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 field-assembled flooring systems can use mold-resistant isolation boards to reduce or eliminate the chances of the onset of mold and effects from water damage. The isolation boards can be installed on floors alone, on walls alone, or on both floors and walls to reduce the onset of mold and/or to reduce damage from water.