The emissivity of a surface depends not only on the material but also on the nature of the surface. For example, a reflective foil may have a low emissivity, whereas a plasterboard has a high emissivity. The emissivity also depends on the temperature of the surface and wavelength and angle.
Adjust the surface emissivity by selecting the material and changing the direction of the facing surface. Click 'To External' for exterior facing or 'To Internal' for interior facing surfaces.