This research investigates the correlation between tensile strength, tensile strain, and porosity in chemically foamed recycled poly(ethylene-terephthalate) (rPET) samples. Tensile strength and strain were measured on non-foamed samples to predict the porosity of the foamed samples. Utilizing various flame retardants and additives, we analyzed their impact on mechanical properties and structures. However, neither tensile strength nor strain demonstrated a reliable correlation with the resulting porosity. Strain values did not consistently predict porosity, indicating the significant role of additive types and concentrations.