Quantifying cooling effects of facade greening: Shading, transpiration and insulation

• Quantification of cooling processes for the urban street canyon and the building.
• Distinction between shading and transpiration effects.
• Demonstration of possible disadvantages through the greening's insulation effect.
• Quantification of transpiration rates and water demand of 3 facade greening species.

Facade greening is expected to mitigate urban heat stress through shading, transpiration cooling and thermal insulation. This study quantifies cooling effects of facade greenings for the building and the street canyon and distinguishes between transpiration and shading effects. Additionally it discusses insulation effects.Outdoor experiments were conducted during hot summer periods on three building facades in Berlin, Germany.We determined transpiration rates (sap flow) and surface temperatures of greened and bare walls as well as of plant leaves (temperature probes) of three climbing plants: Parthenocissus tricuspidata, Hedera helix and Fallopia baldschuanica. Furthermore, air temperature, relative humidity and incoming short-wave radiation were measured.No cooling effect was detectable for the street canyon. Surface temperatures of the greened exterior walls were up to 15.5 °C lower than those of the bare walls, while it was up to 1.7 °C for the interior wall (measured during night-time). The cooling effects mainly depended on shading, whereas a lower proportion was due to transpiration. Insulation of the direct greenings reduced radiation during night-time. We conclude that greening can be an effective strategy to mitigate indoor heat stress as long as the plants are sufficiently irrigated with up to 2.5 L m−2 d−1 per wall area.