Modular building methods and systems using lightweight modular panels, and specially configured transition panels for transitioning from wall to floor, or from wall to roof. Identically configured standard panels are used for constructing the walls, floor, and roof, with transitions from one structure to the next (e.g., wall to floor, or wall to roof). Each of the variously configured panel types includes channels (e.g., 2 pair of channels) formed through the length of the foam body, where the channels are configured to receive splines (e.g., flanges of an I-beam) therein. In the standard panel, the channels may include pairs of top and bottom channels, with the channels offset towards the respective panel major faces. In the transition panels, the channels may be similarly configured, but positioned differently to make the appropriate transition. The splines are connected to a frame, which acts as a template and transfers loads to a foundation.