A theoretical nongray rigorous model was constructed to study the radiative heat transfer through different greenhouse covering materials by using Radiation Element Method by Ray Emission Model (REM2). This model was applied to find the difference in thermal performances performed by greenhouses that were covered with different claddings such as silica glass, Polyvinylchloride (PVC) and Low Density Polyethylene (LDPE) materials. By utilizing the wide-range spectral radiative properties (0.22-25 mu m) for these materials and taking into account the absorption and emission within the covering material, precise estimations of greenhouse temperatures have been achieved. Moreover, differences in greenhouse (enclosures) temperatures have been confirmed between the semi-transparent plastic films and opaque glass. In addition, outdoor experiments were conducted to measure how much heat can be trapped inside the three identical rectangular enclosures covered by the above mentioned materials. Enclosure inside air, ground surface and cover temperatures' measurement showed a good agreement with the calculated ones by using the rigorous model. (C) 2014 Elsevier Ltd. All rights reserved.