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Green Roof ROI Not for House-Flippers

| Thursday May 20th, 2010 | 4 Comments

Expert interviews and analysis of the latest studies indicate that plant-covered green roofs can save both individuals and society money over the long run, but seeing a net return on investment is a long-term proposition.

Green roofs can lower energy bills significantly, especially in the summer. Brad Rowe, a professor of horticulture at Michigan State University, said that savings can range from 7 to 50 percent, depending on the type of vegetation, the depth of soil, the local environment and other factors. “Every roof is different,” said Rowe.

There are other advantages as well. “A green roof is the last roof you need to put on your building,” said Professor Stuart Gaffin of Columbia University, who recently completed a study (PDF) of green and white, or high-albedo, roofs in New York City. The vegetation shields the top of the building from the sun’s UV rays, which degrade building material over time.

But installing a roof with its own mini ecosphere can cost more than twice as much as a traditional one, which makes narrowing the cost benefit gap with traditional roofs tough. Not only do green roofs cost more to install, they also cost money to maintain.  

A study (PDF) released in 2008 by Portland State University concluded that, based on the assumption that green roofs last 50-100 percent longer than traditional roofs, “it is likely that the amortized expenditures approach parity,” the study said.

Translation: a green roof can break even over the long term — 50 years or so. For shorter time spans, the ROI may not be quite there.

Another cheaper, environmentally-friendly roofing fix is a white, or high-albedo one. White roofs reflect back more of the sun’s energy, lowering summer cooling costs (they actually have a slightly detrimental effect on heating in the winter).

Gaffin said that white roofs cost a dollar or less per square foot to paint on, and achieve savings of about 13 cents per square foot a year, for an ROI of seven years. They also extend the life of a roof, though not as much as a green one.

White and green roofs also have externalized benefits, savings that are not realized by the owner, but by society at large.

The plant life on green roofs absorbs rain water, reducing the amount of waste water flowing into municipal sewer systems during storms. In New York City it has been estimated that a 50 percent green roof infrastructure would save the city $18 million per year in storm-water treatment.

In Germany, home owners are assessed a storm-water drainage fee, spurring the installation of green roofs there, according to Rowe of Michigan State.  “It’s like a toll road,” he said. “If you use it, you pay for it, if you don’t, you don’t.” He said some municipalities in the US are toying with the idea.

Both green and white roofs also reduce the urban heat island effect, and, because they reduce building energy demands, reduce carbon emissions as well.

Of course there are other benefits to a roof covered in lush green vegetation that cannot be so easily quantified. From the Portland State study:

Other potential benefits of green roofs, such as air/water quality improvements, commercial crops, wildlife habitat, recreational areas, improved esthetics and an overall improved sense of wellbeing clearly exist but credible monetary values have not yet been established.

Portland State offers a Green Roof Energy Calculator on its website, which can estimate energy savings.


▼▼▼      4 Comments     ▼▼▼

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  • Dan_DHRT

    ““A green roof is the last roof you need to put on your building,” “

    I would rephrase it to say 'A green roof is the last energy conservation investment you need to make for your building'. Sure, it will reduce the amount o fair conditioning needed in the summer and help keep heat inside in the winter because of the increased insullation it provides.

    However it is costly and you have to ensure your roof can take the extra weight.

    And it will eliminate the possibility of placing solar PV panels or solar water heating or solar air heating devices on the roof.

    There are many, many, many other energy steps a household can take which are simple to do, cost no money at all or just a little money before investing in this. There are many, many, many collections of such money savings and energy easily available.

    http://dailyhomerenotips.com/energy-conservation/

    This free collection, for example, contains more than 500 energy savings and clean water savings ideas for the home of which:

    400+ are simple and easy to do
    275+ cost absolutely no money to do
    115+ cost just a few dollars each to do
    115+ are ideas to reduce electricity usage in the home
    110+ are home heating savings tips
    80+ are home cooling savings tips.

    Yes, a 'green' roof is 'cool'. However, there are many, many other ways to go IMO.

    • Zach

      Actually, Roof Garden and Solar PV panels have a symbiotic relationship. All PV panels are rated at 25C (77F). For every 1C increase, they lose between 0.25-0.5% in efficiency. Roof Garden surface temps rarely reach more than 2C over ambient temperature, whereas even a reflective white roof will easily reach 20-30C over ambient temperature. I have seen several instances where a gain in output of 8% is achieved by combining these technologies. This is a common practice in Germany.
      On the intangible side, roof gardens make the building very, very quiet which is a huge bonus. ROI is fairly long but I have seen ROI’s under 5 years when one gets close to the equator and the cooling savings approaches 50% for 10 months out of the year.
      Zach

  • Thomas

    Green roofs and Solar Photovoltaic Panels go together perfectly on Roofs. The Green Roofs lower the Roof Surface Tempature which is beneficial to Photovoltaic Systems because they produce more energy when the Temperature does not Exceed 95 degrees. Combining a Green Roof with Solar Energy Systems can boost Energy Production by up to 20% with the lower Roof Temperatures.
    Germans do it all the time, they have Green Roofs and Solar Photovoltiac System Combinations both on Houses and Commercial Buildings. Over 20% of all German Buildings have Green Roofs while many other European Countries are Expanding Green Roof Programs.
    The overall Cost for a Green Roof in Germany is less then America because the Industry in Germany is mature so the costs have come down with so much business while in America it has just started and we have not reached the the proper Industry size to lower costs. The more Roofs and companies involved, the lower the prices like all businesses. Niche Products cost more then massed produced products so once the proper laws are passed for Green Roofing to Thrive costs will come down with the additional business and it will be more then worth putting a Green Roof on Buildings and some homes.

  • Thomas

    Green roofs and Solar Photovoltaic Panels go together perfectly on Roofs. The Green Roofs lower the Roof Surface Tempature which is beneficial to Photovoltaic Systems because they produce more energy when the Temperature does not Exceed 95 degrees. Combining a Green Roof with Solar Energy Systems can boost Energy Production by up to 20% with the lower Roof Temperatures.
    Germans do it all the time, they have Green Roofs and Solar Photovoltiac System Combinations both on Houses and Commercial Buildings. Over 20% of all German Buildings have Green Roofs while many other European Countries are Expanding Green Roof Programs.
    The overall Cost for a Green Roof in Germany is less then America because the Industry in Germany is mature so the costs have come down with so much business while in America it has just started and we have not reached the the proper Industry size to lower costs. The more Roofs and companies involved, the lower the prices like all businesses. Niche Products cost more then massed produced products so once the proper laws are passed for Green Roofing to Thrive costs will come down with the additional business and it will be more then worth putting a Green Roof on Buildings and some homes.

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