The reason these areas are more challenging than a standard curtain wall condition has to do with the amount of area of the curtain wall that is exposed to exterior temperatures in lieu of internal conditioned space.
Please note the thermal models of the parapet pictured at right. The top thermal model shows a standard detail for a curtain wall parapet. Because the curtain wall is completely exposed to external temperatures at the parapet (i.e., the front, top and back of the curtain wall are each exposed to cold temperatures due to the lack of conditioned space at this location), the internal surface temperature of the curtain wall at the ceiling line is 26.7°F. This temperature is below the dew point of 37.1°F; therefore, under these conditions this elevation will form condensation on the interior of the building.
The thermal model on the bottom depicts the same situation except that a couple of adjustments have been made to the elevation. Both fiberboard and batt insulation are shown at the ceiling level of the detail. This addition of insulation to the detail increases the surface temperature of the curtain wall to 39.2°F, which is above the dew point of the elevation. Therefore, under these conditions condensation will not develop on the interior surface of the curtain wall.