Researchers warn plastic-potted green walls could pose "Grenfell nightmare"

Green walls could present a similar fire risk to the highly combustible cladding used on Grenfell Tower due to the plastic used in their potting systems, research shared exclusively with Dezeen has found.

Sometimes called living walls, green walls allow plants to be potted vertically up the facade of a building.

They have become popular among architects as a way of introducing greenery into urban environments, and are marketed based on their environmental credentials and positive impact on biodiversity.

Test failure in under four minutes

But new research by the Fire Safety Engineering Group (FSEG) at the University of Greenwich suggests that green wall systems using plastic polypropylene plant pots could fail key fire tests in less than four minutes.

This is a faster failure rate than the aluminium composite material (ACM) cladding system used on Grenfell Tower, identified as the main reason that a deadly fire at the building in June 2017 was able to spread so rapidly.

The lead author of the new study warned that green walls could therefore risk another “Grenfell nightmare”.

The research was based on results from a computer simulation model specifically developed to test the fire behaviour of a plastic-pot green wall system.

It was designed to accurately measure the ignition and burn-through of the plastic potting modules, as well as the impact of the vegetation, with the researchers claiming the computer test represents “a realistic prediction of facade-scale fire behaviour”.

Significantly, the modelling showed that more than 94 per cent of the heat release from a burning green wall would come from the polypropylene support modules, not the plants.

The researchers said this finding challenges the common belief that the plants present the main fire hazard in a green wall system, and flies in the face of claims from some manufacturers that green walls are fire safe as long as the plants remain watered and healthy.

“This means that even well-maintained, moist vegetation does not mitigate the overall fire risk,” said the researchers. “Instead, the polymer infrastructure is a driver for fire growth.”

However, the research did suggest that the risk is reduced by keeping the plants wet, as dry plants ignite more quickly and allow fire to spread laterally and vertically more rapidly.

“Aesthetic and environmental appeal must not compromise fire safety”

A facade system comprising the materials used on Grenfell Tower failed after six minutes and 45 seconds in a British standard large-scale test commissioned by the UK Government in the wake of the Grenfell disaster.

In the recent FSEG study, the computer model predicted that the green wall system would fail the same test in just three minutes and 45 seconds, while a system consisting of the potting alone would fail in only two minutes.

“While external living walls have aesthetic and environmental appeal, these characteristics must not compromise fire safety,” FSEG group leader and lead author of the research professor Ed Galea told Dezeen.

“The hard lessons learned from the Grenfell Tower fire demand that fire safety considerations override all other factors. We don’t want the dream of a greener environment turning into another Grenfell nightmare.”

UK building codes allow developers to show compliance using either large-scale testing of entire facade systems like the one modelled in the FSEG study, or smaller-scale tests on the individual products in a system.

Some green wall manufacturers advertise the “Euroclass B” fire rating their products can obtain in smaller-scale tests to promote them as compliant for certain higher risk projects such as mid-rise housing or taller commercial buildings.

Last year, Dezeen revealed that a leading manufacturer called Viritopia obtained this result by testing a system which was so wet that 45 per cent of its mass was water.

The cladding used on Grenfell Tower was also justified using the small-scale testing route prior to the fire.

Post-Grenfell rules restricting the use of combustible materials on tall buildings mean green walls would not be permitted on new residential buildings in England above 18 metres in height. However, they are found on older residential buildings and are still used on hotels, offices, schools, universities and data centres.

Call for ban on combustible potting systems

In response to this new research, a spokesperson for Viritopia said safety is its “priority”.

“Fire safety is fundamental in any living wall system,” the company said.

“We assess fire performance at a system level, including materials, structure, and installation context, and how systems integrate with the wider facade build-up. We support evolving research and testing standards to ensure living wall systems continue to be deployed safely at scale.”

After reviewing the research, James Hanna, a facade specialist at UK architecture studio TODD who has previously raised concerns about green walls, said the findings reinforced his view.

“Suppliers have often stated that where living walls are appropriately irrigated, installed and maintained, they can be suitable for use across a range of building types – including some high-rise applications,” he told Dezeen. “However, this report indicates that while higher moisture content may delay failure, it does not prevent it.”

“The rapid failure observed in large-scale living wall testing, and now reflected in the modelling for both wet and dry conditions, highlights the inherent combustibility issues in certain systems,” he added.

“It is also worth noting that a number of plastic modular living wall systems have been installed across the UK on a range of high-rise office, industrial, and residential buildings over the past 10 to 15 years. As with the ACM issue, this could indicate a wider legacy of buildings that may warrant further consideration.”

The University of Greenwich researchers said their findings were “stark and carry major implications for regulators, manufacturers, building owners and insurers”.

They called for a new testing methodology which adapts large-scale testing specifically for green walls “as a matter of urgency”, as well as a requirement to test them in their reasonable worst-case conditions.

Additionally, they said that the structural components of all green wall systems should be required to be made from non-combustible materials, as this can “significantly reduce the fire load”.

The photo is a stock image via Shutterstock.

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