An insulating cover for assemblies including pipe elements, pipe couplings, elbows, Tees and valves also acts as a vapor barrier. Cover portions are joined along a seam which provides for a continuous seal between both the cover portions and the pipe elements which extend through channels defined in the cover. The seam includes a furrow into which sealant is forced when the cover portions are joined around the assembly. The furrow is defined by an asymmetric tongue and groove joint. Canals in the channels, in communication with the grooves, also receive the sealant to provide the continuous seal.