Your are in ideal weather for condensation in any shelter with high humidity and no wind. Moving air is what keeps condensation out of your tent, but if no air is moving by wind, and what air there is is high in humidity that's a condensation breeding ground. Ventilation is what moves air through your tent. The double wall or fly over a tent is supposed to create and trap a pocket of warm air that will assist air flow in your tent. That does not exist in single wall tents. Also that "warm air gap is often non-existent in double wall tents especially in weather as you described. Read steps 7 and 8 for the theory explained at warmlite:
http://warmlite.com/tents.htm