A cap for forming a sealed cavity around one end of a fastener, the cap having a two-piece construction comprising an outer cap member and an inner cap member. A quantity of sealant is provided between the inner and outer cap members which, upon installation of the cap, is urged to flow between the inner and outer cap members and collect at the base of the outer cap member, to form a bead of sealant around the circumference of the cap, sufficient to adhere the cap to the structure when the sealant cures. A plurality of channels are provided between the inner and outer cap members to facilitate the flow of sealant.