Actually, from exerience, I found that to really get the ambient temps good in a glass enclosure it is better to get a Ceramic heat emmiter on the top. I have a terrible range of temps in my appartment. But using a cheap Repti 500 thermostat and a Repti 100 watt CHE, I was able to keep a 18x18x18 glass vivarium at a toasty 82f all the time. I used UTH pads to warm up 1/4 of the bottom for a hot side.

Good luck.