The thermal efficiency of air lime-based mortars was improved by directly integrating varying amounts (5 wt. %, 10 wt. %, and 20 wt. %) of a biobased Phase Change Material (PCM) into the fresh mortars. The composition of this PCM is vegetable oils and other organic wastes from the agri-food sector. To optimise the mortar formulation, different chemical additives and mineral admixtures were added. The mortar formulation was meticulously designed to produce rendering mortars that are easily workable, crack-free, and fully adherent for use in building envelopes. Positive outcomes in thermal efficiency tests have demonstrated the ability of these materials to store thermal latent energy, offering an environmentally friendly alternative to enhance the thermal comfort of building inhabitants.