The heat storage performance of internal envelope plays an important role in improving indoor thermal comfort for a passive solar room, which is related to the thermal properties of building internal envelope. The previous researches are all case studies. The paper aims to analyze the intrinsic performance of internal envelope controlling indoor air temperature and optimize the specific heat. The results show that the optimal specific heat of internal envelope is nearly (5 function which approaches to the equivalent specific heat form of phase change material. It is helpful to understand the temperature control performance of internal envelope further and guide the building material design and preparation. (C) 2012 Elsevier Ltd. All rights reserved.