Abstract
Building roofs play a very important role in the energy balance of buildings, especially in summer, when they are hit by a rather high solar irradiance. Depending on the type of finishing layer, roofs can absorb a great amount of heat and reach quite high temperatures on their outermost surface, which determines significant room overheating. However, the use of highly reflectivecool materials can help to maintain low outer surface temperatures; this practice may improve indoor thermal comfort and reduce the cooling energy need during the hot season. This technology is currently well known and widely used in the USA, whilereceiving increasing attention in Europe. In order to investigate the effectiveness of cool roofs as a passive strategy for passive cooling in moderately hot climates, this paper presents the numerical results of a case study based on the dynamic thermal analysis of an existing office building in Catania (southern Italy, Mediterranean area). The results show how the application of a cool paint on the roof can enhance the thermal comfort of the occupants by reducing the operative temperatures of the rooms and to reduce the overall energy needs of the building for space heating and cooling.
Highlights
IntroductionMost of the energy regulations in Mediterranean countries currently prescribe high thickness for the insulation of the envelope, especially for roofs
The roof surface represents about 20–25% of urban surfaces and 60–70% of the building envelope on average in Italy, depending on the building typology [1]; it plays a very important role in the energy balance of buildings, and it is important to find appropriate solutions to improve its energy performance, in relation to the specific climate.In particular, the solar radiation impinging on the roofs can raise their outer surface temperature up to 50– 60∘C, that is to say, 10–15∘C higher than the surrounding green areas [2].most of the energy regulations in Mediterranean countries currently prescribe high thickness for the insulation of the envelope, especially for roofs
One of the most noticeable aspects related to the use of a cool paint on the finishing layer of a roof is the sharp reduction of its outer surface temperature; according to several case studies [29,30,31], a mean reduction of 12∘C is expected when using a product with average quality (r = 0.45), whereas the use of a high-reflective paint (r = 0.85) can introduce a temperature reduction up to 25∘C
Summary
Most of the energy regulations in Mediterranean countries currently prescribe high thickness for the insulation of the envelope, especially for roofs. This approach is appropriate to reduce the energy needs in winter, but it is not very effective in summer as a tool for reducing the room overheating. As a matter of fact, the use of high insulation levels in hot climates strongly reduces the effectiveness of passive cooling strategies, traditionally based on high thermal inertia, air permeability, and light colors as far as roof is concerned [3]. As discussed by Masoso and Grobler [4], it is not always true that lower values of the thermal transmittance of the envelope reduce the annual energy consumption for space heating and cooling. It is possible to determine a threshold value of thermal transmittance (point of thermal inflexion) that, if overtaken, brings to negative energy savings on an annual basis
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