Fire Hazards with Vertical Greenery Systems

By C.L. Chow, S.S. Han, K.C. Dahanayake and W.K. Chow

Vertical greenery systems (VGSs) have been widely used¹ in new green construction projects in dense urban areas with limited green living spaces to mitigate the urban heat-island effect. A VGS features plants grown on façades inside or outside buildings. Several examples are shown in Figure 1.

Green façades are popular because they can reduce wall surface temperatures in tropical urban areas up to 12°C.² However, Fischetti has described the possible adverse environmental effects of greenery systems.³ Additionally, vine-type plants⁴ installed for vertical growth can be ignited by a window plume⁵ from a post-flashover room fire, as shown in Figure 2. The fire hazards associated with VGSs that are demonstrated in scale modeling experiments (see Figure 3) show that fire spread pattern and rate depend on the plants and their geometrical arrangements.

Fire hazards of VGSs are usually not the primary considerations on building stakeholder minds when developing green construction.^{6, 7} Fortunately, the fire hazard concerns of green architecture are often identified by fire department officials.

In many locations, building codes and standard fire tests are not available for such green architectural features.^7-9 For VGSs, fire behavior has only been evaluated on the basis of preliminary studies of standard tests for normal roofs and walls^{8, 9} as recommended by the Green Building Council in the United States. Approval Standard FM 4477¹⁰ was proposed to evaluate the performance of vegetative roof systems related to fire, and the fire performance of green roofs and walls was included¹¹ in the United Kingdom and in Sweden¹² based on preliminary studies with existing fire tests.

Appropriate fire scenarios for testing green constructions are not yet available, nor is there any systematic compilation of published articles reporting fire incidents in green constructions that include VGSs.

Knez has pointed out the fire risks of climbing, hydroponic and modular VGSs.¹³ However, the consequences of burning plants grown on VGSs have not yet been investigated thoroughly. A possible reason is that very few fire incidents have been documented in green construction⁸ — not enough to attract sufficient public concern. For example, a fire incident involving a VGS was reported¹⁴ in a semi-enclosed beer garden in August 2012 in Sydney, Australia. At this incident, a patron tried to light a cigarette with a candle and a nearby fern ignited. The fire spread quickly in only a few seconds. It was found that the green wall did not contain a proper irrigation system and the plants had been hand-watered. Furthermore, some synthetic plants were also integrated into the green wall system.

Poorly irrigated VGSs can be a possible ignition source since dry plants burn faster and produce more heat compared to wet plants. This behavior was demonstrated in the scale modeling test on the green wall as shown in Figure 3. In this experiment, plant samples of selected species with different moisture contents were tested with a cone calorimeter.¹ The transient heat release rate of fresh plants was not significant upon ignition, but the transient heat release rate increased when moisture content decreased. Key fire parameters f, including¹ peak heat release rate, average effective heat of combustion, peak carbon dioxide concentration and peak carbon monoxide concentration, decreased with moisture content (MC).

An empirical curve on f can be fitted in terms of f_o, the value f at zero MC and an experimental parameter a as:

f = f_o exp (-MC/a)

It is obvious that vegetation grown on building façades provides fuel for the spread of fires. The direct action of a window flame plume⁵ can ignite plants, including dry or dead leaves, planter boxes, foams, and laminar layers of felt sheets. Heat, flame, smoke and hazardous chemicals generated from burning plants and VGS accessories can spread to different parts of the building and to nearby areas. The heat and toxicity of the smoke can affect firefighters if they lack appropriate personal protection equipment. All of this means that the scientific aspects of VGS fires should be further investigated and better understood.

More attention^{7, 8} should be paid on assessing fire hazards of green buildings. Although there have not been many exterior wall fires, their resulting consequences in terms of the extent of fire spread can be very serious. For VGSs that are integrated into exterior walls, fire spread through the VGS should be carefully monitored. In particular, the scenario identified earlier⁵ involving a window plume from a post-flashover room fire, as in Figure 2, can ignite plants in the VGS, resulting in different fire hazards under different conditions. The fire load is low when the plants are green and there are few dry leaves or branches,⁵ but can be high otherwise. Thus, exterior vegetative covering on a façade requires a more-detailed fire hazard assessment.

C.L. Chow, S.S. Han, K.C. Dahanayake are with the Department of Architecture and Civil Engineering, City University of Hong Kongand and W.K. Chow is with the Department of Building Services Engineering, the Hong Kong Polytechnic University.

¹K.C. Dahanayake, C.L. Chow, “Moisture content, ignitability and fire risk of vegetation in vertical greenery systems”, Fire Ecology, 14(1): 125–142 (2018).

 ²N.H. Wong, A.Y.K. Tan, P.Y. Tan, A. Sia, N.C. Wong, “Perception studies of vertical greenery systems in Singapore”, Journal of Urban Planning and Development, 136(4): 330–338 (2010).

³M. Fischetti, “Trees that pollute”, Scientific American, June 2014, p. 14.

⁴K. Perini, M. Ottelé, A.L.A. Fraaij, E.M. Haas, R. Raiteri, “Vertical greening systems and the effect on air flow and temperature on the building envelope”, Building and Environment, 46(11): 2287–2294 (2011).

⁵L. Miao, C.L. Chow, “A study on window plume from a room fire to the cavity of a double-skin façade”, Applied Thermal Engineering, 129: 230–241 (2018).

⁶W.K. Chow, (FEA1)“Fire safety in green or sustainable buildings: Application of the fire engineering approach in Hong Kong”, Arch. Sci. Rev., 46(3): 297–303 (2003).

⁷B. Meacham, B. Poole, J. Echeverria, R. Cheng, “Fire safety challenges of green buildings“, Springer briefs in Fire. J. Milke, Series Editor, Springer, 2013.

⁸National Fire Protection Association, The latest statistics, research and news about fire 2015. Retrieved from http://www.nfpa.org/research.

⁹ASTM E2777-14, Standard Guide for Vegetative (Green) Roof Systems, ASTM International, West Conshohocken, PA, 2014.

¹⁰Approval Standard for Vegetative Roof Systems, Class Number 4477, FM Approvals LLC, June 2010.

¹¹Fire Performance of Green Roofs and Walls, Department for Communities and Local Government, UK, August 2013.

¹²M. Strömgren, P. Mindykowski, “Next generation Nordic fire safety engineering”, Brandposten, No. 55, p.18–19 (2017).

¹³N. Knez, Reaction to fire of green façades and roofs (No. 2015), Solvenian National Building and Civil Engineering Institute, Brussels, November 2014.

¹⁴Landscape Association NSW & ACT, “Green walls need building code for fire hazard”, Landscape Connection, p. 8, 2012.