AbstractShape‐stabilized phase change materials (SS‐PCM) are promising materials given their potential to control leakage of liquid PCM. However, SS‐PCM still has low thermal conductivity and high flammability, which are important properties for several applications, such as the thermal indoor comfort in buildings. In this study, two new polymeric SS‐PCM were developed and their properties were optimized by the use of different additives. Both high‐density polyethylene (HDPE) and polyoxymethylene (POM) work as matrix materials and MPCM 28 from Microtek acts as PCM. Besides, the graphite was used as an additive material to increase the thermal conductivity, and the magnesium hydroxide to minimize flammability of the composite. Both inorganic fillers also help in the PCM dispersion within the matrix. To evaluate the effect of each component, seven formulations were manufactured by a single screw extruder and a set of characterization (Differential scanning calorimetry, Thermogravimetric analyses, for thermophysical evaluation, and dynamical mechanical analyses, for thermomechanical evaluation over 1000 thermal cycles was performed. The main outputs of the investigation are the proposed formulations that have a good fire reaction performance, whereas their thermal and chemical stability are guaranteed up to 1000 cycles. The HDPE samples present around 12 kJ/kg melting enthalpy when 10 wt% microencapsulated PCM is included in the formulation. In addition, the POM samples present around 7.5 kJ/kg when 10 wt% microencapsulated PCM is included in the formulation. For all formulations, the melting enthalpy obtained is around 27.5°C, in concordance with the reported by the manufacturer.