The increase of thermal insulation of existing buildings is considered a fundamental operation for reducing building energy consumption for heating and cooling. Internal insulation systems represents, in some cases, the only possible solution. Historical refurbished buildings represent a typical case when local regulations forbid external insulation placement. However, sometimes designers compute insulated wall thermal conductance disregarding the effect of structural elements employed to fix the insulation layers to existing walls. In the present paper, we analyse a typical internal insulation system made of steel studs fixed to the wall with interposed insulation layers covered by plasterboard. Using previously obtained experimental measurements of the heat flux at the centre of insulation and on steel studs, the validation of a three-dimensional numerical model and a parametric analysis has been carried out. Five cases have been analysed, showing that the main parameter to be taken into account is the radiation heat exchange between the steel studs and the wall, on the contrary, air convection inside the stud resulted less important in evaluating heat transfer. The numerical model has been used to compute a mean conductance of the wall showing that neglecting the steel stud leads to the underestimation of wall conductance by about 20 percent. (C) 2017 Elsevier B.V. All rights reserved.