From my own experience, this could be caused from a few things. All cause fogginess to a different degree. As Shiftoholic stated, this could be from a leak in the cooling system, possibly the heater core. To check this, you can warm the vehicle up. After it's at running temperature, keep your eyes on the windshield for a few min. Then try turning on the defrost for a short while and be smelling for anti-freeze or humidity. You also might be right about the seal being bad. I would hook up a spray nozzle to the end of a garden hose and force the water pressure into the old seal for about 5-10 min. Check inside for any drips or standing water. This sounds like a very minor ordeal and believe you should have it resolved in a short period of time.
Please let me know if I was any help. I am new to this site and want to start this off right. Good luck.