The Size 4 Carcoon sold in the US is WIDER than the ones sold in the rest of the world. Technically referred to as a Size 4 (wide). Two fans work better than one because of the problems associated with too little airflow...the guys at Carcoon have been doing this for years and knowwhat they are doing. Humidity isn´t the issue, David. I mean, we´re based in one of the mosthumid parts of the USA in Houston, Texas! What´s important is stopping condensation, which is caused by warm and cold air meeting. The two fans mean that temperatures changes are much less dramatic, and prevent moisture from ¨settling¨ on the car. If you´ll be at the Fall Foliage Tour, I can explain it to you there. James Espey Replying late from Costa Rica on vacation!