Living walls were all the rage a decade ago—we showed dozens of them. I was skeptical about their value, noting that “living walls are expensive to buy and expensive to maintain because plants tend to want to live in the ground.” And while I appreciated their beauty, their biophilic effects, and their ability to cool a building, I have often questioned their utility on the exterior of buildings, and whether they were worth the cost and effort. I preferred “green façades” like those planted in the ground by French architect Édouard François or good old-fashioned vines.
However, a new study, “Living wall systems for improved thermal performance of existing buildings,” by researchers at the University of Plymouth found that adding a living wall to existing buildings can significantly reduce heat loss– by a very dramatic 31.4%.
The study authors took an ugly ’70s university building built with uninsulated masonry cavity walls, the same building technique used in 70% of United Kingdom dwellings, and installed a living wall on a portion of it. The cooling effects of living walls are well known and easy to understand: The leaves shade the wall and the moisture evaporates, cooling the air around them.
But keeping a building warmer is more complicated. There are studies that looked at the insulating value of the mats that hold the living wall up, but they can be full of water which is a good conductor. Other studies found that the foliage created pockets of still air and reduced wind-driven convective cooling. The purpose of this study was to figure out what the effect would be on those masonry cavity walls so many British buildings are made with. The researchers write:
The living wall used was a “fytotextile” system with felt pockets filled with potting compost, and planted with a mixture of evergreen plant types. Heat sensors were set up inside and out in different areas, one set where there was the living wall and the other where it was just the masonry.
Note the red line at the top representing the rate of heat loss through the masonry wall, the blue representing the interior temperature, and the orange line representing the rate of heat loss through the section with the living wall. Note also that the outside temperature was pretty warm.
It should be noted again that the addition of substrate and plant layer is not simple. This is expensive, it requires plumbing, continuous running water, and serious maintenance. The temperature in this exercise does not appear to have gone below freezing, which will create a whole different set of conditions. But still, the numbers are significant, even if Dr. Matthew Fox, the study’s lead author is overstating the case in the press release:
A 31% reduction in heat loss is going to get British buildings anywhere near net-zero, but there is no reason that one can’t stick insulation behind it and bump that number up. And as a bonus, you get a lovely green living wall to boot, with its support of biodiversity, biophilia, cooling in summer, and the possible dramatic aesthetic improvement of so many awful British buildings. To paraphrase architect Frank Lloyd Wright, “A doctor can bury his mistakes, but an architect can only advise his client to plant living walls.”
