An interlocking modular building block system for mortarless cavity wall construction having blocks configured such that upon grouting, the cavity is solidly filled in a unified mass. Four block configurations are provided, namely runner, half, corner, and end block units. The runner has a length two times its width, and the other blocks have the same width and length as the width of the runner. Common to all blocks is a pair of parallel offset solid walls forming the outside and inside walls while creating vertical and horizontal positive and negative alignment features. The offset walls also form flat horizontal and vertical surfaces where successive units meet upon alignment. These walls are connected by one or more transverse membranes, with the exception of the corner block, where the walls are joined end to end at right angles.