Energy: 3°C cooler in the home in the summer with green roofs and walls

A reduction in the indoor temperature during the summer of up to 3°C thanks to “green insulation” consisting of plants on the roofs and walls of buildings, which reduces heat flow by almost 50% through the shading and transpiration of plants arranged to protect against solar radiation. These are some of the latest findings of the ENEA project entitled “Green infrastructure to improve the energy efficiency of buildings and the quality of the microclimate in urban areas”, financed under the Programme Agreement for Electricity System Research 2019-2021 of the Ministry of Economic Development, now under the Ministry of Ecological Transition.

The layer of plants acts as a thermal insulator year round, with increased effects in the spring and summer when the vegetation also acts as a natural extractor of heat from the environment. In general, the beneficial effect of thermal regulation is due to summer shading, evapotranspiration and the chlorophyll photosynthesis of plants.

“Thanks to a sophisticated system of sensors for microclimate monitoring, we found that the layers of vegetation covering the roof and external walls of the prototype building at the ENEA Casaccia Research Centre near Rome are able to keep the surface temperatures below 30°C, and thus prevent the sharp thermal fluctuations that the roofs and walls are usually subjected to without vegetation, reaching peak temperatures of more than 50°C during the hottest hours”, explained Arianna Latini, researcher at the Energy Efficiency Unit Department. “And that’s not all. Preliminary data suggest that a reduction in electricity consumption of approximately 2 kWh/m² can be achieved. On average, this translates into electricity savings of around 200 kWh for the summer air conditioning of a 100 m² house, taking into account an indoor comfort temperature of no more than 26°C”, Latini added.

In the experimentation started in 2013 by the then ENEA Agricultural Service led by Carlo Alberto Campiotti, succulent plants of the Sedum genus of the Crassulaceae family were used on the green roof, as they were considered more suitable for the Mediterranean area due to their shallow root system, efficient water use, tolerance to extreme drought conditions and CAM (Crassulacean Acid Metabolism) to fix carbon. “In addition to a rich collection of Sedum, we also used a mix of Festuca and Poa grasses in a section dedicated to Gramineae, with results showing how the contribution of plant species is related both to their characteristics in themselves and to local microclimatic conditions", explained Patrizia De Rossi, researcher at the ENEA Energy Efficiency Unit Department.

Starting with the species most commonly used in green roof coverings, the ENEA study was further extended by testing some spontaneous and native Mediterranean species, such as Echium plantagineum and Echium vulgare, plants that also favour pollinator biodiversity.

On the south-east and south-west façades of the prototype building, ENEA researchers used Parthenocissus quinquefolia,^[1] known as the Virginia creeper, a flowering vine that is very resistant to both heat and cold (in the fall its leaves turn deep red). “We found that the surface temperatures of the green wall are up to 13°C lower than those of the non-vegetated façade, reducing heat flows to the interior by approximately 7 kWh/m² and emissions by up to 1 kg CO₂/m² due to lower energy consumption”, Latini remarked.

Shifting from a building to the city, greening 35% of the EU's urban area (over 26,000 km²) would reduce energy demand for summer cooling of public, residential and commercial buildings by up to 92 TWh per year, with a net present value (NPV) of over €364 billion, and avoid greenhouse gas emissions equivalent to 55.8 million tonnes of CO₂ per year.^[2] To get a realistic idea of the emissions avoided, consider that the agricultural sector in Italy emits 30 million tonnes of CO₂ equivalent per year (ISPRA data 2021).

Hence the need for action in urban areas as soon as possible, launching initiatives and actions to counter the negative impacts of global warming, impacts that include excessive consumption of fossil energy (summer air conditioning accounts for about 30% of total consumption, and the figure is rising), heat islands and heat waves that are increasingly frequent in the summer months, environmental pollution and loss of biodiversity. In fact, green roofs not only reduce temperature increases due to the heat island effect in the city, but also improve air quality. In a study of tree and shrub species commonly found in urban greenery, the average capacity to mitigate air pollutants was found to be 58-140 g of ozone (O₃), 17-139 g of particulate matter PM10, 11-20 kg of carbon dioxide CO₂ per plant per year. Vegetation on buildings is also useful in the absorption of volatile organic compounds (VOCs): ivy and other climbing plant species on the green wall at the ENEA Casaccia Research Centre have led to a reduction of around 20% in benzene, toluene, ethylene and xylenes – the most common VOCs in the urban environment – despite the fact that the area under consideration is not highly exposed to these gaseous compounds that are polluting and unhealthy for humans. Moreover, urban greenery also performs a number of services for the ecosystem such as improving the beauty of living and working environments, protecting biodiversity and slowing excess rainwater runoff.

For more information:

ENEA Report “Plant Ecosystems for the Ecological Regeneration of Cities”.
The technical paper was awarded a special prize at the ECOtechGREEN International Forum in recognition of the importance of the research conducted, aimed at promoting the use of green walls and roofs on buildings as innovative elements to improve the energy and environmental sustainability of the building sector.

Arianna Latini, ENEA Energy Efficiency Unit Department,

Patrizia De Rossi, ENEA Energy Efficiency Unit Department,

[1] Experimental data from the green wall consisting of Parthenocissus quinquefolia are the subject of an article being published in a special issue dedicated to the topic of green infrastructure in cities for sustainability and energy saving (https://www.mdpi.com/si/88334).

[2] Quaranta E, Dorati C & Pistocchi A. 2021. Water, energy and climate benefits of urban greening throughout Europe under different climatic scenarios. Sci Rep 11, 12163 (2021).