A living wall in Paris.

The green walls do more than we previously thought.

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%.1

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.1

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:

"Whilst traditional strategies for improving the thermal resistance of such walls might have added insulation, literature suggests that LWS [Living Wall Systems] could offer an alternative solution for thermal improvement, whilst also providing other unique benefits such as biodiversity, aesthetic and air quality improvements. Furthermore, understanding the scale of the thermal improvement offered in this setting will help define the sustainability potential of this approach given the potentially high environmental lifecycle and overall energy burden this system may exert."

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.

"By reviewing the moving average results over the five-week study period, it became even more apparent that the final U-value for the wall with the addition of an external LWS façade was lower than the U-value for the wall without the LWS. This is significant, since it represents a 0.35W/m2K improvement by simple addition of substrate and plant layer to the outside of the wall. This equates to a 31.4% improvement over the original wall state."

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:

"Within England, approximately 57% all buildings were built before 1964. While regulations have changed more recently to improve the thermal performance of new constructions, it is our existing buildings that require the most energy to heat and are a significant contributor to carbon emissions. It is therefore essential that we begin to improve the thermal performance of these existing buildings, if the UK is to reach its target of net zero carbon emission by 2050, and help to reduce the likelihood of fuel poverty from rising energy prices."

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.”

Fact checked by Haley Mast