Abstract: A rigid insulating panel comprising an insulating material core with an R-value of at least 2.5 (hr·ft2·° F.)/BTU·in. The insulating material core has opposed first and second surfaces, a pair of spaced-apart longitudinal edges, and a pair of spaced-apart lateral edges extending between the pair of longitudinal edges. At least one of the pair of longitudinal edges and the pair of lateral edges comprises connecting members including a tongue and groove assembly with an inner groove and an outer tongue separated by a substantially S-shaped median wall. The tongue and groove assembly is engageable with the tongue and groove assembly of an adjacent insulating panel to provide a flexible interconnection therebetween. The rigid insulating panel also comprises at least one membrane covering one of the first surface and the second surface. An assembly method for insulating a concrete surface of a building using an assembly of insulating panels is also provided.