Why my windows freeze up !!

I just wanted to clear some thing up about 3045 and the windows freezing up in the morning. The reason for that is because i have 2 sets of really thick (vinylly) curtains and i always use them both for insulation.

First of all there is a set that goes round the windows, then there is a set behind the driving seats then the night heater blows out from under the bed.

For that reason i am only heating half the cab so that makes the heater so much more efficient, take a look at the pics.