Green roofs and living walls were developed by modern architecture to find sustainable solutions for modern construction. We know green roofs and living walls have many economic, ecological and social benefits. So we thought we’d compare the two using data from green roof studies. As New York living wall installers, our primary interest is in living walls, but since green roofs have been around longer there is a lot more research on them. Below are facts on green roofs drawn from studies we have read. We added our thoughts on how living walls compare.
Reduction of Urban Heat Island Effect
Green roofs are one of the most effective ways in the reduction of the ambient air temperature in urban areas. In summer, temperatures in cities are about 5-7 ° C higher than in the countryside due to the heat absorption of the buildings and roads and the temperature on the traditional roof can be up to 40 ° C higher than that of a green roof.
According to research by the Tyndall Center for Climate Change, there is a need of 10% more greenery in towns to mitigate the UHI Effect.
Living walls also are an effective solution to the heat island effect. By covering building walls, they keep the walls cool. Plants naturally transpire to regulate their temperature so apart from the living wall simply acting as an insulator for the building, the plant transpiration has the added effect of cooling the building.
Side note: Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth and metabolism. The remaining 97–99.5% is lost by transpiration.
Rainwater retention
A major benefit of green roofs is the reduction of surplus rainwater, which results in a reduction of 50 to 90% of the wastewater load in the summer, depending on the roof construction and the local climate. Green roofs have an impact on reducing costs due to low or no need for rain tanks and similar equipment generally used for stormwater management. The ability to retain rainwater can limit accidents caused by heavy rainfall.
Because living walls are by definition vertical, they don’t gather rain as much as a green roof. Only when the rain is driven sideways by wind does the living wall absorb any substantial amount of water.
Rainwater purification
Through natural bio–filtration, green roofs prevent contaminants and toxins from reaching rivers and streams. A study by Kohler & Schmidt (1990), 95% lead, copper and cadmium sulfide and 19% of the zinc from the remaining in the substrate, which helps to improve the quality of local water.
Living walls do purify water like a green roof would, but again, not at all in the same quantities.
CO² reduction
Green roofs help reduce the amount of CO² concentration in the air, which is considered as one of the main causes of global warming. 1m² of green roof absorb 5 kg of CO² per year. Moreover, the reduction in energy consumption due to building insulation has an additional effect on the reduction of carbon dioxide 3.2 kg per year. For the future, 1m² of a green roof can absorb the same amount of CO² as an ordinary car would emit during an 80 km drive.
Living walls match green roofs in this aspect. Living wall plants absorb just as much CO² as a green roof. Because living walls tend to have larger plants than green roofs that just have sedum, it is possible living walls absorb more CO² than your typical sedum green roof.
Cleaner air
Plants on green roofs can also capture airborne particles such as heavy metals, smog and volatile organic compounds from the local atmosphere, which has a positive effect on the air quality and the health of the residents. Researchers estimate that a green roof of 1m² can help absorb 0.2 kg of dust particles in the air every year **.
Living walls do the same. Again, because living walls have larger plants than your average sedum green roof, it is possible that living walls clean the air more than a sedum green roof.
Natural Habitat
As urbanization increases, ensuring biodiversity is one of the key requirements of local councils. Green roofs can provide a habitat for different species and restore the ecological cycle that is disrupted by the urban infrastructure.
Living walls definitely do the same. Because they are vertical they have the added benefit for birds to safely nest in the wall.
Reducing energy use
Green roofs have been shown to have a positive effect on a building’s energy consumption by improving the thermal performance of roofs, although the resulting level of difference depends on daily and seasonal weather conditions. Poorly insulated roofs dominate the spaces underneath in the summer, which means there is a greater need for artificial cooling and excessive demand for heat in the winter. Modernizing the green roofs reduces the consumption of air conditioning and heating. Flat gravel roofs can be warmed up to 21 °C more than green roofs (Kaiser 1981). Studies at Trent University in British climatic conditions have shown that green roof terraces in all roof layers can have considerably lower temperatures than non-planted roofs.
In the summer plants use solar energy for evaporation, reducing the temperature of the green roof and the surrounding microclimate. During the winter months, a green roof can contribute to the insulating properties of the roof. The thermal performance, however, depends to a great extent on the amount of water in the green roof substrate. Water has a negative effect on thermal conductivity. Thus, in a humid winter climate, such as in the United Kingdom, a green roof will
add little to the overall thermal performance of the roof. No fixed U-value is assigned to green roofs as they are assumed to hold water.
Living walls do the same job, only to the walls of the building instead of the roof. The growing medium we use, coconut fiber and a binder, is a relatively good insulator in itself. This has the benefits of keeping the heat out in the summer and the heat in during the winter. The plants themselves don’t do much to insulate in the winter. They do break the wind however, which reduces heat loss. In the summer they definitely help to reduce the heat by transpiration.
Clearly green roofs and living walls go hand in hand in increasing the value of city architecture. Together they keep cities cooler and cleaner. Our living wall installations in New York, one of the densest cities in the world, are a good fit. Green roofs take up valuable real estate, whereas living walls not so much. That is why we think living walls offer so many great benefits while taking up so little space. Living walls and New York City are a great fit.