A joint for fluid storage tank insulation systems. A central expansion joint forms a fluid-sealed recessed channel having a ridge-like cap. Water and moisture are directed away from the central expansion joint by the ridge-like cap. Any water that breaches the cap enters the recessed channel and flows out of the expansion joint without damaging tank insulation material. With installations having multiple expansion joints, at least one of the expansion joints can be equipped with an inverted cap to form a gutter within such expansion joint.