Some of the greatest potentials for improving building energy efficiency are found in the residential sector. Social housing, in particular, has drawn heavy research interest because it affects the welfare of large populations, is the source of significant energy consumption, and has outsize importance in the construction and regulatory sectors. Energy regulation in Mexico focuses on reducing the energy needed to cool buildings down, neglecting the importance of heating buildings built in the colder regions of the country. To address this gap, the present work focuses on the thermal behavior of social housing in the regions of Mexico with cold semi-arid climate. We found that thermal discomfort inside houses is primarily driven by low temperatures. We calculated annual heat flows in houses, visualizing heat gains and losses through each part of the building envelopes, and found that the highest heat flows occur through the floor. We also found that windows have the greatest heat transfer per unit area of all construction elements. We estimated the energy that each building would require if heating and air conditioning were used throughout the year to bring indoor temperatures within the range of thermal comfort. Finally, we used evaluation schemas from several countries to evaluate the energy demand per unit area (kWh/m2) of several local houses in a typical year. The houses analyzed here presented low scores under these schemas.