Abstract: A prefabricated panel for a post and sill panelized log wall system comprises vertical mullions having generally parallel first and second side faces and inside and outside faces. Each mullion has a first and second grooves extending lengthwise thereof in the first side face and a third groove extending lengthwise thereof in the second side face. Each mullion further has a recess at its lower end extending up from its lower end and extending toward the outside from its inside face providing a vertical face facing toward the inside and an upper shoulder extending from its inside face. A rigid insulation panel extends between the mullions having its ends sealed and adhered in the first grooves, the insulation panel extending from the top of the prefabricated panel down to and terminating at the level of the shoulders.