I have always used recirculation in the winter to avoid heating up the outside cold air. On my 2010 I noticed windows fogging up constantly in cold weather until I switched to outside air and the problem was 90% resolved. Could the Prius have a too efficient recirculation option?
Zeke, You will find multiple cautions in the Owner's Manual against this. Recirc should be used sparingly, especially when it's cold outside, as it will contribute to excessive humidity inside the vehicle, causing the problem you're experiencing. You'll notice that Recirc is not an available option when defrost is selected for this very reason.
