Hygrothermal and environmental performance of a perlite-based insulating plaster for the energy retrofit of buildings

In the last few years, thermal insulating plasters have started to be an attractive solution for the insulation of already existing wall structures, especially old masonry ones, where refurbishment interventions can involve the replacement of damaged plasters. Intensive research efforts are being made to reduce the thermal conductivity and the environmental impact of these materials by optimizing their mixtures (combination of lightweight aggregates, binders and additives). In the present study, the hygrothermal performance and environmental impact of the different perlitebased plasters that are currently being developed have been investigated. A series of analyses has been carried out, at a material scale, by means of heat flow meter apparatus, to determine the relationship between the perlite content and the thermal properties. Moreover, the effect of the moisture content on λ has been analyzed, and the embodied energy and embodied carbon of the four mixtures have been assessed using both the cradle-to-gate and the cradle-to-site approaches. Furthermore, in-situ measurements have been conducted at a demonstration site, at a component scale, and a series of heat and moisture transfer simulations has been carried out to evaluate the actual thermal behaviour of the plaster under real operating conditions. The thermal conductivity values of the four plaster mixtures ranged from between 0.118W/mK and 0.059W/mK, thus demonstrating that the perlite concentration had a significant impact on the reduction of thermal conductivity and that the embodied energy of the applied material (5 cm thickness) decreased as the perlite content increased. Moreover, the results of the measurements on the demonstration building and the hygrothermal simulations have revealed that the thermal insulating plaster is able to reduce the U-value of the wall. However, an increase of 26–30% of the actual thermal conductivity should be considered when the material is exposed to real operating conditions.