Green Roofs: How Much do They Really Conserve?

Green roofs are often billed as a beautiful way to reduce a building’s energy cost and fight climate change. While they may be a physically attractive addition, some argue their usefulness as a tool to reduce energy consumption is exaggerated.

For the past few decades, green roofs have surged in popularity. They take up an ever-growing percentage of city rooftops and are frequently encouraged by municipal governments. In New York City, for example, legislation passed in 2008 provides a tax abatement of $4.50 per square foot for a year. This incentive and increased attention to environmental stewardship has helped build a market for green roofs. Over a dozen companies that specialize in or provide green roof design services have opened in New York City in the past ten years, exemplifying this shift.

Why are green roofs beneficial? How have they been so successfully marketed to such a wide variety of audiences? Green roofs were designed to provide a sort of insulation for the building they cover, keeping them cooler in the summer and warmer in the winter. During times of intense heat, a standard dark colored roof will heat up quickly and in turn raise the temperature of the building under the roof. The cooling systems in turn must work harder to maintain a standard temperature. Of course, this immense cooling requires more energy.

For years, green roof advocates have argued that green roofs can reduce this kind of wasteful energy consumptions, thus lowering the carbon footprint of the building they are on top of. Proponents claim that green roofs’ steep upfront cost reduce a building’s energy consumption and even pay off financially in the long run.

However, some climate change activists argue that the benefits of green roofs have been largely oversold. Tom Sahagian spent over thirty-five years of his career conducting energy audits of mostly residential buildings and advised his clients against building green roofs. His job was to look for ways for owners to reduce energy costs and consumption, but he didn’t think green roofs were the way to do it. Sahagian contends that while green roofs can be enormously useful for things like helping manage excessive storm water, they are not the best tool for reducing energy consumption.

“They’re a wonderful idea but it’s important to make a well-informed decision,” Sahagian said about green roofs, “don’t do it if you think it’s going to save energy in your building.” Sahagian, though in the minority opinion within the environmental community, has spent years arguing this point. Much to his chagrin green roofs remain popular and are marketed as a way to reduce energy consumption.

According to Sahagian, green roofs are not as insulating as many think for a few reasons. In a typical multifamily building, “the proportion of the building envelope that is the roof is pretty small,” he explained. Even if energy is escaping through the roof, insulating such a small portion of the building will not have a huge effect on the overall amount of energy lost. Additionally, between the top floor ceiling and the roof of many buildings, there is some open space that can range from a few inches to a few feet. On the sides of the building there are frequently air vents into this open space. This means that any insulation that green roofs can provide is essentially negated because the air they insulate is not in the core of the building, the part that is heated and cooled.

In buildings not structured like this, Sahagian said he saw more potential benefits for green roofs keeping things cool in the summer months. However, he noted that “it would be much more effective to paint the roof a reflective color,” referencing initiatives like New York City’s Cool Roofs program. Other ways to reduce energy consumption may be less exciting but more effective than green roofs. Things like eliminating drafts so the heat isn’t overused in the winter, for example, can go a long way.

Though Sahagian wrote off green roofs for reducing energy consumptions, he admitted that they could be hugely useful in managing storm water in cities like New York. Since so much of the city is paved over, little water can be absorbed directly into the ground the way it is in a more natural environment. More green roofs entail a greater proportion of space that can absorb water, effectively taking some stress of the sewer system.

It is not surprising that green roofs are as popular as they are, but perhaps their popularity should be examined more carefully. Green roofs can seem to someone considering installing one like the win-win way to fight climate change. While many other popular ways to fight climate change involve personal burdens, green roofs do not. Cutting out meat from a diet or switching to public transportation, though reducing energy use come at a personal cost. Installing a green roof (when financially feasible) is only beneficial; no one ever complained about having an extra garden in a city with minimal green space. Green roofs can be a pleasant addition to a building and will help a city manage water, but installing them may not be the best way to reduce energy cost.