Described herein are methods for installing field-assembled flooring systems that include isolation boards on a wall frame, a subfloor, or on both the wall frame and the subfloor. The underlayment can be a hybrid design that includes a combination of isolation panels and cementitious product. The flooring systems are configured to reduce or eliminate the curing time requirement after pouring the cementitious product (e.g., gypsum concrete). The field-assembled flooring systems can reduce or eliminate the chances of the onset of mold due to high moisture levels by removing the cementitious product from the prone areas and replacing it with structural boards. The isolation boards (e.g., cellulose fiberboards) can be installed in non-critical areas such as underneath cabinets, around the perimeter of the floor, under bathtubs, in non-walk-in closets, anywhere drywall reaches the floor, or the like. The isolation boards can be installed on walls rather than or in addition to the isolation boards installed on the subfloor.