January 2013
Publication year: 2013
Source:Building and Environment, Volume 59
Green roof plants alter the microclimate of building roofs and may improve roof insulation. They act by providing cooling by shading, but also through transpiration of water through their stomata. However, leaf surfaces can become warmer when plants close the stomata and decrease water loss in response to drying substrate (typically associated with green roofs during summers), also reducing transpirational cooling. By using a range of contrasting plant types (Sedum mix – an industry green roof ‘standard’, Stachys byzantina , Bergenia cordifolia and Hedera hibernica ) we tested the hypothesis that plants differ in their ‘cooling potential’. We firstly examined how leaf morphology influenced leaf temperature and how drying substrate altered that response. Secondly, we investigated the relationship between leaf surface temperatures and the air temperatures immediately above the canopies (i.e. potential to provide aerial cooling). Finally we measured how the plant type influenced the substrate temperature below the canopy (i.e. potential for building cooling). In our experiments Stachys outperformed the other species in terms of leaf surface cooling (even in drying substrate, e.g. 5 °C cooler compared with Sedum ), substrate cooling beneath its canopy (up to 12 °C) and even – during short intervals over hottest still periods – the air above the canopy (up to 1 °C, when soil moisture was not limited). We suggest that the choice of plant species on green roofs should not be entirely dictated by what survives on the shallow substrates of extensive systems, but consideration should be given to supporting those species providing the greatest eco-system service potential.
► Of all the species tested, leaf surface temperature was lowest in Stachys , even when water was limited. ► On warm days, both Stachys and Sedum cooled the air above the substrate compared to bare soil. ► On several hot afternoons in the glasshouse Stachys provided more aerial cooling than other species. ► In outdoor conditions we recorded one incidence where Stachys provided additional localised aerial cooling. ► On a warm day, temperatures below Stachys and Sedum canopies were 11 °C and 4 °C lower than of bare soil.
Publication year: 2013
Source:Building and Environment, Volume 59