Abstract: A pre-insulated prefab wall panel comprising of a rectangular wall frame having top and bottom rail members and a plurality of spaced apart stud members aligned between the top and bottom rail members. A polystyrene boardstock is affixed to a first side of the rectangular wall frame, thereby defining with the top and bottom rail members and the plurality of stud members a plurality of rectangular cavities, wherein each cavity has a depth of the thickness of a stud member. The prefab wall panel further has a layer of foamed-in-place polyurethane covering a portion of each cavity adjoining the boardstock, and bonding the structural wall frame to the polystyrene boardstock. The layer of polyurethane foam has a thickness which is substantially less than the depth of each cavity, whereby each cavity has available space for accommodating sub-trade installations.