Living walls and green roofs

The first thing that strikes you when you enter the lobby of Arup’s San Francisco office is the mass of vegetation mounted behind the front desk.


Engineer Kirstin Weeks in front of Arup San Francisco’s living wall

When staff members installed the living wall five years ago, they were motivated primarily by its potential for use as a research tool. Soon, however, they came to appreciate its aesthetic and air-quality benefits as much as its experimental possibilities.

Weeks and other San Francisco staff members building the living wall in 2009

As research increasingly confirms the critical importance of natural systems within the built environment, green walls and living roofs have become popular tools for integrating ecology and architecture. San Francisco engineer Kirstin Weeks told us more about their history and future.


Green roofs and walls are becoming more and more popular. What makes them so appealing?

Obviously, this concept has been around for thousands of years, especially in northern cultures, where you see farmhouses covered in grass, or actual subterranean buildings, throughout history.

Traditional Norwegian architecture.

Traditional Norwegian architecture

What’s changing now is the extent to which we understand how these systems are able to contribute to building performance in different environments. Green roofs and living walls have arguably way more benefits for urban environments where we have an excess of hard, impermeable surfaces and not enough vegetation. So we’re seeing lots of benefits, especially with regards to the urban microclimate; urban livability and air quality; stormwater management; and thermal buffering.

Through new research, we’ve been able to substantiate what people know intuitively about the benefits of building-integrated green infrastructure. For example, we ran a study for the US GSA to survey and document all the benefits and challenges of green roofs and came up with a cost-benefit analysis that shows that cities offering incentives for green roofs stand to get a great return on their investment: $30/sf worth of heat island mitigation; biodiversity and habitat; and stormwater management, just to name a few of the benefits.

Green roof in Victoria, BC

We also did a study for Walmart looking at a store in Chicago with half green roof and half white, or cool, roof. It showed that green roofs can be a sound investment and even save money upfront in municipalities with stringent stormwater requirements.

The same type of research is happening with living walls. There’s some great research out recently from the University of Maryland that quantifies the energy savings a building can achieve by installing an exterior living wall.

Building-integrated vegetation seems like a no-brainer; it seems like every building could benefit from it. Is that true?

What you get out of a vegetated building does depend upon the structure, as well as the design, obviously. For example, if you put a green roof over an unconditioned parking structure, it will offer you thermal and energy benefits, but you don’t care about that because you’re not cooling or heating the space below. But it will still provide great stormwater attenuation.

Similarly, if you put a living wall on a concrete surface that’s freestanding in a park you would get some microclimate benefits for the air immediately surrounding the structure, but you’re not going to be getting any direct energy savings.

It’s far more beneficial to tailor systems to the site and seek to solve problems with vegetation. For example, a building with a lot of glass on the west façade likely has a heat gain and glare problem. A trellis-based living wall system can help.

The Consorcio building in Chile is a great example where they have mounted planters and are screening all the building’s western-facing windows with deciduous vines. So in the winter the building gets the heat it needs because the leaves are off the vines, but in the summer the leaves significantly decrease the solar gain to the building.

If you have a green roof on a single-story building, the energy benefit is going to be more than if you have it on a 30-story building, where it’s only able to directly impact the floor below.

California Institute of Sciences’ green roof.

California Institute of Sciences

So does that mean green roofs aren’t actually that beneficial for tall buildings?

Well, no, the roofs are still beneficial, particularly in certain configurations. What we’ve learned recently is if buildings have their HVAC air intakes on the roof, a green roof will have a huge impact on the whole building, making them a great opportunity for energy savings.

In the recent Walmart study, we were able to specifically monitor the air temperature at the point of air intake and really show that in the summer the air up there is significantly cooler, so the equipment brings in much cooler air on the side with the green roof. In the winter it’s not quite as pronounced, but the air is warmer on the green roof side, so there is a heating energy savings too. However, if the air intakes are placed too high, these benefits dissipate quickly.

This is a great example of why it’s important to have diverse design teams — someone who understands both mechanical engineering and green roofs, for example — so you can get the maximum performance benefit for your investment in a living system.

What about retrofitting with living systems? Is that not as effective because you don’t have control over the design of the building?

These systems are absolutely viable strategies for existing buildings. In those instances you have the benefit of knowing what specific benefits you want to achieve, whether it’s thermal insulation, stormwater retention, or maybe usable or farmable open space, so you can design specifically for those.

But there are also additional challenges. Structural engineers need to verify the roof’s load-bearing capacity and maybe plan a retrofit if the capacity isn’t there already. A lot of buildings do have spare capacity, though, and you can usually design a very lightweight green roof system that provides the basic benefits a client is looking to achieve.

Living wall at the Musée du quai Branly in Paris

With living walls it would be a question of determining how to mount it. A freestanding living wall is an option for a building whose façades cannot accept additional load or can’t be modified.

Right now, at least in the cities that I’ve worked in, there tend to be more drivers and incentives in place for people to institute green infrastructure on new buildings than existing, and more so green roofs than living walls. Living walls are a bit newer, but they are becoming vastly more popular in the last few years as their benefits are better understood, systems improve, and the industry matures.

How important are incentives in driving this trend forward?

Right now, a lot of work is focused not only on designing and installing systems, but, really importantly, on removing barriers and better understanding the benefits and incentives.

I think one of the big drivers motivating people to install any kind of green infrastructure in cities is stormwater management ordinances, which typically only apply to new buildings. It would be great if there were more incentives for existing buildings to implement green roofs or living walls. For example, some cities separate their water bills into a fee for water use and another fee for stormwater discharge and outflow to sewers. That creates a huge incentive for existing building owners to retrofit green systems to reduce their flow to sewers, because they will immediately see the savings on their water bills. In San Francisco we don’t have that at the moment, so there is less incentive to make a change in an existing building.

It would be great if there were more incentives for existing buildings to implement green roofs or living walls

As far as incentives go, I think the trend is in the direction of government being concerned about problems that are related to urbanization, such as ground- and surface water pollution; climate change and air quality; biodiversity loss; and many others. The GSA study that we did was commissioned by the US government to try to quantify the performance benefits of green roofs, presumably because they’re interested in whether they should promote or incentivize, or even require, green roofs for some buildings, although it’s not actually been said that’s the goal.

Other than incentives, what would you like to see happen in the building-integrated green infrastructure industry?

I really look forward to seeing some great advances in design for green roofs and living walls — simpler and at the same time more site-tailored and climate-adapted projects; projects that are sort of loaded up with intelligence so that they integrate with building systems and create a lot more benefit per cost.

Interior living wall

Up until about five or ten years ago, the influx of green roofs to the US was mostly technology transfer from Europe. Now we are beginning to adapt and innovate in terms of system support, plant selection, climate considerations, habitat benefits, indoor living walls that maximize pollutant removal — the list goes on and on, really. For our western climates, we are really interested in designing for reduced potable water use: improving plant selection, designing better methods of irrigation, and using stormwater.

Indoor living walls tend to be resource-intensive as well, so implementing recirculating irrigation systems, using LED lighting: we are working to create more benefit for a smaller footprint. Optimization is the focus.

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