Decorative Molding: Exterior Use
Cornices are generally found at the very top of an exterior wall, just under the roof line. Historically cornices were designed to prevent rainwater from running off the roof and hitting the walls. Most buildings today solve the rainwater problem with gutters, however cornices are still used as decorative elements. Exterior cornices are often constructed out of aluminum to withstand weather conditions.
Decorative Molding: Interior Use
Just as cornices top exterior walls, cornices are often installed where interior walls meet the ceiling for decorative reasons. This application is most often called crown molding. Molding comes in a variety of sizes and styles and can be constructed of plaster, solid wood, MDF, polyurethane, PVC, flex or polystyrene. Consult with a building professional to determine the best material for your location.
Cornice Box for Curtains
A third application for cornices in the home comes in the form of curtains and window treatments. In this application, a cornice is a box that's constructed to hide a curtain rod. These boxes can be constructed out of decorative wood trim or built out of simple plywood and covered with fabric to match or coordinate with your window treatments.
Check out this related video from Homesteady on Youtube.