Paraffin and paraffin mixtures that are preferred as phase change materials in many thermal energy storage applications are highly flammable. Microencapsulation of paraffin in a polymeric shell can decrease flammability, however, breaking of the shell under fire conditions can still cause a high risk. In the current paper, microencapsulated paraffin with a polystyrene shell is prepared and halogen-free flame retardants (ortho-phosphoric acid and pentaerythritol) were applied with the novel approach of direct incorporation during the microencapsulation process. Thermal energy storage and fire retardancy properties were characterized before and after fire-retardant addition. The fire behavior of samples in concrete blocks was determined with standardized methods in order to assess their suitability in building applications.ortho-Phosphoric acid as a flame retardant in microencapsulated phase change material was tested for the first time in this study. The results support that the improved flame retardancy and thermal energy storage properties were achieved with the incorporation of a flame retardant on microcapsules for energy storing concrete samples.