A system for ventilating the cavity of a fixed or movable window or door construction of a building, the window or door having an upper sill or header and a low sill provided with weep holes and a pair of jamb elements, the system comprising a vent member operative associated with the upper sill portion, the vent member is in fluid communication with the upper sill at one end thereof and also with the interior of the building at an opposite end thereof, the vent member extends vertically above the upper sill a distance sufficient to vent pressure within the cavity to the interior of the building so that water is caused to be readily conveyed out of the weep holes regardless of any pressure differential between the interior and exterior of the building.