In recent years, the building sector is consuming high amount of energy; therefore, low energy buildings are promoted by the European Directives. In order to reduce the energy demand, improvements in the building envelope have been studied based on different aspects such as thermal insulation, thermal inertia and combining both of them. From the results obtained in the experimental set-up of Puigverd de Lleida (Spain) which analysed the thermal performance of different constructive systems, a similar methodology was used to consider internal heat loads, simulating scenarios with occupancy. This paper pretends to analyse the thermal performance of an insulated constructive system and another one with phase change materials (PCMs) located in the envelopes as passive cooling system. The experiments were done during the summer period. The results of the experimental campaign show that the insulation effect when internal gains are involved is harmful because heat loads cannot be easily dissipated to outdoors, increasing the energy consumption. Moreover, when adding PCM to an insulated constructive system, this effect is maximised because the PCM stores the heat produced by the inner loads and the external conditions; hence, the heat dissipation to the outer environment is limited. The work partially funded by the Spanish government (ENE2011-28269-C03-01 and ULLE10-4E-1305). The authors would like to thank the Catalan Government for the quality accreditation given to their research group (2009 SGR 534) and the city hall of Puigverd de Lleida.