Hot summers can be expensive, with air conditioners and fans driving up household electricity costs; however, researchers from the University of South Australia (UniSA) have identified a scientifically valid and inexpensive cooling method humans have known about for nearly a century.
In a study from the UniSA, living walls or vertical gardens—walls that are entirely covered with vegetation— were shown to lower the overall temperature of houses on hot summer days by up to 12 degrees Celsius (around 54 degrees Fahrenheit). The vegetation on these walls is grown in pots, felt pockets, or planter boxes and is irrigated on structures attached to the wall.
Originally first commercially marketed in 1937 by Illinois Professor Stanley Hart White with his patented Botanical Bricks, living walls are common in Europe and in some Asian countries, but they have yet to gain popularity in Australia because of high upfront costs.
The average prices of the planter boxes that are used to house the vertical vegetation range from $500 to $700 (US $400-$500) a square metre. However, researchers said that over time the energy savings are considerable and more than exceed the initial cost outlay.
Additionally, the co-author of the recent paper, UniSA Emeritus Prof. Simon Beecham, said in an email to The Epoch Times that in winter, living walls have a positive influence as a thermal insulation layer— similar to a blanket.
Experimenting with Living Walls
In 2018, Rosmina Bustami, who, at the time, was a UniSA civil engineering PhD student, conducted an experiment to determine the potential energy savings of vertical gardens on the external walls of western-facing houses.
The experiment observed a 144-pot living wall located in Adelaide for over 18 months.
In Adelaide, over the course of the experiment, the average, minimum, and maximum daily temperatures were, respectively, 18.7, 2.6, and 45.6 degrees Celsius (65.7, 36.7, and 114 degrees Fahrenheit), and the average air humidity was 64 percent.
In the study, Bustami compared the temperatures on living walls to those on control walls without plants and recorded a difference of up to 12 degrees between the walls. The cooling effect of living walls can be explained by the shading that sunlight absorption for photosynthesis affords, insulation, and evapotranspiration.
Evapotranspiration refers to the processes that release water, from the soil and plants, into the air as a vapour.
Furthermore, this study also allowed researchers to demonstrate the efficacy of living walls in comparison to other water-sensitive urban designs, like permeable pavements.
Water-Sensitive Urban Designs
Water-sensitive urban designs are used to mitigate flood risks, moderate temperature, enhance urban biodiversity, and improve water quality. they are also another way to reduce heat in the urban environment.
Permeable pavements like porous concrete can act as a non-vegetated water-sensitive urban design system because they are able to absorb stormwater, unlike conventional impermeable concrete or asphalt pavements.
“Evaporation of water from porous concrete occurs after rainfall, and this results in the ambient air temperature decreasing,” Beecham said in a UniSA news release.
“Both living walls and porous concrete roads are now being investigated for their ability to cool the urban environment,” he said.
Additionally, Beecham said that both living walls and permeable pavements have a number of built-in mechanisms for pollutant removal.
“The predominant mechanism in porous pavements is mechanical filtration, which removes sediments and associated pollutants such as heavy metals,” he said.
“The soil in the living walls also serves the same filtration purpose, but the vegetation provides additional pollutant removal mechanisms such as nutrient removal.”
Porous Concrete vs Living Walls
Bustami, who is now based at the Universiti Malaysia Sarawak, is the lead author of the paper publishing comparisons between porous concrete and living walls, which is co-authored by Beecham and UniSA researcher Assoc Prof. James Hopeward.
In this paper, the researchers compared the evapotranspiration rates of plants within living walls to the evaporation rates of permeable pavements. The experiments demonstrated that porous concrete is, at best, only 15 percent as effective at cooling as vertical gardens, and in the worst cases, only four percent as effective.
“The porous concrete pavement was able to evaporate infiltrated rainfall through the air gaps in the material,” Beecham said.
“This evaporation did cool the air temperatures above the pavement but only 15 percent as much as an equivalent area of the living wall.
“This was because the vegetation in the living wall was also able to transpire water (that had been taken up by the plant roots) during daylight hours and for much longer periods (than for the pavement),” he said.
Beecham said that in the case of porous concrete, once the upper 13mm or so of stored rainfall is evaporated, very little of the lower stored water can escape or evaporate.
“The upper/available rainwater typically takes only a few hours to evaporate, depending on the ambient temperature and humidity,” he said.
“This process is even more enhanced by living walls because the vegetation transpires water for much longer periods. Also, living walls are irrigated more frequently than it rains.”