In this paper influence of house walls on heating energy consumption and indoor thermal comfort was investigated. The simulations were carried out using an extended dynamical model of building-software ZID under transient conditions, like the first heating day after a long non-heating period. Four simulation cases were considered: two-layered thermal-insulation concrete house walls were used, where the thermal insulation was inserted in the inner, outer or middle positions according to the inside of the house, and one-layered concrete house walls. The influence of the house walls on internal thermal comfort and heating process is presented by two introduced factors: thermal comfort degree-hour and building envelope response factor (BER factor). This new BER factor could be a significant variable in considering house walls as a passive heating energy source. The conclusion is that under certain conditions the BER factor is at a maximum, at which point the influence of house walls on inside thermal comfort and heating energy consumption is also at a maximum. (C) 2002 Elsevier Science Ltd. All rights reserved.