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Q&A Spotlight

Do Green Roofs Temper Urban Heat?

Vegetated roof surfaces make compelling green PR, but how do they compare to ‘cool roofs’ in reducing the heat island effect?

A vegetated roof reduces rainwater runoff and reflects less heat than a conventional dark roof, thereby lowering air temperatures in urban areas. But the beneficial impacts of vegetated roofs vary by region.
Image Credit: Creative Commons license / Flickr

Luke Morton sits on a green building committee that’s been asked to advise local officials on a green building code. The code will feature both mandatory and elective features. One of the electives currently on the list is for a “green,” or vegetated, roof, but Morton has his doubts whether the case for this type of roof is very compelling.

A green roof consists of soil or some other planting medium and living plants installed over a waterproof membrane. Two benefits typically attributed to a green roof are the ability to control water runoff and, more important to Morton, the fact they contribute less reflected heat to ambient air temperatures than conventional dark roof coverings.

But, Morton asks, how much less?

Writing in a Q&A post at GreenBuildingAdvisor, Morton looks for guidance on whether the case for a vegetated roof is strong enough to justify its place in the green building code, and whether other roofing strategies can be just as successful at a lower cost.

“There a bit of good discussion already on this forum about green roofs,” Morton says. “I note that the benefits cited by advocates are usually poorly defined, or can only be ascribed to specific green roof compositions in certain climates. And there’s vigorous counterpoint about whether or not the amenities of stormwater management and energy efficiency could not be better and more cheaply provided by more appropriate materials and assemblies. (I’m largely convinced by these views).

“So, the question is actually taking on the last ‘benefit’ of green roofs, and wondering if it can be shown to be better than a cool roof at lowering the urban heat island effect,” he continues. “I’ve narrowed the question a bit by saying I’m thinking about a hot/dry climate (e.g. many parts of California).”

That’s the question for this Q&A Spotlight.

Green roofs have higher maintenance costs

Conventional roofs don’t require a lot of maintenance or attention after they’re installed, points out Stephen Sheehy, while a vegetated roof has to be watered and weeded. And it almost certainly costs more to install a green roof, in part because the added weight of the soil and plants may require stronger roof framing.

“Any roof leak is usually hard to find,” Sheehy adds, “but a leak in a roof covered with soil and plants would be harder to find and much harder to fix. Do a white roof and grow your pot inside like everyone else.”

A green roof also has a lower albedo than a cool roof, says Dana Dorsett, meaning it reflects less solar energy back into space than reflective roof materials. But it still does a better job than typically dark roofing materials and, he adds, “evapo-transpiration is your friend.”

“The evapo-transpiration benefit makes a real difference on the heat island effect, and it can even trump the high albedo of a titanium-white cool roof finishes (particularly in dry climates),” Dorsett writes. “But of course it comes with water and maintenance costs that the cool roof doesn’t have.”

Dorsett cites two studies, a report on green and cool roofs from the Natural Resources Defense Council, and a research paper published by the International Building Performance Simulation Association.

The watering question

If green roofs help lower air temperatures through the process of evaporation, the plants still need to be watered, and in a dry climate that’s a drawback, Sheehy says.

“I’d be concerned you’d need to water the roof to keep the plants alive and in a dry climate, you’d be fighting against evaporation and need to water even more to get the benefit of any cooling,” he says. “Sprinklers on the roof? I guess you could use drought tolerant-plants, but that sounds really pointless.”

Drought-tolerant plants need less water, but Morton admits that people don’t always like the way they look.

“Of course, most of my projects have those,” he says, “but they still manage to vastly overwater them. It turns out that people think drought-tolerant plants are ugly when they actually experience a drought, so people run their irrigation systems to make sure they don’t.”

Morton also has unearthed some research of his own in the form of a report from the Lawrence Berkeley National Laboratory. It suggests that both green roofs and white (cool) roofs are effective in cooling city air, but that white roofs are three times more effective at countering climate change.

Dorsett adds: “Green roofs are both more expensive and much higher maintenance than cool roofs, which is why I’ve never been a big fan. If rooftop gardening or urban rooftop farming floats your boat, great — have at it! But for the other 99% of the population, a cool roof would be a better choice.”

Is there such a thing as too much evaporation?

Green roofs help to lower air temperatures through evaporative cooling, which makes Morton wonder whether a large concentration of vegetated roofs would add noticeably to humidity levels.

“I still have one question remaining that I believe is beyond this forum,” he writes. “At what point does extensive deployment of well irrigated green roofs actually start measurably effecting the local relative humidity and turn into the urban ‘latent’ heat island effect?”

Chris M isn’t sure it’s possible to answer that question precisely, but he doubts that a wide scale deployment of green roofs would create a humidity problem.

“I doubt you could move the needle very much where it would be noticeable, since humidity levels are regional,” he says. “Cities just aren’t that big and the air can only hold so much moisture at a given temperature.”

More effective at controlling runoff

Because a green roof is able to absorb a certain amount of water during a storm, its real benefit may be in helping to control water runoff and lowering demand on city storm drains and sewage treatment plants, one GBA reader suggests.

“I studied them a bit as I wanted to be able to use one on my personal addition and found the concept wider than I anticipated, varying from underground parking garage [like under the Boston Common] to lightweight trays on the roof systems,” writes Keith Gustafson.

“I see green roofs more from the perspective of water control than energy efficiency,” he continues, “meaning in an urban area if all of the roofs could handle the first 1/4 inch of rain it would have a huge impact on water control in the city, and if it is a positive in other aspects — awesome.”

Agreed, adds Charlie Sullivan, who sums up the situation this way: “If you have problems with stormwater management, the green roof helps; if you have problems with water shortages, it hurts.”

To Morton, however, stormwater management isn’t the key issue when it comes to roof design. All projects already are required to manage water on site.

“This forum has elicited enough information to convince me that the policy I should propose is indeed a prohibition on green roofs,” Morton writes. “(Don’t worry — this proposal won’t win.) The first key factor is water: the city in question is in a deep drought, and they are really trying to move towards banning landscape irrigation now and in the future.

“Secondly,” he continues, “the city is among those that is trying to be as aggressive as possible in demonstrating its leadership towards climate change. Green roofs come with an opportunity cost of having a higher albedo cool roof, which is already required locally (though high-mass roofs are exempted).”

Our expert’s opinion

Here’s what GBA technical director Peter Yost has to add:

It won’t surprise folks on GBA that has quite a bit of information on the various aspects of green roof performance. It also won’t surprise any GBAers that the answer depends quite a bit on climate, both from the perspective of energy and water.

The stormwater retention performance of green roofs varies a lot with the type of green roof system being used. In studies being conducted at the University of Texas-Austin, some green roofs did only marginally better at stormwater retention than traditional membrane roofs.

While vegetated roofs have a much bigger impact on keeping roofs cooler and shifting the peak temperature than white roofs, white roofs have a significant impact compared to darker membrane roofs.

White roofs have clear and significant energy benefits in Climate Zones 1-3, but the further north you go, the more those benefits become eroded by increased loads during the heating season, according to an article posted at Environmental Building News.

An interesting note: The province of Almería, Spain, is said to be the only human settlement that can be seen from space, because of its gleaming white reflective greenhouses. The farmers whitewash their greenhouse roofs with slaked lime every June to keep out heat during the summer months, and the lime is later washed away to capture the winter sun for heat. These reflective greenhouses create roughly 100 square miles of high-albedo surface area (Environmental Building News, July 2014).

Whitewashed greenhouse roofs reflect summer heat. The coating washes away later in the year so the winter sun can be captured for heat.

The drought-tolerance of vegetated roofs depends on the type of green roof system, soil depths, and plant species. Intensive roofs with a much greater variety of plants (including trees) require much deeper soil depths and hence much greater load design for the roof (up to 150 pounds per square foot).

Vegetated roofs, especially in hot-dry climates, need a year or two of irrigation to become fully established, and during droughts will almost certainly need some level of irrigation. This does not necessarily mean they aren’t worth the irrigation they need.


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