Experimental Assessment of the Effects of Low-Emissivity Paints as Interior Radiation Control Coatings

Radiation Control Coatings (RCC) are commonly recognised as paints, in which the long-wave radiation emissivity can be dramatically reduced from 0.9 to below 0.25 due to the dispersion of aluminium flakes inside the base paint. The low emissivity (Low-E) feature makes these materials particularly suitable for reducing the radiative heat exchange in building components and worthy of being used in roof attics, pipes, heat storage tank, etc. However, in the last few years, the application to the indoor surfaces of the building envelope has become quite popular, because the reflective properties can be exploited to increase the thermal comfort and reduce the winter heat losses. Except for aluminium based paint, that, for their strong metallized effect, suffer from some aesthetical limitation, the claimed performance of most of the other commercially available reflective paints are not universally recognized and in most of the cases their properties are misled, referring to visible and short wave infrared reflectivity. In this paper, a new methodology for assessing the long-wave thermal emissivity by using a heat flow meter apparatus is proposed. Moreover, the thermal emissivity of different paint mixtures with reduced metallised effect is assessed. The results allow for affirming that paints with acceptable aesthetic value (limited metallized effect) can reach an emissivity of ~0.60 instead of a typical emissivity of paint between 0.85–0.90. Furthermore, the partition wall of a double climatic chamber apparatus was painted with different low-E paints to evaluate whether an increase of the indoor operative temperature would have been observed. A slight, but not negligible, increase was shown of up to 0.3 °C and 0.6 °C for paint with an emissivity of ~0.6 and ~0.4, respectively.