Differences on Results from Steady-state and Time-dependent Wall/roof Heat Transfer Models in Mexican Climates

Abstract

Mexican regulations aiming to reduce the air conditioning energy consumption in buildings use methods based on the steady-state heat transfer model. Thus, they take into account the thermal resistance of the envelope elements, but do not consider their thermal storage capacity (or heat capacity). This model is useful to estimate the needs of heating in winter for countries in the temperate or polar areas where the temperature oscillation during one day is small and solar radiation is low. When the temperature oscillation during one day is large and the solar radiation is high, like in Mexican climates, the wall/roof thermal storage capacity is very important. In these cases, a method based on the time-dependent heat transfer model must be used. In this work, the results of the wall/roof heat transfer from the steady-state model are compared with the results from the time-dependent model. Four roof constructive systems have been considered, in three different Mexican cities. Their thermal performance has been analyzed for air-conditioned buildings and for non-air-conditioned ones. It is demonstrated that the differences on the results from the steady-state model and the time-dependent model can be greater than 80% for air-conditioned buildings, and greater than 800% for the non-air-conditioned case. These results show the need to change Mexican regulations incorporating methods for evaluating the envelope walls/roofs heat transfer based on the time-dependent model.