A sound damping wallboard for installation on an installed wallboard, a sound damping wallboard system, and a method of constructing a sound damping wallboard on a building structure are disclosed. The sound damping wallboard comprises a gypsum layer having a gypsum layer inner surface and a gypsum layer outer surface, a sound damping layer disposed at the gypsum layer inner surface for installation between the gypsum layer and an installed wallboard and having a sound damping layer inner surface, a first encasing layer disposed at the gypsum layer outer surface, and a second encasing layer disposed at the sound damping layer inner surface.