Energy prices are a major concern for the Chilean population. Electricity is more expensive than in countries with higher incomes such as the UK. People are very sensible about energy bills and save energy by under heating their dwellings, sacrificing thermal comfort and affecting health. Through a case study, this book analyses different cost-effective measures to improve thermal comfort in apartment buildings in Valparaiso. Two apartment buildings were used as case study, apartments where monitored and post occupancy surveys carried out. A flat was modelled to simulate the effect of different energy efficiency measures. Finally, cost evaluation of different alternatives was carried identifying the most cost effective improvements. These measures could be applied to other apartment buildings in the central area of Chile, which concentrates half of the country’s population. Thus, this analysis should be especially useful to professionals in the building industry in Chile, real estate developers, policy makers or anyone else concerned with improving thermal comfort and energy efficiency in the built environment.