Hopefully this will make sense....
It is open, but if you have the rear glass in place and the 3 covers on the amount of cold air you will experience would be minimal. I'm guessing that if your cargo area was a lot colder than the cab it might be cool for a while, but with your heater on it would balance out pretty quick, because with the rear glass in place as well as the cargo covers there is no new cold air coming in, if that makes sense. If you have the rear glass off or the cargo covers off then I'm guessing it will get a little chilly, depending on how cold it is outside.