A joint, in one embodiment, between adjacent panels of sheet encased insulating material in which one lateral edge has an essentially frustoconical nose, and the other edge has a frustoconical pocket is disclosed. In an alternative embodiment, essentially frustoconical noses are at both lateral edges and the two adjacent panels are joined by means of an I beam embodying the interlock of the present invention. In both embodiments, the interlocking relationship is a function of the lateral edges of the cladding sheet, or in the flange of an I beam, in which one edge has a sealant pocket, and the opposite edge has a locking sealant press which engages the sealant pocket and thereby interlocks the panels as well as presses the sealant in the sealant pocket to compress and secure the same and to form a sealant gasket between the lateral edges of the sealant pocket and the locking sealant press.