I used a heat lamp at first when I got my BP a little under 2 years ago. I found that keeping humidity up was next to impossible. Instead I just put a second heat pad in under a second, smaller plastic container that was sealed with holes in the top. I also put some aluminum foil over 3/4 of the tanks screen top.

The water evaporates into the tank and keeps the humidity at almost perfect levels all the time. I also have a glass tank, but a tub would definitly help keep humidity up if you had real humidity problems. I also live in SouthWest Florida so keeping humidity up isn't too hard