Vertical Greenery Systems (VGSs) enhance the thermal performance of the built environment by controlling the heat transfer through the building envelope. The vegetation and substrate of VGSs absorb solar radiation and create a cooling effect through evapotranspiration. These systems further improve shading as well as insulation effects of the building envelope. This study focuses on two important aspects of VGSs in modern buildings: (i) the thermal effect as well as the passive energy benefits of VGSs and (ii) the possible fire hazards associated with vegetation in VGSs.  

The passive energy performance of VGSs is studied using building energy simulations. A mathematical model based on the heat balance principle is developed in this study in order to calculate the surface temperature of building facades with VGSs. The model accounts for shading effect, insulation effect, and evapotranspiration effect created by VGSs. The model was then integrated into EnergyPlus, a widely accepted building simulation program, to simulate VGSs together with a building model. The integrated VGS model has been validated against four reported experimental studies, under different climatic regions. The simulation results showed good agreement with the experimental results.

The simulation results showed that the exterior surface temperature of building facades with VGSs are significantly lower compared to that of a bare facade. In accordance to Hong Kong climate the exterior surface temperature of facade with VGS shows up to 24 ^oC lower compared to a bare facade, on the day with highest outdoor air dry-bulb temperature, and up to 16.9 ^oC on the day with lowest outdoor air dry-bulb temperature. These results clearly show that VGSs can control the heat gain by the building. Furthermore, simulations were carried out to study the energy benefits of VGSs among four cities in different climatic zones based on Koppen climate zone classification: Tropical moist climate, Dry climate, Sub-tropical humid climate, and Mediterranean climate. The highest annual energy benefit of 64 kWhm^-2 was found in Dry climate of Abu Dhabi, UAE. These results demonstrate that VGSs can provide significant energy benefits in climates with higher solar radiation and outdoor air dry-bulb temperatures. However, energy benefits decrease when solar radiation and outdoor air dry-bulb temperature decreases.

With respect to the effect of the characteristics of VGS, results show that energy saving potential of VGSs are influenced by certain plant species, area of greenery cover, and orientation of VGS. For example plant species with higher leaf area index (LAI) provide higher benefits. The increase of LAI from 1 to 5 would increase the annual energy benefits from 0.4% to 1.8%. The energy benefits show a positive linear relationship with the area of greenery cover. For instance, increase of the area of greenery cover from 25 % to 100 % result would increase the energy benefits from 0.42 % to 1.62 % on west-facing wall. In terms of orientations of VGS, the west-facing facade enjoys the highest saving of 1.62 %, followed by the south-facing (1.58 %), east-facing (1.31 %), and at last north-facing (0.51 %) facades respectively for a multi-story building in Hong Kong. When comparing the energy use of buildings with VGSs to green roofs (GR), VGSs are more beneficial for multi-story buildings. A GR with area of 2500 m² shows 0.73 % of energy saving, whereas an equal area of VGS on west-facing facade shows 1.62 %. It should be noted that all percentage values are based on a multi-story hypothetical building in the Hong Kong climate. Moreover, with the increase in building height, more area is available for VGSs, whereas the area for GR is limited. In summary, this study suggests that VGSs effectively reduce the cooling load of a building, with benefits depending upon the plant species, area of greenery coverage, orientation, scale of the building, surrounding environment, and the geographical location.  

Possible fire hazards associated with the vegetation in VGSs were studied using a set of experiments with a Cone calorimeter. The moisture content (MC) of a plant is identified to be the most critical factor affecting its ignitability. Three commonly used plant species for VGSs were selected for the study namely, Hedera helix, Peperomia obtusifolia, and Aglaonema commutatum. Hedera helix and Aglaonema commutatum started to ignite once the MC was lower than 243 % and 316 % respectively at heat flux of 50 kWm^-2. Peperomia obtusifolia started to ignite once the MC dropped below 200 % at heat flux of 20 kWm^-2. MC is expressed as the percentage of water over the dry mass of the vegetation. The peak heat release rate, emission of smoke and gaseous effluents, ([CO₂] and [CO]) notably increased with the decrease of MC. The results of the present study clearly indicate the importance of maintaining a healthy and live VGSs to ensure a minimum risk of fire. Inadequate irrigation or malfunctioning of the irrigation system would lead to the plants drying out in the VGSs, creating a favorable condition for fire.