then it matters little what colour your roof is. - In winter the sun is at a lower angle in the sky, roofs are often in shade more, and houses have a greater proportion of sunlight on walls and windows and a lesser proportion on the roof. Many houses have eaves designed to shade the windows in summer when the sun is more overhead, and to let the sun through the windows in winter when it is at a lower angle. It seems the amount of solar heating coming directly through windows in winter may be the main source of winter heating. - In winter, most of the heating is in early morning and early evening, and at these times the sun on the roof, so at the times your house needs heating the most the sun is not there and it makes no difference what colour the roof is. - For houses in very cold climates where there is rooftop snow in winter, the roof cannot 'see' the sun and it does not matter what colour your roof is. - Scientists have measured that a white roof may actually make your house slightly warmer in winter at night because white roof doesn't radiate heat (from inside the house) as fast as a dark roof. Researchers at the University of Columbia were surprised to measure no winter heating penalty of a white roof compared to a black roof right next to it, in a real-world 2009 study, and they found the reason was that that the white roof slowed the rate of heat loss through the roof at night, compared to the black roof, because it had a lower emissivity. This study was in New York and was commissioned by Con Edison, click here for the article. On balance it seems that a white roof might have only marginal effect on heating cost and emissions in winter and that this is far outweighed by the contribution to cooling the planet by reflecting sunlight back into space. If a house has a dark roof then it's going to be hotter in the summer sun. A white roof gives you free cooling in summer which is why it has been used for thousands of years in Mediterranean and Middle East countries. What energy savings might be typical from a white roof in New Zealand? While there has been a rigorous energy efficiency study looking into the consequences of installing white roofs on commercial buildings across the USA, so far the results of this work have not been applied to New Zealand conditions. However, comparing the results for a location such as New York which has a climate similar in some ways to Auckland's, one can get an idea bearing in mind that Auckland does not have snow in winter for example. Studies show that commercial buildings in New York typically have positive savings in energy and emissions after installing white roofs, found by rigorous simulations and also in real-world measurement on actual roofs. By comparison, Auckland has more intense sunlight than New York and AUckoland's temperature differences between white roofs and dark roofs are measured to be greater in the summer sun, which implies greater air conditioning savings from white roofs. This is partly because the ozone layer in New Zealand is thinner and does not filter out so much sunlight, and partly because of our position on the Earth in relation to the sun. So from that point of view Auckland might have greater white roof savings than New York. Though other factors need considering as well, such as differences in typical NZ commercial building construction, insulation, roofing materials and slope, etc. The software as used in the US 2010 Energy Efficiency study is available to carry out such studies for buildings anywhere in New Zealand, and include for local building construction and climate database. It would be great if such a study were made applying the methods and results of overseas studies to NZ conditions. However, many businesses in New Zealand deliberately choose white roofs in the knowledge of the thermal benefits. What is the difference between white roofs and green roofs in terms of their positive contribution to a reduction in global warming? Together white and green roofs can achieve more for climate change mitigation than either by themselves Green roofs are roofs with vegetation. They cool by evaporation of water via the leaves of plants, and also from the soil itself. Cooling is highly dependent on plant density and rainfall. Phenomenal during wet summers (and dense planting)4,5 but poor during dry summers (and sparse planting)5. Significant winter heating savings when nighttime temperatures go below freezing4 . For a moderate climate like Auckland's, where summer rainfall is moderate and winter frosts are rare, the green and white roofs appear roughly the same on energy efficiency, and heat island reduction4. Green roofs require buildings strong enough to take the extra weight of soil and are expensive to install. However, they have other benefits such as biodiversity, amenity value, and holding back rainwater. In cities like New York where sewerage and stormwater are combined in the same drains, heavy rainfall often results in effluent overflows into rivers. However for a city like Auckland where the stormwater is being separated from the sewage drains, this may be less of an issue. Green roofs cool by relocating heat by evaporation into the atmosphere, but it still mostly remains in the system. White roofs cool the planet as a whole, reflecting sunlight back out of the system into space, and offsetting CO2. Cost/benefit decisions mean that more businesses choose white rather than green roofs But each type appeals mainly to different people for different reasons. White roofs + Green roofs -->> maximum roof area for climate change mitigation Like in New York where mandatory requirements allow choice between white or green There is an established trend for white roofs in New Zealand with well-known businesses having quietly used them for years to increase energy efficiency and manage heat loads long before it was known this also helps cool the planet. Most people are unaware of this trend but it potentially is a resource that can be amplified to convert a greater proportion of roofs for climate change mitigation. 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 offer different positive outcomes. Never before has mankind faced such stark choices in outcomes, and both of these roof types can play positive roles. How much cooler is white paint?  Measurements were taken of colour samples laid out in the midday sun in mid February 2011. White was 39ºC (blood temperature) while black right next to it was 74 ºC! You could cook an egg on black. The difference as they say is black and white! New unpainted galvanised iron was 63ºC, ie about the same as dark red. The temperatures of standard ColorSteel samples are also in the photo. The samples were mounted on a polystyrene sheet to simulate an insulated flat roof, and laid flat in the midday sun. Temperatures were read using an inexpensive non-contact infrared thermometer (see photo) which reads the temperature of a surface without having to touch it. The measurements were taken in mid February, at around noon, with a slight breeze present. If they were taken in December or January when the sun is more intense, then a higher temperature difference would be expected. The evidence for global warming is very clear  The graph (at right) shows the trend in the temperature of the globe over the past 1000 years up to the year 2000, published by the IPCC (Intergovernmental Panel on Climate Change). It shows that from the start of the industrial revolution there has been a sharp rise in in global temperature. This coincides with the records of rise in atmospheric carbon dioxide. Natural large variations in global temperature occur slowly over 100-150 thousand years between ice ages and are driven by small changes in the earths orbit around the sun, (since the orbits of all the planets vary slightly over time and they all exert a slight gravitational pull on each other in a cyclical way). However, the increase that we are now seeing in global temperature is occurring in the “blink of an eye” and 98% of climate scientists are in agreement that this is the result of humankind’s activities.Around the world alpine glaciers are in retreat, the polar ice sheets are melting by 475 cubic kilometers per year [ref 1] and faster than previously predicted. In the last ten years in particular we see effects of global warming often in the news with extreme weather events occurring more frequently and with greater magnitude, such as floods, droughts, record temperatures, and cyclones becoming the new “normal”. . The temperature rise is already about 1ºC above pre-industrial revolution levels and it is predicted that if goes over 2ºC we may see the start of tipping points which may lead to runaway global warming. A tipping point example would be the melting of the North Pole ice so that the sunlight falling in that area no longer gets largely reflected off the ice and back into space, but gets absorbed by the sea increasing the rate of global warming. It is said that the next 20 years are critical. The New Zealand Ministry for the Environment website has similar comments, (see here), and the Prime Minister’s Chief Scientific Advisor, Professor Sir Peter Gluckman, in his report on Climate Change says “If temperature rose by this amount (2ºC) then the scenarios become quite scary in terms of climate change, flooding of low-lying areas, new patterns of infectious disease, and reductions in the capacity of many parts of the world to support agriculture and therefore to support our continued existence as we know it.” See here for his complete report. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - |
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