The on-site measurement of the thermal properties of existing building envelopes is of utmost importance to fairly accurately calculate the thermal loss by transmission to the outside environment and so to define the building overall energy performance. This paper presents a preliminary investigation concerning the use of a new indirect method for measuring the thermal characteristics of building materials throughout the year based on the analysis of the material electrical properties. The main goal is to identify a relationship between thermal transmittance, moisture content and electrical impedance through relative electrical permittivity measurements.