The increasing demand of reducing energy consumption in buildings has led to the implementation of insulation material in roof assemblies to reduce heat gain through the roof. This paper investigates the thermal performance of reflective insulation and radiant barrier systems in a gable roof assembly by using computational fluid dynamics (CFD) simulation. In the first part of the paper, the experimental measurements using test huts under actual outdoor environment were detailed. The CFD simulation was then conducted and the results obtained from the simulation were validated against the experimental measurements. In the second part paper, the validated CFD model was then used to evaluate the reduction of heat flux transmission through the roof and the thermal resistance (RSI value) of the roof assemblies for various roof configurations, which are the thickness of reflective air space, the roof pitch and the roof material. The study showed that increasing the reflective air space thickness from 25 mm to 100 mm has increased the total RSI by 26％ for both roof pitch of 30° and 45°. Besides that, increasing the roof pitch of the roof from 30° to 45° showed that biggest impact on the thermal performance of the roof, as the RSI value increases by 30％. In terms of roof tiles material, only small difference was observed in terms of thermal resistivity when switching the roof tiles material between clay tiles and concrete tiles.