The provision of proper indoor thermal conditions in winter is a requirement for health and comfort that is seriously compromised in Southern European countries by poverty and inadequate housing. According to Eurostat (EU-SILC, 2016), 10.1% of the population in Spain state they are "unable to keep their home adequately warm" in winter, unveiling one of the critical consequences of energy poverty. Although relevant field monitoring assessment have been conducted in other European countries, very little research has been carried out in the context of Spain. In this study we collected winter indoor temperature data, socio-economic, building and heating system characteristics from 112 multi-family dwellings in social housing built in the 40-80s in the North of Spain. The main building determinants of temperature variation were examined and assessed according to the socioeconomic situation through a mediation analysis. The principal findings showed that households living within district heating networks maintained temperatures above 18 degrees C on a continuous basis, whereas 54% of households living outside these heating networks presented temperatures below this threshold during night periods in bedrooms, while 25% had temperatures below 18 degrees C during both day-time and night-time occupied periods. Within this 25% of cold homes, 16.4% corresponded to households under the poverty threshold. As the mediation analyses showed, poorer households in the sample were associated with lower temperatures and higher temperature contrast throughout their dwellings for the following reasons: a lower incidence of vulnerable cases within district heating networks, heating restriction behaviours, higher dependency on room-heaters, lack of central heating and more thermally inefficient dwellings. The evidence presented in this paper offers a new framework for comparing the assumptions derived from other contexts to the situation in Spain about the effects of space heating and housing deprivation on thermal conditions, provides a basis for reviewing some possible intervention, and aims to raise social and political awareness further about this issue. (C) 2018 Elsevier B.V. All rights reserved.