Abstract In this work, an innovative modular heat storage system is investigated experimentally and by numerical modeling. A single storage module consists of a helical heat exchanger in a water saturated porous cement matrix. The experiment comprises a 5 day thermal loading stage, followed by 16.5 days of passive cooling, and was especially designed to quantify the thermal insulation efficiency. An inverse modeling approach was applied to successfully match temperature measurements within the storage by numerical simulation. The thus determined heat loss rates amount to 130 W for the fully loaded storage and to 50 W on average during passive cooling.