News from the Columbia Climate School

Solving Urbanization Challenges by Design – The Science of Green Roofs (part 2)

Continued from Part 1

Patricia J. Culligan is a professor of civil engineering and engineering mechanics at Columbia University and the Vice Dean of Academic Affairs for Columbia Engineering

In part one of this interview Culligan discussed her her work with the Columbia University Green Roof Consortium to quantify the ecological benefits of green roofs.

In part two she talks about the challenge of quantifying the economic benefits of green roofs, the potential for rooftop agriculture, and what it means to “solve urbanization challenges by design.”

Do you also take into account things like building insulation and the heat island effect?

Wade Gillis and Stuart Gaffin have been working on that. Stuart’s recent results indicate that, from the perspective of building insulation, they might not be cost effective; the R-value of a greenroof can be matched by the R-value of modern roofing materials.

But then they started to focus on is the impact the green roof might have on the HVAC handling system. If you could locally reduce the temperature the HVAC is pulling against, you could have significant energy reductions from that. So the research group is trying to quantify that right now. We suspect that from the HVAC energy perspective, greenroofs will provide a benefit. But from the perspective of insulating the building, not so much. However, the claim is that green roofs will last longer that most modern roofing materials, so that there’s potential cost-saving there.

Wade and Stewart are also looking at evapotranspiration, and measuring that both directly and indirectly. We can do a water balance where the unknown is the ET. Wade also has an instrument that literally is a dome, and every now and again it traps air and can measure the vapor pressure, etc. So he’s been measuring the breathing of the roofs. It’s quite interesting; the ET data could go into urban heat island calculations.

We’ve recently instrumented a roof that isthe size of a city block. Thus, the roofs that we’re instrumenting are getting big enough to measure the impacts of evapotranspiration on local air temperature directly, as opposed to modeling this effect, which is what we had been doing.

We’ve also been very interested in air quality issues, whether green roofs do actually trap particulate matter. We’ve made some initial measurements on one of the bigger roofs we’ve instrumented at Columbia to explore this.

The question is, if you have the capacity to put these systems up, do you cluster them? Or do you have one per block? What’s the spatial strategy? But obviously that’s also going to depend on what you want to achieve.

We have got to the point where, as a research group, I think we’re considered fairly unique in our capacity to cover all of these various attributes for green roof systems. I don’t know that we’re necessarily advocates for green roofs. We’re more interested in properly quantifying what the benefits of this technology is.

An instrumented green roof on a Con Edison building in New York City.

Things can get faddish – it’s good to have the science to back up claims.

That’s how we got the NSF funding, actually. Everyone opens whatever paper they’re writing and says: “green roofs do a-b-c-d-e. ” We basically argued that if you trace back a lot of the claims that are made about green roof performance, they’re in gray literature that is not well peer reviewed. But, nonetheless, these claims get perpetuated.

And then there’s trying to understand the performance metrics of the different systems. We’ve actually found that from the perspective of stormwater management, the mat system that Columbia has installed, which is the lightest and the cheapest, seems to actually be comparable to the tray system. And we think it’s because the trays are almost like the small box systems we set up, so that the water goes down, and travels underneath the drainage mat, as opposed to having to travel some distance horizontally through the greenroof growing medium itself.

So the other thing that people are not necessarily asking is well, I wish to install a green roof, but I have a suite of options – which is better? The natives outperform any system so far. But they’re more expensive.

What about growing food or rooftops? There’s a lot of talk of urban agriculture these days. Is there a potential added benefit there, or not really?

I don’t really know. It would be interesting to figure that out. The Urban Design Lab has been doing some very exciting initial work in this areato estimate the capacity for urban agriculture in New York City. In some neighborhoods roofs are 13 to 35% of the land area. So strategies like rooftop agriculture would factor into such an estimation. At the site of the native green roof system that we instrumented in Brooklyn, the owner does grows tomatoes on the roof. So, yes, it’s feasible. It’s just not the focus of our work.

Matt Palmer is very interested in ecosystem restoration in urban environments, and he’s studying many of the native plant systems. As part of this study he’s also interested in the role that greenroofs might play in supporting bee and insect life; he’s not soley interested in horticulture. I imagine that beesand insects are part of the ecosystem that would be need to be supported if the city was at all serious about urban agriculture.

I think urban agriculture is an interesting question, I really do. I’m certainly an advocate for exploring it. I know some people think there’s no point because there’s no capacity. But if you look at Victory Gardens, they supplied a significant fraction of the City’s food needs at the time.

Especially produce needs.

Yes, I believe so. I talked with Manu [Lall] some time ago about how to quantify the water footprint of a city like New York City. Calculations that I have seen done for London suggest that London’s water footprint was the size of England. If you think about it, if we consume a tomato that’s grown in California here, we could be said to be also consuming water from the aquifer in California that was used to grow the tomato.

We’re depleting that aquifer in California.

Yes, exactly. And how to quantify that? And then you start thinking of urban agriculture as not necessarily being about addressing issues of food scarcity as much as about addressing issues of reducing water and energy consumption for urban populations, which are dominant on the globe.

That would be my interest but I can’t say that’s part of this project.

Another view of the Con Edison Green Roof.

One thing at a time!

Well, most of the systems we use are the thinner substrates; they just wouldn’t be suitable for growing food.

So I come from a background of looking at porous media flow and transport. And this project is focused on urban water systems. But the complexity of urban watersheds! My goodness! It’s mindboggling! Often when we teach hydrology, we’re teaching it in the context of pristine environments. My daughters are in school, and they’re already doing the hydrological cycle, and you’re thinking, “Oh, that’s great. How many places on the Earth does that model that they are learning actually work in reality?” so few! I mean, this – the NYC urban water system — is the new reality, and I don’t think we understand it very well at all.

So the green roofs have stepped me into that. It’s been interesting. I think researching water management in an urban environment involves a very challenging set of scientific questions that interface with policy and economics.

You have talked about “solving urbanization challenges by design.” What does that mean?

If you look at pristine watersheds and you look at their hydrologic cycle, the water balance that goes on is explained by so much percolates underground, so much runs off, so much evapotranspirates, etc. You look at that balance versus the balance of an urban environment, and we’ve skewed it. And I think the idea is, let’s design our way back to the “natural” world. We’re already designing our environments. So let’s design to mimic the functionality of the natural environment if that improves the health and sustainability of our cities. In the case of the water cycle, many people now believe that replicating the perviousness and evapotranspiration balance of the natural environment in an urban context is an important step toward sustainability So, let’s not sit back and wait for this to happen, let’s design it.

I have worked with scientists who believe that you cannot solve a problem until you first thoroughly understand it. But then I look at the complexity of the urban watershed and I don’t know that we’ll ever thoroughly understand it. Maybe sometime in the future when the smart sensors actually arrive. But we’ll understand it empirically, I think. We’ll understand it through data.

At some stage I think you’ve got to step back from wanting to study a problem and you’ve got to actually try and solve it. Maybe that’s the Engineer in me!

Columbia Water Center demonstrates research-based solutions to global freshwater scarcity.  Follow Columbia Water Center on Facebook and Twitter

Science for the Planet: In these short video explainers, discover how scientists and scholars across the Columbia Climate School are working to understand the effects of climate change and help solve the crisis.
Notify of

1 Comment
Inline Feedbacks
View all comments
Would love your thoughts, please comment.x