Abstract: A prefabricated, low-energy building adapted to be erected on a usual foundation wall, has a prefabricated modular post and beam frame of heavy wooden timbers fastened together with hardware concealed from within the building. The timbers are relatively short and light in weight permitting the construction to be completed on the site one floor level at a time and allowing the frame to be extensible in length, width and height to form buildings of diverse exterior shape and size. The side walls, end walls and roof of the building are completed by a prefabricated modular panel system including frameless sandwich type panels which are secured to the outside of the framework and envelop the latter from the roof ridge to and below the top of the foundation wall. The panel system also includes means for sealing against infiltration and conduction heat losses through the walls and roof and also between abutting panels.