Gypsum plaster is a building material used in walls or false ceilings. The aim of this paper consists on the improvement of thermal properties and lightness of gypsum plaster by combining it with granular cork collected from Moroccan Maamora’s forest. This composite material is intended to be used in false ceiling such as cork–gypsum board instead of plasterboard; its use will be a contribution to improve energy efficiency in buildings. By varying the granular cork size, an experimental investigation of thermal proprieties of gypsum based composite material with embedded granular cork was mainly performed using the asymmetrical transient Hot Plate method. A comparative study based on other experiments (Differential Scanning Calorimeter and Steady state Hot Plate) was realized and the thermal conductivity results were confronted to different theoretical models of equivalent thermal conductivity determination. The experimental results exhibit a good agreement with the weighted geometric mean equation.