Heat stress is a growing problem in the urban environment. In this TKI project, industry, real-estate managers, and government and research entities joined forces to optimise the cooling function of evaporation from blue-green roofs. The project produced a widely applicable roof system, as well as unique measurements of the water and energy balance in blue-green roofs. The system also contributes to the reduction of rainwater runoff and makes a greater diversity of vegetations possible, which also sustains a rich insect life.
Cooling the urban environment with an optimally evaporative blue-green roof system
Heat stress in the urban environment is a growing problem worldwide. By evaporating water, plants can make a significant contribution to lowering air temperature. Green areas are however quite limited in cities, and temperatures can rise sharply and negatively impact the living environment. Given the huge surface they represent, roofs have the potential of making a great contribution to the greening of a city and to making it climate-robust.
That green roofs are technically feasible and roofs can be covered in lush and flowering vegetation is no longer news. Conventional green roofs tend however to be planted with species that require little water. The cooling function of such roofs is therefore minimal. Planting other types of plants is possible, but this requires a thick, and therefore heavy, granulate layer, which is frequently not technically possible in today’s buildings. In this project we developed a system which optimises the water supply to the plants, and thereby minimises the thickness and weight of the required granulate layer, leading to a maximal cooling rate and lowering of rainwater runoff levels.
Optimisation and validation of passive irrigation system for urban cooling with blue-green roofs
In this project we optimised and validated a passive irrigation system in which water, via special fibres, is recirculated in a capillary manner to the substrate layer when the plants need it. In this way the plants, in contrast to conventional systems, can optimally transpire over the long term, and thereby contribute to cooling the urban environment.
An essential element of the project was the quantification and scientific underpinning of these properties. To this end, measurement instruments were developed for specific application to roofs in inner-city areas. By setting up test plots and conducting innovative measurements using advanced lysimeters, which are specially adapted to the roof environment, the water and energy balance of the different variations of the system were studied closely. Below on this page, some of these measurements can be followed live.
Less heat stress thanks to optimally evaporative roofs
The objective of the project was to improve the cooling properties of green roofs in urban areas. Through rainwater storage and capillary delivery, the evaporation from a blue-green roof can be substantially increased. This was illustrated by the measurements taken during the two-week heat wave in June 2017. The conventional blue-green roof test plot evaporated 18 litres per m2 over the period. The test plots with storage and capillary irrigation evaporated more than double as much over the same period: no less than 42 litres per m2. Calculations based on the measurements show that the actual evaporation from a Sedum vegetation increases from an average of 290 mm/p.a. in a conventional system without water storage, to 386 mm/p.a. with capillary irrigation and water storage of 80 mm, with a potential evaporation of 401 mm/p.a.
The blue-green roof system with capillary irrigation, when compared to conventional green roofs, converts over the long term an average of almost 50% more incoming (solar) energy into water evaporation, thus reducing the amount of energy converted into air-heating. Moreover, during dry, hot periods, conventional roofs quickly run low on water, which further increases the performance gap between the two systems. In conventional roofs during such periods, most of the incoming energy is converted into air-heating, while the evaporation, and thus the cooling effect of the blue-green roofs, remains steady over the long term.
Besides the realised research objectives, a number of other observations deserve mention. Because of the continuous availability of water, the soil humidity in the plots with capillary irrigation and storage remains stable. This produces a clearly visible difference in vegetation and a high species diversity. The plots with capillary irrigation have a rich mixed vegetation of Sedum, grasses and herbs, while the variety on the conventional green roof does not go beyond the planted Sedum. This is also reflected in the insect diversity: a very high number (42) of species were found on the research roof over 24 hours.
Since the installation of the Project Smartroof 2.0, the air-conditioning units in the spaces below the roof have not been used. The installation of the blue-green roof on top of the non-insulated roof kept enough heat away from the building to suppress the need for air-conditioning. Temperature measurements support this: even on the hot summer days in 2018, the temperatures in the water storage layer didn’t rise above 23-24 °C.