incurring additional cost or compromising other performance parameters objective no 3 is accomplished fig5 is a typical fragmentary cross sectional view taken along line 5 5 of fig1 of the vertical butt joint in a horizontal application the end of panel 71 is butted tight against the end of panel 72 over the mullion 14 to form a panel butt joint 73 before erection the exterior side of the panel ends are partially notched up to the entire depth containing the exterior metal skin to form a vertical joint cavity 70 each panel end is fastened into the mullion 14 using a washer plate 41 and a fastener 43 the washer plate 41 is extended partially into the vertical joint cavity 70 to hold the joint cover securing member 74 in position the joint cover securing member 74 can be a continuous member or intermittent short segments occurring at each four corner intersection point as shown on fig6 the member 74 can be installed with the panel fastening process and in the case of intermittent short segments they can be installed after the panel erection the vertical joint cover 75 having a face covering span larger than the width of the vertical joint gap is designed to have spring legs to cause engagement with the top part of the member 74 in a snap in fashion members 74 and 75 can be made of extruded aluminum or plastic materials vertical joint sealant 76 is provided between the mullion 14 and the interior panel surfaces at butt joint a depression on the face of the mullion 14 is desirable to maintain a pocket of sealant along the vertical panel butt joint marriage sealants not shown between the vertical sealant 76 and each horizontal interior.