The use of optimization techniques has been recently advocated in order to improve the energy performance of the design of buildings. The objective function in our model accounts both for the heating cost and the cost of insulation materials so that its minimization, with respect to a meaningful set of technological and architectural variables, yields a sequence of designs of decreasing "cost", converging to that design which ensures, for the weather conditions of the site of the buildings, the optimal balance between the cost of additional insulation and the related energy saving.