Rear Glass / Condensation

[mattylondon]

I also get some condensation on the rear window. It doesn't happen when raining only when the temperature drops. I've double checked there are no leaks and the seals are good. So I've put the problem down to the fact that the A8 is highly sealed and doesn't allow many air changes to give proper ventilation when parked up. As a crude calculation of only 1 air change per hour we would need about 5000litres of air movement, which I doubt the A8 achieves. Ideally 4 changes per hour would be needed. Also it depends on the relative humidity within the cabin. So if there are HVAC drain problems it will exaggerate the condensation problem because the dew point will be higher inside the cabin.
In my old A4 I used to leave the sun roof open ever so slightly, even when it rained, and that car was dry as a bone with zero condensation problems. Sadly my '8 doesn't have a sun roof.
From what I know in the building industry a highly sealed environment with poor ventilation is a breading ground for condensation problems and then worse, as Mike discovered in his S8.

Edit:
Do you have the ski hatch into the boot? Try leaving this open and the boot side panels removed to encourage more air changes as behind the rear bumper there are two 'hit and miss' vents. It's worth a try to see if it lessens the condensation