In Italy, an important percentage of historical buildings was built with stone masonry. It is necessary to correctly understand its thermal behaviour in order to evaluate suitable retrofit interventions. Stone masonries can present very complex and variable thermal fields, due to the different geometry of stone blocks, to the various type of stones used and to the presence of mortar. Twelve samples are analysed with a Finite Element Modeller to investigate its effect on dynamic properties. The results show that mortar positively influences periodic thermal transmittance and decrement factor of stone walls.