The joint assembly comprises a pair of strips 13, 12 secured to the edges of respective panels by folding the outer skins 10 and by a dovetail joint 20 with the foam insulation material 11. The connecting strips provide a curved tongue 27 and groove 21. The groove strip has a pivotal body 23 of part-cylindrical shape so at one strip can be pivoted relative to the other for tongue 27 to enter groove 21. Pivotal body 23 lies to one side of tongue 27 and intermediate the height thereof so that in the desired orientation, the curved surfaces of tongue 27 and groove 21 prevent withdrawal of tongue 27 by linear movement. A locking strip 33 can be inserted into a groove; it includes spaced clips 34.