The company now offers a quick and easy method of installing crown moldings. Trimroc Interior Crown Molding consists of interior acrylic modified gypsum-plaster moldings extruded over an expanded polystyrene base that is ideal for residential and commercial applications. Trimroc inside corners can be installed by using a 45-degree miter cut on each piece. Any gap that forms between the mitered pieces is then filled in with the joint compound used to install the molding.