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Con Edison ‘Cool’ Roofs Reduce Energy Costs

Con Edison’s green and white roofs atop of its training and conference center in Long Island City help prevent energy losses, provide other environmental benefits, and reduce heating and cooling costs, compared to traditional dark roofs, according to research from Columbia University.

A key finding of the study (PDF) shows that the green roof, consisting of 21,000 plants, on The Learning Center reduces summer heat gains by up to 84 percent and winter heat losses by up to 37 percent, compared to a black roof. The white roof reduces summer heat gains by up to 67 percent. NWC says these figures represent only the reduced amount of heat flowing through the roof, not a building’s energy consumption.

This translates into an estimated annual cost savings of $330 to $350 for heating and $225 for cooling, according to the study.

The green and white roofs also prevent “heat island effect,” where dark roofs absorb sunlight during the day and radiate heat back into the atmosphere at night, say researchers.

Con Edison has installed nearly 250,000 square feet of white roofing, including on its Manhattan headquarters, and has plans for at least another 220,000 square feet by the end of 2010.

The energy provider commissioned a study by Columbia scientists to measure temperatures and other data on the green, white and dark sections of the training center’s roof. The goal was to determine the environmental and energy-consumption performance of each type of roof as well as the cost-benefit estimates for energy conservation.

While green roofs are more energy efficient and offer benefits such as water runoff control and noise reduction, it is more expensive than a white roof, according to the study.

During the next phase of the study, researchers will measure the quantity and quality of the water runoff from the green roof and compare it to the control roofs. Excess storm water runoff in urban areas leads to combined-sewage-overflows that pollute surrounding waterways, according to researchers.

Similarly, the University of Oklahoma National Weather Center (NWC) plans to evaluate its recently installed Experimental Green Roof as potential contributors to green urban development.

The green roof, located on NWC’s sixth-floor outdoor classroom, comprises 1,280 square feet and consists of 160 planted green roof trays. NWC says vegetative or green roof systems cool structures during the summer months, and reduce annual storm-water runoff as well as pollutants in the storm-water effluent.

The state’s first experimental university vegetative roof system will be evaluated for energy and water efficiency and water quality. As part of the primary evaluation, researchers will study plant performance, changes in the radiation balance, cooling efficiency for various climate conditions and impacts on building day lighting.

A second phase of the project is scheduled to begin later this year and will add 220 square feet to the existing green roof. The roof will remain at NWC for up to three years for investigation.

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5 thoughts on “Con Edison ‘Cool’ Roofs Reduce Energy Costs

  1. In the undated report A Temperature and Seasonal Energy Analysis of Green, White, and Black Roofs, there is no measurement of any energy savings. There is no basis for the estimated annual cost savings of $330 to $350 for heating and $225 for cooling, or a total estimated savings of about 5¢ per square foot out of a likely total of several dollars per square foot per year.

    Measuring temperatures at various surfaces in the roof sandwich is not a valid method for determining or estimating energy use. Internal heat gains from lighting, people, and equipment means that to at least some extent, buildings are self-heating. So long as the indoor temperature remains reasonably constant, whether from internal heat gains or from an intentional space heating system, measuring temperatures as you did can be meaningless.

    For example, take a day when the indoor temperature is 75F and the outdoor temperature is 0F. Assume the interior surface temperature of the roof is about 75F and the exterior surface temperature is about 0F. Those assumptions would indicate a heat loss from the space. However, it is a heat loss only if the internal heat gains in the space are not sufficient to provide the amount of heat required to keep the space comfortable.

    If the lighting is 1.1 watts per square foot, the lighting heat gain alone is 3.75 Btu per square foot. Under the current energy code in New York and most other states, the allowable lighting for schools is 1.2 watts per square foot, or a heat production of 4.1 Btu per square foot. If the roof (of any type) has insulation of R-20, the heat loss at 75F inside and 0F outside is 3.75 Btu per square foot. Yes, when the space is unoccupied and the lights are off, some lesser amount of heating energy may be needed. Thus, at 0F outdoors there is no purchased heating energy required. Yet, the measurements made and the conclusions in the paper would indicate a heating requirement when there is none.

    Therefore, I conclude the results and conclusions in this paper are fatally flawed because the scientific method used does not account for the practical every day interactions in buildings. To one degree or another, most buildings are inherently self-heating. The only questions are how much and for how long. The measures that claim to reduce heat flow through the roof assembly do not necessarily reduce energy use. Just because there is heat flow through a roof does not mean the heating energy is purchased.

    If anything, the reduced heat flow through green roofs can be counterproductive and increase energy consumption for cooling. This happens when a roof with low heat flow traps internally generated heat in the building, causing more cooling and cooling energy to be used, compared with a high heat flow that allows the internally generated heat to flow out of the building.

    When internally generated heat is trapped in the building due to insulating values, and cooling is required, during many hours of the year that cooling is provided with no energy expenditure by using economy cycle or free cooling, which is also required by the current energy code in New York and most other states.

    Finally, I am not aware of any peer reviewed, measured experimental data showing substantial changes in roof membrane life by virtue of color or green, contrary to the estimate in the paper of two to three times the life for green roofs. Even if the roof life might be extended, one must consider and evaluate the additional cost for both routine maintenance over the years and the substantial premium cost for roof replacement when that becomes necessary.

    In conclusion, just because there is heat flow through a roof does not mean that energy is purchased for the purpose of compensating for that heat flow.

  2. Direct solar insolation is of the order of 1000 watts per square meter. At a average angle of incidence throughout the day, we might cut that value by two thirds for a reasonable estimate of power incident on the horizontal roof surface.

    So 333 watts per square meter is equal to 32 watts per square foot. That’s an order of magnitude higher than the heat generation numbers that Larry Spielvogel worries about above. And exterior roof surface temperatures can be upwards of 150 degrees for black roofs as a direct result. For white or green roofs, the exterior surface temperature is greatly reduced.

    So during the cooling season, the advantage of a green or a white roof can be considerable, in terms of reduced energy spent to cool the interior space.

    During the heating season, a white roof radiates less heat energy in the form of infrared radiation than does a black roof. So the temperature difference between the interior and exterior roof surfaces is less for the white roof. Again, heat flow through the roof is reduced.

    I am aware of few buildings that require no energy consumption to heat interior spaces to comfortable temperatures in the heating season. Other interior heat sources like lighting and people are generally not sufficient to maintain interior temperatures alone. Therefore, green and white roofs very likely save energy during the heating season as well.

  3. Having read many such articles and being highly involved in roofing design and specification some questions on the truth behind all the numbers arise. Has anyone considered the energy and labor cost involved in the increased maintenance of the roof surfacing? Green roofs will need weeding etc throughout the growing season to prevent the roof from becoming a fire hazard. White surface roofs in many areas will need frquent cleaning to maintain the reflectivity. These jobs are typically performed by maintenance people in older low efficiency pick-up trucks. Extra insulation and using the best grade of roof material available with a competent applicator is much more sustainable and cost efficient

  4. White roofs need virtually no cleaning. Yes, the reflectivity suffers somewhat, but usually not enough to warrant the cleaning effort. And re-coating for a white roof is not needed much more frequently than for a black one. And the considerably lower summertime temperatures experienced by a white roof mean that it ages much more slowly than an equivalent black roof would.

    I don’t know about the maintenance needs for green roofs.

  5. Again we are reinventing the wheel in the US … while vegetated roofs have been common in other parts of the world for decades!
    Instead of being fire hazard, green roofs help prevent fires by cooling the roof due to taking up large amounts of latent heat for the evaporation of stored water in the soil before it finally CAN catch on fire!!
    It depends on the types of plants, if the correct plant mix for the particular location is chosen, NO maintenance is necessary.
    Additionally to the energy savings that can be realized by green and white roof surfaces, the reduced maintenance due to considerable lowered temperature extremes can be large and greatly extent the life of roof systems and structures!

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