Investigating the performance of reflective insulation and low emissivity paints for the energy retrofit of roof attics

The energy retrofitting and conversion of attic spaces into liveable dwelling units is a widely diffused practice in EU countries, and it leads to an increment in urban density, without having to construct new buildings. Roof surfaces are the parts of buildings that are exposed the most to the sun in summer season, and for this reason, they are responsible for high cooling loads, which can determine overheating phenomena in the dwellings below the attics. Different strategies that involve the use of extra-insulation or an increase in the solar reflectance of the outer layer can be adopted to resolve this problem. However, another solution that is sometimes adopted is the introduction of long-wave infra-red reflective insulations, which are generally applied below the roof tiles, facing an air cavity. This strategy allows the thermal resistance of a roof to be increased, as a result of a reduction in the radiation heat exchange, without having to increase the insulation thickness. In this study, an in-field experimental campaign has been carried out in Turin, Italy. The analysis was aimed at investigating the thermal behaviour of traditional roof adopting reflective insulation, made up of aluminium sheets, and of a new strategy, based on a treatment with low emissivity paint below the roof tiles. The experimental data were used to verify numerical 1D heat transfer simulations to account for the effect of the radiation heat exchange in the airspace below the roof tiles. The verified simulation models were used to analyse the effects of the insulation levels and of the emissivity properties of the reflective layer on the roof heat exchange. A reduction in the indoor summer heat gains of between ∼10% and ∼53% was found for roofs treated with refective insulation, depending on their emissivity properties, while the contribution to reducing the winter heat losses was negligible. The obtained results are encouraging and highlight that reflective insulation, used together with Low-E paints, represents a promising solution that can be used to help design attic retrofitting interventions, with the aim of achieving a better thermal performance in the summer period, and to address the requirements of indoor space saving.