Abstract: Pipe insulating units are disclosed which can be slipped radially over the pipe. The units are formed such that two adjacent pieces interlock to form a set which completely surrounds the pipe and which has parallel faces which are abutted by the parallel faces of adjacent sets. The units and sets are then held together by bolting or other means. Each unit comprises a circular piece of insulating material having a hole in the center thus forming an annular disc. This annular disc has a portion of the annulus omitted with this omitted portion having sides which are parallel to each other and which are tangent to the periphery of the hole in the center. The sides of this omitted portion extend outwardly to the periphery of the disc. Formed integrally therewith on one face of the disc is an outwardly extending portion in the area bonded by extensions of the parallel sides of the omitted portion and on the opposite of the center hole from the omitted portion.