Role of green coverings in mitigating heat island effects: an analysis of physical models

Worldwide, governments are introducing several rules and standards in the aim of limiting the quantity of primary energy for air conditioning of buildings and supporting the use of renewable source of energy for generating thermal and electric energy to be utilized in buildings such as the European Directive on the Energy Performance of Buildings [1]. Along with these interventions, another important action takes currently place in the building sector, that is the introduction of passive structural components of the envelope, able to reduce the requirements of energy for air conditioning purposes. Among these building components, the so-called green roofs are becoming more and more interesting for designer and buildings owners. This simple technical solution, in fact, apart its effectiveness in lowering particularly the summer cooling loads, could make more attractive the shape of each single building and, when adopted on a large extension, of an entire city district, reducing the risks for urban heat island phenomena that represent a severe problem in urban contexts. In this work, a short review of the literature models for computing the energy balance of roof coverings will be proposed, with particular reference in modelling the heat exchange of a canopy.