It could be that any moisture in the car (including and especially from occupants breathing out) builds up quickly because the route that air is expelled from the car (to be honest, I'm unsure where from - perhaps via the back seats and the boot?) is blocked somehow to some degree, and will only be made worse if the air recirculator is on.

I wouldn't be surprised if water ingress into the car (or blocked drain holes in the doors, or in the air intakes by the windscreen wipers [leaves etc]) also contributes by effectively reducing (even when the air recirculator is off or car is sitting idle) fresh, drier air coming into the car and reducing the relative humidity. This will be all the more evident in colder weather, when the dew point is lower and condensation forms far more easily.

A damp filter could indeed be a contributory factor in the OP's case, if it hasn't been changed out on schedule. The water causing the moisture has to come in somehow, and stay within the ventilation system, likely at a low point where it can pool. A small gap in some ductwork may be the entry point. Several possible causes as I and others have said earlier, and may be a pain to find, though perhaps not so much to fix if all it requires is changing a filter, cleaning intakes or sealing up a hole of two in air ducts.

As well as the windows misting up, excess moisture in that system can also cause/exaccerbate significant bacteria build up, which will not be good for the respiratory health of any person in the car.

When the fault or faults causing the original problem are discovered, I would strongly recommend the ventilation system be disinfected/deodourised, at least by 'burning off' any residual bateria on the inside of the air ducts by running the system on maximum heat and full fan speed for 10-15 minutes once warmed up.

Often such procedures (including best practice in using the A/C system to keep it in good condition) are given in the car's manual.

Had similar on all my cars in cold weather going back long before aircon. Problem is moisture from occupants breath condensing on cold glass. Typically occurs more on driver's side if no passenger. For good scientific reasons I cannot now remember chemical de-icer makes it worse.

Solution is to use warm (not hot) water to de-ice. The warmth raises the internal temperature of the glass above condensation point sufficiently for blower and or heater to keep it clear. When I commuted to London used to fill a thermos with warm water before leaving the ofice so as to de-ice/demist at station. On sunny winter day like today thermal gain then rapid cooling at sundown would mean screen was misted before I'd left London!

If you've got water penetration by any of routes mentioned in other posts the 'cook/chill' effect above will have rivers of condensation on interior glass. As I found when my Xantia was shipping water via it's tailgate.

Edited by Bromptonaut on 09/12/2017 at 11:04

I've had a similar problem on my Octavia. What prevented it on that was selecting front windows de-mist even if it wasn't apparently needed, and opening the front side windows about an inch.

I think what happens is that the 'climatronic' is configured to heat the car from the footwell up, unless you tell it the windscreen needs de-mist.