Green Wall Research, Full Steam Ahead!
By George Irwin, The Green Wall Editor
April 11, 2009
Updated April 26, 2009

The Green Walls Column
All Photos Courtesy George Irwin

The reduction of urban heat islands, usage of fossil fuels, increased storm water retention……..sound familiar?  If you’re a green roof fan you will recognize these are some of the benefits of green roofs.  Green roof data has a long history.  What about green walls?  Some of the earliest research depicts the use of green façades as a means to cool buildings by shading.

In reality it was the grape that was planted close to the building.  The vine produced more fruit faster, it ripened with added sweetness and produced an excellent wine with high alcohol content. With a long history of facades, where is the modern data?  More scientific research is being done as I write.  For example, Green Roofs for Healthy Cities announced a formal research fund at the 2008 Greening Rooftops for Sustainable Communities Conference, Awards and Trade Show in Baltimore.  I know of a few others that are more specific to individual manufacturers.  With all the available data on green roofs, green walls are lacking.

Some of you may recall a study published by Drs. Brad Bass and Bas Baskaran titled “Evaluating Rooftop and Vertical Gardens as an Adaption Strategy for Urban Areas,” (2003).  In this column I include some of the paper as a pre-cursor that identifies green walls and their ability to cool the walls of buildings.  Involved directly with many research opportunities, I find the common question is relating green walls to green roofs.  How can green walls compare to green roofs in saving energy?  Here are some of the earliest modern references to green walls that are compared to preliminary short term data that will provide the ground work for additional long term studies.

According to Bass and Baskaran, "Higher than average temperatures within city limits known as Urban Heat Islands (UHI) are a direct result of replacing vegetation with typical urban surfaces also creates an elevation of temperature relative to the surrounding rural or natural areas.  The UHI occurs because more of the incoming solar radiation is absorbed by dark surfaces such as rooftops and pavement in the city and reradiated as longwave radiation or heat. 

Green Roof Data Collection

"Below a certain temperature, the demand for electricity is inelastic.  Above this threshold, every degree C increase can increase electricity consumption by 5%, increasing emissions of the fossil fuels required for its generation.  Although the UHI may be as small as 2° C, that may be sufficient to move the temperatures above this threshold due the additional demand for air conditioning and requirements for refrigeration.  The increased temperatures also increase the problems associated with heat stress and the rate of ozone formation."

Vegetation can reduce all of these impacts.  The focus is vegetation reducing the UHI and thermal elevations is because of evapotranspiration. Incoming solar energy that is used for evapotranspiration cannot be absorbed and re-radiated as heat. Studies in Oregon demonstrated that non-vegetated areas could exceed temperatures of 50° C (122° F) in July while vegetated areas remain at 25° C (77° F) (Luvall and Holbo, 1989).

Vegetation can also further alleviate air and water quality problems by filtering pollutants through the leaves or the roots.  In addition, vegetation in urban areas has been shown to increase mental well being, biodiversity and residential property values.

"Most discussions of the UHI focus on the temperatures of surfaces or the canopy level UHI, which occurs at the level at which most people live.  We only feel surface temperatures directly when in contact with these surfaces, but they heat up the surrounding atmosphere.  For the canopy level, the primary affect is experienced in the evening.  Heat from rooftops affects the temperature of the boundary layer or upper layer of the atmosphere, the layer of the atmosphere extending roughly from rooftop level up to the level where the urban influence is no longer ”felt" (Oke, 1976).  This additional heating occurs throughout the day and influences the chemistry of air pollution and temperatures above the roof."

Nakamura and Oke (1988) found that temperatures in the urban canyon and temperatures in the lower part of the urban boundary layer, are usually very similar.  Thus, higher temperatures above the roofs can affect temperatures at canopy level, where we live, and in areas with only one or two story buildings, the roofs may be at the canopy level.

"Reducing the rooftop temperatures would further reduce the use of energy for space conditioning in both the summer and the winter.  In the summer, a typical insulated, gravel-covered rooftop temperature can vary between 60° C (140° F) and 80° C (176° F) (Peck et al., 1999).  These temperatures increase the cooling load on a building in two ways.  Since the internal temperature underneath the roof is typically lower than the temperature above the roof, the heat will always flow through the roof into the building.  In addition, modern high-rise buildings are constantly exchanging the internal and external air.  Because of the high roof temperatures, the temperature of this external air that is brought into the building’s ventilation system may be warmer than the ambient air, requiring additional energy for cooling.

"Evapotranspiration from rooftop vegetation could cool the roof, reducing the amount of heat flow into the building through the roof.  The lower rooftop temperature would also reduce the temperature of the external air that is exchanged with the building’s air.  The temperature of this air could also be reduced if the rooftop garden is designed so as to shade the intake valves."  Summer temperatures as low as 25° C (77° F) have been observed. (Peck et al., 1999).

Most of the above is taken from the publication of Bass and Baskaran, and some of the data is well over 10 years old.  Ten years later the technologies, materials, and design techniques have also evolved.  There has been no slowing down the green roof momentum and green walls are not far behind.  The focus to combat the issue of UHI was primarily on green roofs until additional technologies were also being recognized for the ability to cool the Urban Heat Island.  The walls are heating up and reflecting UV rays just as much as a roof top, depending upon the color and surface material, location, etc.  Green façades (trellis structures with climbing plants) were utilized to shade the sides of buildings much in the same manner green roofs were used for the roof tops.

Here's the catch: on average buildings have much higher wall-to-roof ratios in most cases.

An even greater amount of space for vegetation may be available on the exterior walls of the buildings in urban areas, and growing vegetation on walls could create vertical gardens.  Vertical gardens increase the amount of vegetative surface in urban areas, increasing evapotranspiration and evaporative cooling, and can be used for direct shading as well.  In comparison, green roofs directly affect the boundary layer UHI, and vertical gardens can reduce the canopy level UHI.

Previous observations indicate that vertical gardens do reduce the heat flow into the building, and their surface temperature is lower than a bare wall, which is necessary to reduce the urban heat island (Bass and Baskaran, 2003).  A series of experiments in Japan suggested that vines could reduce the temperature of a veranda with a southwestern exposure (Hoyano, 1988).  Vines were effective at reducing the surface temperature of a wall.  In Germany, the vertical garden surface temperature was 10° C (18° F) cooler than a bare wall when observed at 1:30 p.m. in September (Wilmers, 1988).   The study does not state how mature the plants were.  Theoretically speaking, the potential for additional shading would be accomplished with with fully grown plants.

Holm (1989), demonstrated a reduction of 2.6° C (4.7° F) behind the vegetated panel.  For a building consisting of two 10mm fiber-cement sheets with 38mm of fiberglass insulation, a computer simulation estimated that a vertical garden reduced summer daytime temperatures on the surface by 5° C (9° F).  These results are not as dramatic as the cooling effect on a horizontal surface, such as a roof, but given the amount of wall space in urban areas, the potential impact of vertical gardening is expected to be quite dramatic.

These results were utilizing green façades and the primary method of cooling was shade and the process of transpiration accounting for the movement of water within a plant and the subsequent loss of water as vapor through stomata in its leaves.  Mentioned earlier were two ways of cooling a roof top.  The first is shading with vegetated coverage and the second is through the process of evapotranspiration to dissipate accumulated heat energy.  Technical breakthroughs have a few companies manufacturing and producing Green Living Walls or Living Walls - defined as wall structures that support rooted plant coverage. This is different than a green façade that can be identified as having a climbing plant at the base of a support structure. The majority of living walls are media-based except for a single hydroponic wall. The premise is that the media will also retain water available for evapotranspiration. Utilizing the living wall, both shading and evapotranspiration are implemented.

Many models exist that analyze numerous variables to determine the rate at which water evaporates and creates a cooling effect.  Let's keep it simple and provide tangible examples.  Since we established that vertical surfaces can be comparable to horizontal roof tops, can we assume a living wall with the same depth will provide the same cooling results, only vertically?

I would have to say "yes" as I leave myself open to debate and welcome other opinions.  Unlike green roof research, there is a lack of defining green wall data.  Green Roofs for Healthy Cities has implemented a research program for living walls and green facades. I have implemented thermal testing specifically for green living walls and will be analyzing data after completion of a 1-year study (July 2009).

The preliminary short-term thermal testing showed that a 3” (7.6 cm) deep green living wall provides similar results under the same environmental variables as a green roof with 3” growing medium depth.  The initial test plot was painted black (behind the living wall) to match the EPDM rubber membrane.

The preliminary short-term thermal testing showed that a 3” deep green living wall provides similar results under the same environmental variables as a green roof with 3” growing medium depth. The initial test plot was painted black (behind the living wall) to match the EPDM rubber membrane.

Preliminary Temperature Comparison Test Results

The preliminary testing shows an average surface temperature difference of 75° F (41.7° C) between the exposed rubber roof and the protected green living wall. This observation supports more advanced research. With more detailed testing and longer trials comparing 3", 4" and 6" rooting depths, I feel confident that the findings will show even better data as a direct result of evapotranspiration and shading.  The mentioned preliminary study is being conducted in Rochester, NY with a short cooling season.  This study will be compared to other thermal testing I will be conducting over the summer 2009 near Miami, FL, where we expect the green wall’s cooling capabilities to rival that of green roofs in locations with cooler growing seasons.

Recall Hoyano, Wilmers, and Holm had recorded 10° C, 2.6° C, and 5° C reductions in surface wall temperature utilizing green facades with shading as the primary means of temperature reduction.  Our initial observations indicate similar thermal mitigation by green walls compared to green roofs.

Why?  I hypothesize that medium depth, hydration layer, and rate of evapotranspiration each contribute to reduced wall surface heating.  However, the extent of these and other influences remain an open question until ongoing and future research can provide much needed data.  The eventual comparison will be between green façades and living walls.

George Irwin, The Green Wall Editor

Literature Cited:

Bass, B. and B. Baskaran, 2003: Evaluating Rooftop and Vertical Gardens as an Adaption Strategy for Urban Areas: Impacts and Evaluations Progress Report. April 1, 1999 – March 3, 2001.

Holm, D., 1989: Thermal improvement by means of leaf cover on external walls - a simulation model. Energy and Buildings, 14:19-30.

Luvall, J.C., and H. R. Holbo, 1989: Measurements of short-term thermal responses of coniferous forest canopies using thermal scanner data. Remote Sensing of Environment, 27:1-10.

Oke, T.R., 1976: The distinction between canopy and boundary layer urban heat islands. Atmosphere, 14: 268-277.

Nakamura, Y. and T. R. Oke, 1988: "Wind, temperature and stability conditions in an E-W oriented urban canyon," Atmospheric Environment, 22:2691-2700.

Hoyano, A., 1988: Climatological uses of plants for solar control and the effects on the thermal environment of a building. Energy Buildings, 11:181-199.

Wilmers, F., 1988: Green for amelioration of urban climate. Energy and Buildings, 11:288-299.

Note:  See the Baskaran, Bas and Bass, Brad (2003) "Evaluating Rooftop and Vertical Gardens as an Adaption Strategy for Urban Areas,” References Page listed at the National Research Council Canada - Conseil national de recherches Canada (NRC-CNRC) page.

George Irwin is the President and CEO of Green Living™ Technologies, LLC (GLT) based in NY.  Green Living™ Technologies is the only US manufacturer of growing media based green wall and three types of green roof systems.  Mr. Irwin is also a trainer for Green Roofs for Healthy Cities Green Walls 101.

Contact George Irwin at:,, or 1.800.631.8001.

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