Energy saving in the building sector is one of the key issue to achieve environmental targets at national and EU levels. Even if characterised by a large number of different climatic conditions, Italy energy policies were aimed at reducing the energy consumption related to space heating in buildings, neglecting other relevant energy uses as space cooling, which has dramatically increased in the past years. The recent EU Directive for the State Members is to assess the energy quality of buildings taking into account all the relevant energy uses.
Cool roofs are an old concept merged with new technologies that play a crucial role in the energy balance of buildings especially at Mediterranean latitudes. Reducing the solar gains thanks to the roof high solar reflectance, cool roofs keep the building cooler during cooling season, reducing the cooling demand and increasing the thermal comfort; On the other hand this technology has a negative impact on the winter energy balance. It is be noted that the performance of the roof surface also depends on the thermal emissivity, that drives the radiative exchanges of the roof towards the sky and the surrounding objects. If the performance of the cool roof products is a function of the above cited quantities, the technology impact is a function of several other parameters: climatic conditions, building geometry, thermo-physical characteristics of the building envelope. Energy rating and labelling are a quick solution to compare energy related products, as inferred from recent EU Ecodesign Directive which requires the energy labelling not only for products that consume energy but also for energy related components, as windows.
This study presents the first studies aimed at the definition of an energy rating scheme for cool roof, starting from product properties and reaching the building performances, with a focus on dwellings. The methodology is based on a wide number of energy simulations carried out with an accurate dynamic calculation tools. The variables taken into account are:
- Roof surface radiative properties;
- Reference buildings characteristics - including solar and thermal properties, geometry, orientation;
- Climatic zones: The calculations were carried out for Palermo, Rome and Perugia in order to take into account a wide variety of summer and winter conditions of the Mediterranean areas of the country.
The calculations were performed with an insulated and not-insulated building configurations. Hourly and annual heating and cooling demands were calculated, as well as effective heating and cooling degree days.
The results were used to find out simple linear regressions, expressing the energy performance of the building as a function of the roof radiative properties and, as a consequence, measuring the performance of the cool roof product. Limits and potentialities of the method are discussed.