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Living walls are created in one of several ways:
- Climbing plants - growing directly against the wall or trained against a trellis. The plants can be rooted into the ground or in substrate-filled planters. Such systems are usually irrigated but can survive without irrigation if rooted in the ground.
- Hydroponic green walls - usually constructed from plastic mesh, geotextiles, fabrics, or horticultural mineral wool, or a combination of materials, and fixed to supporting frames or boards. Plants grow without substrate or soil and rely on irrigation and nutrients added to the irrigation water.
- Modular green walls - made from purpose-built HDPE (high-density polyethylene) modules, containing cells that are filled with growing medium and planted. Modules are fixed to a wall or frame and combined to cover large areas. Irrigation water is usually delivered to the top of each module via irrigation lines. Nutrients are contained in the soil or added to the irrigation water supply.
Most living walls are constructed using bespoke carrier systems, and the parts can vary. For example, a system may use the rails that form part of a building's cladding to support modular plant boxes. Other elements can include water-resistant backing layers, irrigation systems, drainage channels, and, of course, the growing medium and plants.
Fire risks with green walls
Like any cladding system, there will be a backing wall with insulation and the possibility of vertical voids through which a fire may spread. Even if the system has suitable cavity barriers installed, some components (including some types of growing medium) may be combustible and contribute to fire spread.
It’s important to look at the whole wall build-up when considering the fire susceptibility of a living wall. Wherever possible, the backing wall, insulation, and support systems should be of non-combustible material to limit the spread of fire as far as possible.
Water and wind damage to the plants and building also need to be managed, as does the potential impact to the building's fabric if the green wall is allowed to dry out. A dried-out green wall also increases its fire risk.
A well-planned maintenance schedule and proper precautions can reduce fire risk:
- Prohibit hot works during the wall’s installation.
- Even when the wall is installed, hot works should not happen within 10m of the facades of the building.
- Regularly inspect the irrigation system to ensure the growing medium does not dry out.
- Situate waste bins, smoking receptacles, and smoking areas at least 10m from the wall to reduce fire risk.
- Forbid the use of BBQs or smoking on balconies in living walls, or take action to mitigate the risks.
- Review your fire risk assessment and update it if the green wall is a retrofit project.
We are starting to see more building specifications that include green walls. The insurance industry has embraced the drive towards achieving Carbon Net Zero, but sustainable options bring their own risks.
For instance, as green walls normally contain combustible materials, they are not normally suitable for use on high-risk residential buildings.
We advise you to involve your insurer in the planning stage of any new sustainability project. That way, you can work together to mitigate risks early in the project and achieve a safe and sustainable building.
The Fire Protection Association has issued guidance on living walls, which you can find here.