In order to reduce energetic consumption, buildings may be retrofitted by covering them with an insulation envelope composed of rectangular parameterizable panels. In such scenario different envelopes configurations are possible to cover the same building. Thus, a crucial aspect of the retrofit is to select, among potential solutions, those ones providing good thermal performance at reasonable cost. In this paper are discussed which characteristics are part of a panels-made envelope, how to compute their performance and associated cost, and a bicriteria function evaluating the envelope. We compute thermal and cost estimations with respect to the spatial positioning of panels configuring the layout. The aim of our contribution is to assist architects in their decision-making process. Conception and implementation of the thermal retrofit are then supported by our estimations.