joint cover 28 it is only symbolically represented in fig3 vertical sealant 26 is provided between the interior metal skin 17 and the supporting mullion 14 marriage sealant 27 is provided to connect the vertical sealant 26 to the horizontal sealant 22 in the interior panel side joint 20 a vertical joint cavity 29 is formed by the four sides including two panel ends 16 mullion 14 and vertical joint cover 28 the arrows 30 represent water infiltrated through the exterior side joints 21 being dumped into the vertical joint cavity 29 the dotted arrows 31 represent water infiltrated through the gaps at the exterior corners of the panels 11 into the vertical joint cavity 29 according to this invention the water infiltrated into the vertical joint cavity 29 will not be permitted to wander along the panel end surfaces 16 to make contact with sealants 22 26 and 27 to accomplish this objective firstly the vertical joint cavity 29 must be pressure equalized to the exterior air if the vertical joint cavity 29 is not pressure equalized to the exterior air any air leakage in the sealants 22 26 and 27 will draw more water into the vertical joint cavity 29 and enhance the tendency of sucking the water inwardly to the sealant locations to equalize the pressure inside the vertical joint cavity 29 to the exterior air exterior air must be allowed to freely flow into the.