Air infiltration of buildings has a considerable impact on the energy performance of buildings. Permeability or airflow leakage can be evaluated by introducing the Infiltration Heat Recovery (IHR) factor in the energy balance equation. Conventionally, this factor is computed using a Computational Fluid Dynamics (CFD) software, that is time consuming and not very useful for a fast evaluation of balance energy of a building. This article proposes a Reduced-Order model (ROM) approach to evaluate the effect of the permeability of the energy balance for a building. The ROM, based on the well-known Proper Orthogonal Decomposition (POD) method, is developed in the Modelica modeling language. It is successfully applied to the case study of the air infiltration in the low energy consumption building.