Some humidity tricks aside from the foil:
1.) T-Rex Cypress Blend substrate holds humidity better than Aspen, in my experience
2.) The Zoo Med Repti Rock water bowl allows water to evaporate faster, thereby increasing humidity, than the Exo-Terra water dish. Place the Repti Rock half-way over the UTH for even increased humidity.
3.) Damp spaghnum moss in little containers in the viv improves humidity as well.
4.) If all else fails, a room humidifier works wonders.