It is not obvious to accurately determine the heat loss under dynamic conditions for building envelopes made of hollow blocks using classical one-dimensional heat flow computations. Consequently, complex three-dimensional heat transfers analyses are necessary to correctly assess their thermal behaviour. This latter approach is characterised by linear state models with high-order matrixes. Therefore, this method is not very practical since it requires cautious numerical implementation and intensive computation time. One way to obtain important decrease of the computation time with no significant losses of precision is to use model size reduction techniques. This study presents such an approach based on Moore's balanced method for two kinds of small-size concrete hollow blocks. The low-order models obtained after reduction for these two hollow blocks configurations are five-order state models (their complete state models had 680 modes and 973 modes, respectively). To estimate the accuracy and the efficiency of each reduction, the results are compared to those issued from the original complete models. The confrontations show that the proposed reduced models provide excellent prediction of hollow blocks thermal behaviour for excitation periods higher than 4 min. Moreover, the numerical results were very satisfactory comparing to experimental data obtained by means of classical calibrated hot box measurements. Finally, it must be noted that the approach developed in this study can be extrapolated to all kinds of heterogeneous walls. This can lead to simple "model libraries" within building simulation codes, based on "tabulated values" according to data issued by small matrixes set for each type of hollow blocks envelopes. (C) 2004 Elsevier B.V. All rights reserved.