It could also be worth a try to cover half of the top with glad press and seal instead of tinfoil - you can really press it down on the screen top and it does a great job at helping me bump up the humidity in my glass tank.

Someone else mentioned it but it is worth mentioning again - go to teh hardwar store and get insulation and try putting that around the sides of your tank - that will help keep the ambient temps up and hopefully you will have to use your lamp less often, since they are murder on humidity.