During the Bat climb during which they see Santa Claus, a wire holding Batman's cape in a climbing up position is visible.
When the dynamic duo ascend to over 20,000 feet in the balloon, they should experience extreme cold and insufficient oxygen.
There would have to be a very loud burner heating the hot air balloon to have them rise to 20,000 feet and you would never hear them talking without shouting. You do see the rope next to Batmans' face that controls the burner .