In tropical countries, the greatest thermal gain occurs through the roof of a house. In Brazil, the use of an asbestos-cement roof without a ceiling is very common. Thus, there is intense heat transfer to the internal environment, which may cause thermal discomfort to the inhabitants. One way to reduce the heat flux is the use of radiant barriers, which reduce the heat flux through radiation. The use of this type of barrier in Brazil began in the mid-1990s and since then it has become popular for the thermal insulation of building roofs. However, there is a drawback regarding the experimental methods for the evaluation of the energy efficiency of these sheets. Many manufacturers show only the thermal conductivity, or give sparse information on the percentage reduction in radiated heat. There is currently no Brazilian normalization regarding the effective reductions in heat due to the use of these sheets. This study presents a compact apparatus for the determination of the efficiency of insulation sheets based on the use of heat flux transducers. Also, two models using thermal resistances which give simple correlations of great usefulness are presented. The sensitivity of the efficiency in relation to the several variables involved can be verified by exploring these correlations algebraically. Test results for different types of insulation sheets are reported and compared with the theoretical models. (c) 2007 Elsevier B.V. All rights reserved.