A frame corner joint includes first and second frame rails having hollow interiors of predetermined contours and ends in mitered abutment. A corner key has legs inserted into the mitered ends of the frame rails. End portions of the legs substantially fill the hollow interiors of the frame rails forming a substantially closed cavity within the frame rails and between the end portions of the corner key legs. Solidified resin foam is disposed within this cavity rigidifying the corner joint, while the remainder of the interiors of the frame rails preferably is substantially free of solidified foam. At least one end portion of at least one leg of the corner key preferably has a flexible wall portion to permit escape of foam from within the cavity in the event of excess foam pressure. The hollow interiors of the frame rails preferably are mirror images of each other, and the legs of the corner key, including the end portions of the legs, preferably are mirror images of each other.