An insulating member comprises a body of rigid foam material having a first connecting edge surface and a second connecting edge surface, a groove formed into the rigid foam material of the body along the first connecting edge surface, and a tongue formed from the rigid foam material of the body along the second connecting edge surface. The groove, when viewed in a transverse cross-section, tapers from narrow to wide in a direction extending from its opening and into the body. The tongue, when viewed in a transverse cross-section, tapers from wide to narrow in a direction toward the second connecting edge surface. The tongue is bifurcated along its length into first and second ridges by a wedge-shaped void. A polymer coating of polyurea or polyurethane substantially covers the tongue and structurally reinforces the tongue allowing the first and second ridges to flex toward one another without breaking.