The Debate over Green Roofs
The Urban Green Council (New York City Chapter of U S Green Building Council) held a debate on the issues of Green Roofs earlier this week. Three panelists were invited to give their view points as well as share their own experiences with Green Roofs.
The term Green Roof refers to the use of live plant material placed on a roof over a prepared surface instead of standard roofing materials. In general, many people find them not only attractive and aesthetically pleasing, but they are thought to have other benefits as well. These include the control of storm water run-off, reduction of heat island effect and energy savings.
First, Signe Nielsen (Principal, Mathews Nielsen) gave a brief history of green roofs. We were told that they go back almost as far as recorded history, in that they were used in Mesopotamia for the purpose of heat control, and to mimic the effects of mountains. During the Middle Ages, monks used them for the purpose of raising herbs, and during the Renaissance era, grand roof-top gardens were planted. New York City began to see them used during the later 1800's in hotels and theaters. The first use of a green roof to aid in the Return of Investment on a real estate project in NYC was probably at Rockefeller Center, where a grand "garden in the sky" was planted in the late 1930's in an attempt to drum up rentals during the Great Depression.
There are two types of green roofs used today. They are: Extensive and Intensive. An Extensive green roof has a soil depth of less than six inches and is usually planted with drought tolerant plantings, while an Intensive green roof will have a greater soil depth, and will often be planted with shrubs and occasionally, trees. In both cases, care needs to taken to see that a proper membrane system and leak-proofing is installed. In addition, attention MUST be taken to deal with the increased weight load on the building.
The roofs may be modular or continuous in nature. Modular roofs have the advantage of being able to removed in sections should a problem with roof substrate develop.
There are alternatives to green roofs. These include Cool Roofs and Blue Roofs. Cool roofs involve the use of a high albedo (light colored) roof material or paint. This will also reduce Heat Island Effect, while a Blue Roof is a system that is comprised of small retention basins integrated into the roof system for the purpose of controlling the release of storm water.
While green roofs have been shown to save on energy costs, the pay-back can be as long as 30 years in some cases. The same is true for storm water control. A major benefit seen with green roofs is the ability to provide, especially in an urban location, a direct connection to the outdoor environment.
Green roofs have been clearly shown to reduce heat island effect, as well a lower the temperature of the surrounding area. This is due to the release of moisture from the growing plant material. They also keep building cooler inside during the summer, and provide up to an R-20 insulation factor during the winter months.
Cool Roofs, while not providing additional insulation effect, can reduce heat island effect and greatly offset CO2 emissions, while having NO incremental cost added on to the building project.
As mentioned above, Return on Investment is often a motivating force in the use of a green roof. They have been found to increase real estate values, as people respond positively to the natural beauty of a green roof versus the stark appearance of a normal roof. In addition, even a cool roof (with its high reflective value) can cause glare issues for neighboring buildings.
Heat Island Effect and Storm Water Control are major driving factors in green roofs, but there is plenty of arguments as to whether they are really cost effective in this case. In addition what works well in NYC may NOT work in Toronto, Canada.
The three respondents were: Leslie Hoffman (Executive Director of Earthpledge) Dr. Stuart Gaffin (Associate Research Scientist, Columbia University) and Dr. Hashem Akbari (Professor, Department of Building and Civil, Environmental Engineering, Concordia University).
Dr. Akbari mentioned the learning curve involved here. While green roofs are not new, they have not been evaluated over a period of time for true return on investment versus other methods, such as cool and blue roofs. Leslie Stuart mentioned the storm water runoff being reduced by at least 50 percent, and soil being far safer than static standing water. She also felt that green roofs do a better job at controlling heat island effect, and that green roofs save up to 85% on energy costs, while white (cool) roofs only contributed a 50% savings during the summer. While the R-20 insulation factor has been mentioned above, this does not seem to be the real issue for a green roof, as this can be done from under the roof at far less cost.
The issue of Cost Benefit Analysis was discussed. Leslie Hoffman said that storm water control may not be cost-effective with a green roof, BUT real estate values will be greater. She said that the intended use of the building will often be the deciding factor as to whether a green roof will make sense. Dr Akbari said that solar panels are also competing for space here,and that cool roofs have NO incremental added cost to the building. Dr. Gaffin said that since 1938, green roofs have seen favorable response in NYC.
Regarding the science of green roofs, Dr Akbari brought out the point that newer technologies have made inroads into the benefits of green roofs, and at a lower cost. One ton of CO2 emissions can be reduced for every 100 SF of cool roof area. The moderator Laurie Kerr (NYC Mayor's Office of Long-Term Planning and Sustainability mentioned that green roofs reflect indirect infrared energy better than other types of roofs, and lower the nearby air temperature due to the release of moisture from the live vegetation. This effect is NOT seen with a white (cool) roof.
Dr. Gaffin said that the future of green roofs in NYC will be a patchwork, as they are dependent on specific building client needs and budget requirements.
The term Green Roof refers to the use of live plant material placed on a roof over a prepared surface instead of standard roofing materials. In general, many people find them not only attractive and aesthetically pleasing, but they are thought to have other benefits as well. These include the control of storm water run-off, reduction of heat island effect and energy savings.
First, Signe Nielsen (Principal, Mathews Nielsen) gave a brief history of green roofs. We were told that they go back almost as far as recorded history, in that they were used in Mesopotamia for the purpose of heat control, and to mimic the effects of mountains. During the Middle Ages, monks used them for the purpose of raising herbs, and during the Renaissance era, grand roof-top gardens were planted. New York City began to see them used during the later 1800's in hotels and theaters. The first use of a green roof to aid in the Return of Investment on a real estate project in NYC was probably at Rockefeller Center, where a grand "garden in the sky" was planted in the late 1930's in an attempt to drum up rentals during the Great Depression.
There are two types of green roofs used today. They are: Extensive and Intensive. An Extensive green roof has a soil depth of less than six inches and is usually planted with drought tolerant plantings, while an Intensive green roof will have a greater soil depth, and will often be planted with shrubs and occasionally, trees. In both cases, care needs to taken to see that a proper membrane system and leak-proofing is installed. In addition, attention MUST be taken to deal with the increased weight load on the building.
The roofs may be modular or continuous in nature. Modular roofs have the advantage of being able to removed in sections should a problem with roof substrate develop.
There are alternatives to green roofs. These include Cool Roofs and Blue Roofs. Cool roofs involve the use of a high albedo (light colored) roof material or paint. This will also reduce Heat Island Effect, while a Blue Roof is a system that is comprised of small retention basins integrated into the roof system for the purpose of controlling the release of storm water.
While green roofs have been shown to save on energy costs, the pay-back can be as long as 30 years in some cases. The same is true for storm water control. A major benefit seen with green roofs is the ability to provide, especially in an urban location, a direct connection to the outdoor environment.
Green roofs have been clearly shown to reduce heat island effect, as well a lower the temperature of the surrounding area. This is due to the release of moisture from the growing plant material. They also keep building cooler inside during the summer, and provide up to an R-20 insulation factor during the winter months.
Cool Roofs, while not providing additional insulation effect, can reduce heat island effect and greatly offset CO2 emissions, while having NO incremental cost added on to the building project.
As mentioned above, Return on Investment is often a motivating force in the use of a green roof. They have been found to increase real estate values, as people respond positively to the natural beauty of a green roof versus the stark appearance of a normal roof. In addition, even a cool roof (with its high reflective value) can cause glare issues for neighboring buildings.
Heat Island Effect and Storm Water Control are major driving factors in green roofs, but there is plenty of arguments as to whether they are really cost effective in this case. In addition what works well in NYC may NOT work in Toronto, Canada.
The three respondents were: Leslie Hoffman (Executive Director of Earthpledge) Dr. Stuart Gaffin (Associate Research Scientist, Columbia University) and Dr. Hashem Akbari (Professor, Department of Building and Civil, Environmental Engineering, Concordia University).
Dr. Akbari mentioned the learning curve involved here. While green roofs are not new, they have not been evaluated over a period of time for true return on investment versus other methods, such as cool and blue roofs. Leslie Stuart mentioned the storm water runoff being reduced by at least 50 percent, and soil being far safer than static standing water. She also felt that green roofs do a better job at controlling heat island effect, and that green roofs save up to 85% on energy costs, while white (cool) roofs only contributed a 50% savings during the summer. While the R-20 insulation factor has been mentioned above, this does not seem to be the real issue for a green roof, as this can be done from under the roof at far less cost.
The issue of Cost Benefit Analysis was discussed. Leslie Hoffman said that storm water control may not be cost-effective with a green roof, BUT real estate values will be greater. She said that the intended use of the building will often be the deciding factor as to whether a green roof will make sense. Dr Akbari said that solar panels are also competing for space here,and that cool roofs have NO incremental added cost to the building. Dr. Gaffin said that since 1938, green roofs have seen favorable response in NYC.
Regarding the science of green roofs, Dr Akbari brought out the point that newer technologies have made inroads into the benefits of green roofs, and at a lower cost. One ton of CO2 emissions can be reduced for every 100 SF of cool roof area. The moderator Laurie Kerr (NYC Mayor's Office of Long-Term Planning and Sustainability mentioned that green roofs reflect indirect infrared energy better than other types of roofs, and lower the nearby air temperature due to the release of moisture from the live vegetation. This effect is NOT seen with a white (cool) roof.
Dr. Gaffin said that the future of green roofs in NYC will be a patchwork, as they are dependent on specific building client needs and budget requirements.
Posted by Alan L. Englander at 11/7/2009 6:40 PM 
Categories: Energy Savings, Storm Water Control, Intensive, Cool Roofs, Return on Investment, Extensive, High Albedo, Heat Island Effect, Blue Roofs
Categories: Energy Savings, Storm Water Control, Intensive, Cool Roofs, Return on Investment, Extensive, High Albedo, Heat Island Effect, Blue Roofs
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