Characterization of thermal insulation products for industry and civil construction
Materials for thermal insulation are characterized basically by two kinds of effect: resistive and radiant.
Resistive insulation occurs by the material’s heat-transfer resistance. Because air is highly resistant to heat conduction, the best resistive insulation materials are those that confine air in small cells designed to eliminate convection movements. Insulation materials such as glass fiber, rock fiber and expanded polystyrene are typical examples of this category and are characterized by their low thermal conductivity. This property is measured in IPT’s laboratories using standard methods such as ASTM C177.
Radiant insulation works by reducing radiation heat transfer. The ability of a material to reflect, absorb or emit radiation depends on the nature of its surface. Here it is important to distinguish between the radiation spectra in question. Materials that work as radiant barriers are characterized by their low emissivity, which is related to the infrared radiation spectrum. IPT performs evaluations of this property in its laboratories, according to standard methods such as JIS A 1423. Materials used on external surfaces, such as paints, are also characterized by their property to reflect or absorb solar radiation. In this case, solar radiance reflectance is also measured (ANSI/ASHRAE 74 method), thus considering the entire spectrum of solar radiation. To characterize the thermal insulation potential of these materials, the “Solar Reflectance Index” is calculated using the ASTM E 1980 method, which considers these two properties.