What is applied over trim molding in a boxed cornice?

In a boxed cornice, fascia is the correct material applied over the trim molding. The primary function of fascia is to provide a finished look and protection at the edge of the cornice where the horizontal trim meets the vertical section. It helps to create a seamless transition between different planes and adds structural integrity to the cornice design. Fascia also serves to conceal the ends of the trim and any rough edges, enhancing the overall aesthetic of the boxed cornice.

The other options do not fit the specific function required in this context. Sheathing typically refers to a layer of boards or panels that provide support and protection to walls or roofs, but it is not specific to decorative trim. Molding is a general term that encompasses various decorative elements, but it does not specifically denote anything applied over trim. Insulation, while crucial in building for energy efficiency, is not related to the finish or aesthetic elements of trim moldings. Thus, fascia is the most appropriate choice for creating the finished look of a boxed cornice.