Growing green walls

Classification and insurance

Green or living walls are not new, but the systems now in common use broadly fall into three categories:

• Climbing plants grown directly against the wall or trained against trellis; these systems are usually irrigated if they are not planted directly into the ground.
• Hydroponic – usually constructed from mesh, geotextile, or a type of mineral wool, with plants growing without soil, relying on irrigation and added nutrients.
• Modular green walls – manufactured from fire resisting plastic or aluminium filled with soil/and or peat and plants, fixed onto a wall or frame.

It does not take a rocket scientist to work out all three systems are likely to perform quite differently in a fire scenario and, given the potential combustibility of the system components, it is unsurprising that whatever documentation you read on the subject of green walls invariably includes large sections on the importance of ensuring that these systems are properly maintained and regularly irrigated. Reassuringly, the green wall designers, installers, and installation contracts do generally recognise this, with most contracts appearing to come with some sort of maintenance provision. But what happens when that provision comes to an end?

Whilst there have been thankfully few incidents involving living walls, the fire at the, then recently opened, Mandarin Oriental 180-bedroom hotel in Knightsbridge in June 2018 sparked interest in how this type of façade could encourage fire spread. The incident, which took 120 firefighters six hours to extinguish, was reportedly caused by welding igniting a lining on a living wall. Although the fire damage was contained, the hotel remained closed for nine months – the financial loss must have been very large indeed.

Given the growth in popularity of these systems and the large potential for loss demonstrated by the Knightsbridge incident, it is probably unsurprising that insurers have become increasingly concerned about the fire performance of these systems, particularly given the lack of large-scale test data available. Understandably, insurers want to know more, given that most of the current testing data, such as it is, relies on small-scale tests of various components of green walls.

Growing the data pool

To date, there has been little to no full-scale testing of these systems and so to begin the process of covering this data gap, a RISCAuthority project has commissioned the FPA to review methods of installation for green walls onto external elevations. This will include fixing methods, planters, pipes, irrigation systems, and specifically looking at materials used in the construction and their typical reaction to fire properties. This part of the research will also consider issues such as the creation of cavities within the green wall system and how these are addressed.

The project will then go on to review irrigation methods for the green wall systems including backups, grey water emergency backups, and tank and water leak detection.

This will be followed by a review into the ongoing maintenance of the green walls – specifically typical plants and growing medium, a change in the distribution and makeup of plant species, and frequency of inspection and replacement of plants. This will then inform the production of guidance or a code of practice for green/living walls and will include a review of typical contract arrangements for green wall systems post-installation, including condition monitoring and wind resistance.

The programme will begin by fire testing the support system typical of a green wall system, but without the growing medium or any vegetation. The fire testing of systems would be restricted to those that do not meet either the Euroclass A1 or A2 classification and are, therefore, defined as combustible. From the research we have undertaken thus far, we understand that at least one of the green wall systems uses a ‘fire-resisting’ polymeric container as a support for the growing medium. We are proposing that the cladding walls are utilised for the testing, but the system is installed on the main wall only to a height of 5m above the combustion chamber (to the line of the level 2 thermocouples). The reason that a full height BS 8414 wall will not be used is down to speed and cost.

It is proposed that a second test with the growing medium installed could then be tested to understand what effect the growing medium has on the fire performance of the underlying support system. Further testing with vegetation may be undertaken if budgets permit and it is thought that the testing would be scientifically significant.

We will establish the rate of moisture loss from a system to simulate the failure of the water supply. The results will provide an indication of the time frame for desiccation of the green walls and the potential increase in fire risk due to a system drying out.

We will also undertake small-scale fire tests on a variety of plant species at decreasing levels of moisture content, with the aim to comment on the practicalities of testing. We need to establish whether green systems can be legitimately classified to BS EN 13501-1 and/or tested to BS 8414 and BR135 and whether the results will be meaningful. The final stage will be to undertake full-scale BS 8414 tests on green walls.

The aim of the research is to promote a better understanding of these systems so that it is clear which occupancies and which systems are best suited to their use, and to ensure that installation and maintenance is undertaken to a standard that reduces risk.

Fire & Risk Management is the UK’s market leading fire safety journal, published 10 times a year, and is available exclusively to FPA members in digital and print format depending on your requirements. You can find out more about our membership scheme here.

Jonathan O’Neill OBE is Managing Director of the Fire Protection Association.