fig3 this further bead of foam 42 is placed on the underside of each wood strap 28 to prevent infiltration of air underneath the wood straps 28 along the nails 40 and through the eps boards 26 a further step in manufacturing a prefab wall panel of the preferred embodiment is to apply a layer of polyurethane foam 30 in all cavities defined by the wall studs 20 top and bottom rails 22 24 and the eps boards 26 during this operation the wall panel is preferably raised in a vertical position on its upper edge as shown in fig4 and the polyurethane foam 30 is applied using a spray gun 44 as is customary in the insulation industry as it was mentioned earlier the top plate 34 protects the upper edge on the eps boards 26 when the prefab wall is raised in this vertical position and for example when it is manipulated out of the fabrication shop and onto a transport trailer for example referring now specifically to fig5 and 6 the prefab wall panel of the preferred embodiment is covered by an eps board 26 having a thickness a of about three inches 3 34 as also mentioned earlier the thickness b of the polyurethane foam 30 is preferably about 1 34 to 11 2.