This paper presents a novel in situ measurement method of the total heat transfer coefficient on a building wall. The device developed is based on harmonic thermal load of the wall surface. In situ tests of the method were performed in a steady environment as well as in off-equilibrium conditions (generated using electrical heaters). The heat transfer coefficient was estimated to 7.5 and 9 W.m−2.K−1 respectively. A parametric analysis allows determining the optimum operating conditions. The results obtained were in good agreement with standard values found in the literature. Measurement uncertainties were evaluated and a simple analytical model of the device was also developed.