Most historic churches are characterised by low thermal efficiency due primarily to their architectural design and the traditional materials used in their construction. Heating the huge volumes of air involved may be more or less effective depending on the HVAC system used. Up until very recently, traditional heating has been the system of choice in Spanish churches. The present study analysed the parish church at Talamanca de Jarama (a village on the outskirts of Madrid), where the forced hot-air system was found to induce wide fluctuations in indoor thermal-hygrometric (T/RH) conditions, in turn translating into the temporal and horizontal stratification of temperature and relative humidity. As a result, the thermal comfort sought for parishioners is not reached, for the air remains cold at pew height while heat accumulates in the upper area of the church. A three-dimensional sensor network was designed to monitor these conditions. Suspended from helium balloons, the sensors could record the indoor air temperature and humidity without causing any damage whatsoever to church property. (C) 2014 Elsevier B.V. All rights reserved.