A cap assembly for enclosing a metallic fastener which extends through a structure. The assembly includes a housing having an inner wall and an outer wall which extends about and is spaced apart from the inner wall defining a space positioned between the inner wall and outer wall. The assembly further includes a plunger member with a side wall portion secured to an end wall portion. The side wall portion of the plunger member has a thickness dimension smaller than a distance between the inner wall and the outer wall to position the sidewall portion of the plunger member within the space. With the side wall portion positioned within the space, the end wall portion is positioned in alignment with the inner wall and with an opening defined by and through the inner wall.