Due to the pressure on CO₂ reduction, during the last years "lightweight" parts have become rather popular, as opposed to "conventional" parts, traditionally constituted by a heavy mass layer on top of a soft decoupler. While "conventional" parts are based on pure insulation, "lightweight" parts propose some kind of compromise between absorption and insulation. This makes their design difficult: designing a "lightweight" part means adjusting in the proper way the balance between the absorption and the insulation provided by the part itself and the search for an optimal balance has to take into account relevant vehicle-dependent boundary conditions. Typically, in the design of a lightweight dash insulator a key role is played by the presence of the instrumentation panel and by the importance of the pass-throughs.This article describes a procedure that can help the NVH engineer in the above-mentioned task. The procedure relies on an SEA model of the vehicle under analysis and it is configured as an optimization algorithm where the frequency-dependent optimal balance between absorption and insulation is determined. After describing the mathematical procedure, an application to a vehicle dash insulator is described. Such application shows how, to obtain an optimal result at vehicle level, it is necessary to tune the balance between absorption and insulation in a different way for the different regions of the part. This indicates even more how complex the design of a lightweight part can be and how risky it is to judge its performance simply on the basis of analyses on flat and constant-thickness samples.