Acoustic treatment development for car sound insulation is a complex task that requires many technical compromises to be found with other specifications. This paper is the result of a three years joint collaboration between FIAT Auto and RIETER on the New FIAT Punto co-design project. All the phases of the project have been developed together: RIETER support FIAT not only for the design of the package insulation but also for target setting, simulation, measurements, analysis of different solutions and problem solving, dealing with airborne noise propagation. The main aim of the co-design project was to control, in certain steps of the project, that all the sub-targets and P/P target assigned at the beginning of the activity were achieved. The empiric approach for this development, used in the past, can no longer be applied because most of the decisions must be taken before correct prototype availability and the analysis of the components is not sufficient to support it. Starting from the earliest phases and accompanying the whole development, SEA simulations overcome these problems and allow an optimisation not only for the insulation package, but also for ether car parts involved in the NVH performance. SEA models have been extensively used for the simulation of P/P transfer functions, showing that a good accuracy can be obtained in the frequency range above 315 Hz-400 Hz. The first step in the SEA model construction process generally consists in building a 'topological model', Le. a model that includes all the panels that can be relevant for the airborne noise transmission into the passenger compartment and the acoustic cavity of the passenger compartment itself. This model includes only the geometry of the vehicle body-in-white and of the passenger compartment, i.e. it does not contain any information about acoustic trim and acoustic loads. As a second step, for each one of the panels around the passenger compartment, an acoustic load has to be defined. Most of the times, this is done on the basis of experimental data obtained by measuring the acoustic field around the car: for each panel an average is calculated over a few points and the resulting spectrum is assigned as an acoustic load in AutoSEA. Finally, for each panel facing the passenger compartment, a TL curve and an absorption curve has to be defined. The TL for the three main components have been taken under control: dash, pc floor, trunk-load floor. For each one of these components SEA model was built and an experimental activity was carried out to validate and refine the model itself. Using the complete vehicle SEA model the global P/P curves have been evaluated, during the whole design phase, allowing a forecast of the global vehicle acoustical performance.