The installation of vertical green systems, also known as green walls, is a growing trend in modern construction and fit-out [1]. In Sydney, the prominent building that comes to mind is Lendlease’s One Central Park in Chippendale (constructed circa 2014) which supports over 1,120 m2 of vertical garden arrangements absorbed into its clean glazed facades.

One Central Park, Chippendale NSW

There are also numerous other examples where green walls have been integrated internally into corridors, entrance lobbies and the like all the while forming a key feature to the final design and overall architectural vision.

Typical lobby green wall.

Green walls come in a variety of shapes, sizes and configurations, however generally speaking, they constitute one or a combination of the following [3]:

  • ‍Climbing: Growing directly against a wall, mesh grid or cable trellis, the plants can be rooted in the ground or from planter boxes. These systems are typically irrigated, but if the plants are rooted in the ground they can survive without irrigation.
  • ‍Hydroponic: Constructed from plastic mesh, geotextiles and horticultural fabrics which are in turn fixed to a cross brace frame. Plants grow without growing medium and rely on nutrient enriched irrigation water.
  • ‍Modular: Typically constructed of high-density polyethylene (HDPE) or some variant, these consist of plastic modules supported by plastic cross frame fixed to the wall. The modules contain the growing medium, minerals and plants. Irrigation is provided via plastic drip line fed from a water supply to provide a continuous means of irrigation. It is not uncommon for these systems to be fully or partially artificial (i.e. incorporating synthetic plants without the need for irrigation or growing medium).
Example of a climbing green wall (left), hydroponic green wall and modular green wall.

The major driver for the green wall trend is sustainability mixed with a good measure well-being. It is a desirable thing to raise the Green Star Rating of your building by improving on energy efficiency, air-quality and a bunch of other less marketable engineering benefits (like cleaning rainwater runoff before it hits the stormwater line). It has also been demonstrated that plants provide a calming effect on our moods and stress levels [2],thus green walls are the perfect design concept for our eco-friendly, anxiety-conscious society.

Problem Definition

With the emergence of these vertical green systems, it has been suggested that they may represent a fire risk. The general line of thinking is that as long as the organic matter is kept moist and the system suitably maintained, it represents a low fire ignition and fire spread risk. The primary concern is where the system gradually dries out (i.e. loss in moisture content), or alternatively, where it is fully (or partially) artificial thus incorporating dry synthetic components. The shift in moisture and availability of synthetic materials shifts the fire risk from low to immediate or even high. Remember, risk is probability x consequence and slightly increasing the ignitability of the system could potentially increase both the probability that the system will ignite as well as the consequences of that ignition.

Historically speaking, green wall fires in Australia are rare, perhaps due to their “moist nature” and presence of rigorous maintenance regimes. On the other hand, green wall fires may have occurred but weren’t recorded because they were too small and/or easily controlled/extinguished. In 2012 a non-irrigated artificial green wall in a Redfern pub caught alight with the ignition source being attributed to a nearby candle being used as cigarette lighter. The fire was said to spread rapidly (estimated within 5-6 seconds [7]) and is indicative of the likely fuel loads involved (e.g. synthetic materials like plastics etc.) along their vertical configuration. This observation is consistent with the limited test studies available [5] [6], whereby rapid spread of fire has been observed across a green wall in the order of 20 seconds.

Examples of fire-tested plants [6]

There has been limited amounts of testing on green walls. And more often than not, the limited data that is available is based on singular components not complete systems. The UK and Europe go as far as to say that there is no significant testing on green walls [3]. Therefore, demonstrating compliance or predicting likely fire performance can be a very difficult task. In the context of our current building code (Volume One of the NCC 2019 Amdt 1), most (if not all) green wall systems would rely on the niche discipline of fire safety engineering to achieve compliance via the performance-based pathway. This is not to say that green walls are the deadliest thing since polyethylene (PE) core Aluminium Composite Panel’s (ACP’s). Under the right circumstances they would most likely represent a low fire risk, however they do require some careful thought and strategic consideration during the design phase of a project. The earlier the fire question is asked of green walls the better.

The Question of Compliance

Green wall construction is captured in either one of two places in the building code and it is dependent on where the green wall is located, internally or externally. With this in mind, let’s review what the building code says.

Internally

In most instances internal green wall compliance is dictated by Clause C1.10 & Specification C1.10 from Volume One of the NCC 2019 Amdt 1, which prescribes wall linings to achieve specific fire hazard properties. These wall lining properties are broadly summed up under the designation of a “Group Number” which can be 1, 2 or 3 (with Group 1 being the best performance category). The required Group Number is influenced by the building classification, wall lining locality as well as the presence of sprinkler protection. The Group Number itself is determined in accordance with AS 5637.1 which in-turn permits three different test methodologies: AS 3837, ISO 5660 & AS ISO 9705 [4]. Essentially, AS 3837 involves radiant heat panel and pilot flame exposure of a sample. Similarly, ISO 5660 requires a sample to be tested in a cone calorimeter. Both the AS 3837 & ISO 5660 tests are small scale. AS ISO9705, which is arguably the best and most expensive of the three, requires a complete wall lining assembly to be set alight in a large-scale room fire test.

The Group Number designation is representative of the time taken for the tested specimen to reach a 1MW fire size under a set of specific fire exposure and time conditions. With nearly all green wall configurations the components usually don’t comprise the right thermal properties to achieve the required Group Number and hence they typically don’t achieve compliance with the building code as an internal wall lining.

Externally

In external situations the rules of the game are dictated by building height and size. The more storeys and the more people within the building the greater the risk, is essentially the way the building code is written. So, for buildings of Type A or B construction, Clause C1.9 prescribes that external walls and their components are strictly to be non-combustible (strike 1 against green walls). Clause 2.4 of Specification C1.1 prescribes that attachments to fire-rated elements must not impair the fire-resisting performance of the element they are attached to. In the majority of cases external walls are required to be fire-rated due to their proximity to adjacent boundaries and therefore fixing a combustible attachment could be deemed to reduce its fire-resisting ability (strike 2 for green walls). The only remaining alternative (from a prescriptive perspective) would be to apply the latest and greatest verification method, CV3. This was introduced as a way to demonstrate external attachments, claddings and the like are still safe whilst not technically adhering to the aforementioned prescriptive provisions. The CV3 verification method essentially prescribes an external wall fire test in accordance with AS 5113 which must achieve an EW rating. Not only this, but the building needs sprinklers to the relevant Australian Standard (not including FPAA101 H/D) along with additional sprinkler requirements such as protection to external areas and other prescribed design criteria (e.g. monitored stop valves at each level, flow requirements) [4].

The AS 5113 test is large scale test apparatus and involves the partial construction of a whole façade system in line with BS 8414 or ISO 13785-2. The form of construction is representative of the ‘as-built’ configuration complete with cavities, substrates, fixings and barriers. The performance and attainment of an EW rating is dependent on a number of factors including (but not limited to) temperature rise at various points along and behind the test façade, continuous flaming and mass of falling debris. The strength of the method is the array of data it yields and the fact that complete systems are tested and not singular components. It would be unlikely that any green wall system on the market would stand up to the AS 5113 test (strike 3 against green walls). Therefore, green walls would not typically achieve compliance with the building code when used as an external attachment.  

AS 5113 / BS 8414 test rig

Performance Approach

The typical path to achieve green wall compliance is via the performance-based approach utilising the niche discipline of fire safety engineering. With respect to fire, the building code is fundamentally based around occupant life safety and protection of property. Most performance-based approaches would be aimed at demonstrating the green walls performance with respect to fire spread and safe conditions under evacuation. These specifics also come with sets of sub-considerations such as evacuation and fire brigade intervention times, fire safety systems, compartment sizes, fire safety systems etc. Essentially what needs to be demonstrated is that when the green wall lights up, fire spread is restricted, everyone gets out safe and fire brigade personnel are able to deploy their operations and ultimately extinguish the fire.

Despite the limited test data that is available it is still possible to draw out some relevant conclusions if a clear understanding of the green wall system, its components and materiality are known. Established fire science principles like flame height/projection, radiant heat flux exposure, fuel load contribution, fuel consumption/burn duration, ignition source identification combined with evacuation and fire service intervention modelling can be utilised to assist in demonstrating performance. However, the key stakeholders (incl. the fire authority in most cases) will ultimately need to be satisfied that the green wall doesn’t constitute an undue fire spread risk. Another aspect of the performance approach is that the end design likely won’t be without additional mitigation measures.

Green walls are primarily architectural and may not be introduced until late in the design or at fit-out stage. By then, most of the detailed design has been completed including resolution of the fire services design. This sometimes leaves little room to address the fire spread risks by incorporating additional measures. This is why the fire question needs to raised as early as possible. Green walls require careful thought and strategic consideration at their earliest suggestion.

Conclusion

So, where do green walls and the fire questions they raise leave us? The most troubling aspect is the ability for a construction trend with demonstratable energy and health benefits to supersede fire safety which is fundamental to the building code. This being said, the current building code is well-equipped to take on green walls, the industry is also well-equipped with talented fire safety engineers who have the ability to answer the fire question. But the question should be asked as early as possible.

References:

[1]             Chow, C.L., Han, S.S., Dehanayake,K.C., and Chow. W.K. “Fire Hazards with Vertical Greenery Systems” FPEeXTRASFPE Issue No. 31

[2]            Lee.M-S., Lee, J., Park, B-J., and Yoshifumi, M. (2015) “Interaction with indoorplants may reduce psychological and physiological stress by suppressingautonomic nervous system activity in young adults: a randomized cross overstudy” Journal of Physiological Anthropology 34, 21. https://doi.org/10.1186/s40101-015-0060-8

[3]            Crown (2013) “FirePerformance of Green Roofs and Walls”, Department for Communities and LocalGovernment, Eland House, Bressendon Place, London UK

[4]             SFS Façade Fire Safety DesignCommittee (2019) “Society of Fire Safety Practice Guide Façade/External WallFire Safety Design”, Revision 002, Engineers Australia Society of FireSafety

[5]             Dahanayake,K.C., Yang, Y., Wan, Y. et al. (2020) “Study on the fire growth inunderground green corridors”. Build. Simul. 13, 627–635.https://doi.org/10.1007/s12273-019-0595-4

[6]             Dahanayake, K.C.,and Chow, C, L.. (2018) “Moisture Content, Ignitability, and Fire Risk ofVegetation in Vertical Greenery Systems” Fire Ecology, Volume 12, Issue 1,pp. 125-142, doi: 10.4996/fireecology.140112514

[7]             McNeilage, A. (2012) “GreenWalls ‘need building code’ to reduce fire hazard” Sydney Morning Herald,September 15, 2012.