Volume decomposition is a technique to decompose a computer aided design (CAD) model into sweepable subvolumes, by which better types of mesh can be generated. A thin-shell part can be divided into a main body (thin shell) and protrusions that reside on the thin shell. When the thin shell and protrusions are separated, it would become easy to decompose each of them individually. Direct recognition of protrusions on a thin-shell part is error-prone owing to the complex structure of protrusions. The recognition of inner and outer faces on the thin shell can help the recognition of protrusions, as well as the volume decomposition of the thin shell. In this study, the complexity of the models considered includes the following: (1) various types of transition faces between inner and outer faces, (2) complex protrusion structures both on the inside and outside of the model, (3) fillets are included, and (4) complex holes lie across multiple faces. The proposed approach is divided into the following four steps: separation of inner and outer faces, transition faces recognition, inner faces recognition, and outer faces recognition. A detailed discussion of the procedures for each of the steps is provided. Also, 25 thin-shell models are employed to demonstrate the feasibility of the proposed method.