2003 Dodge Van defroster excessively fogs up all windows

In the winter in this full size B1500 van when the temp is below freezing all the windows fog up and frost up no matter how much heat the defroster and rear heating vents put out - it gets worse as you drive not better and it feels humid in the vehicle. Has the same problem in the summer only the bi-level A/C setting will eliminate it.

Do you know what the problem is?