This article presents a study on the thermal properties of geopolymer mortars, which are novel materials with lower environmental impact and higher performance than Portland cement. Geopolymers are formed by the reaction of aluminosilicate sources and alkali activators, resulting in a polymeric Si-O-Al network. The study used the non-stationary method of measuring thermal diffusivity to evaluate the heat transfer coefficient of geopolymer mortars modified with perlite powder, cenospheres and perlite sand, which are porous components that affect the bulk density and thermal conductivity of the composite. The study had two specific objectives: to assess the significance of the factors related to the composition and bulk density on the thermal conductivity coefficient and to test the suitability of the non-stationary method for geopolymers. The study found that all the composites met the RILEM standards for a Class II composite, with a thermal conductivity coefficient below 0.7 W/(m·K). The most influential factor was the dosage of perlite sand, which reduced the composite density and increased the thermal insulation. The study concluded that geopolymer composites modified with low-density grain additives are a promising thermal insulation material, but they need more research on their durability.