Importance of taking into account the thermal mass in simulations for a non-air-conditioned house

Abstract

Building thermal simulation (BTS) programs are being increasingly used to evaluate the occupants’ thermal comfort and the building’s energy consumption. Although some authors have pointed out that not taking into account the thermal mass of the building’s components results in erroneous performance predictions, some BTS program users, standards and building rating systems still do not consider the thermal mass. The quantification of the difference in the simulations’ predictions of the performance of a building when its thermal mass is either considered (M) or ignored (NM) have been reported in the literature for air-conditioned buildings (AC). In this work, and for the first time, the quantification is made for a non-air-conditioned (nAC) building. The quantification of the difference between M and NM simulations’ results is also made for the same building but with AC. Simulations were carried out using EnergyPlus and considering four study cases according to the constructive systems of walls and roofs. The building is a house located in a Mexican city with a hot summer and a mild winter. For the nAC house the differences in monthly averages for the daily maximum and for the daily minimum temperatures between M and NM simulations are up to 8.3oC and 6.4oC respectively, and differences for the cold and hot discomfort degree hours are up to 124%. For the AC house the differences between M and NM simulations on the cooling and heating thermal loads are up to 30%. The difference in the air conditioning peak power demand is up to 38%, and the difference in the time this peak occurs is up to 4 hours. These results show that differences between simulations are larger for the nAC house than they are for the AC house.