Informal settlements, are generally considered to provide very unhealthy living environments. Poor sanitation and lack of potable water, as well as limited access to health and education services, mean that occupants are often plagued by an array of appalling illnesses. It is known that poor internal housing conditions exacerbate and facilitate disease and ill health. As yet, little research has been undertaken on the actual internal thermal conditions of informal housing. This paper reports on a comparative study of internal temperature in three urban lowersocioeconomic housing typologies in Buenos Aires, Argentina: incipient shacks, consolidated houses (both informal housing typologies), and government housing. The internal temperature of 24 rooms in 10 case-study houses was measured every 30 min over a 10-day monitoring period in both summer and winter. Incipient shacks displayed poor thermal performance due to their poor building quality: during the winter, large amounts of energy were required for space heating to keep conditions favorable and in the summer these houses became significantly hotter than outside temperatures. Consolidated houses and government houses are constructed of similar materials, and therefore had similar thermal performance, with government houses providing only marginally better internal conditions. This paper concludes by suggesting that, along with the provision of basic infrastructure, the consolidation of incipient shacks with bricks and insulated roofing should be promoted in order to improve the internal environmental health of informal settlements.