Thermal insulation is a good strategy for saving energy and reducing the emission of CO2 especially when produced locally with ecological materials. The hereby document aims at evaluating the thermal and mechanical properties of eco-composite materials based on plaster and wood waste or granular cork. The proportion of used additives were respectively 20 and 30%. Two wood waste types were used as sawdust and wood chips; several granular sizes were used for granular cork. The thermal characterization consists of determining the thermal conductivity, and thermal effusivity of all samples. Flexural and compressive strength were also tested. Results show that the composite materials with 20% wood shaving, 20% granular cork size 3.35–6.7 mm, and 30% sawdust have improved thermal performance with acceptable mechanical resistances. The thermal simulation proves that using these composite materials as plasterboards in buildings can generate thermal energy gain about 38%, compared to reference building. These results indicate that the use of ecological composites based on plaster has a promising future in thermal building insulation.