Sarge is correct.

With glass cages, it is best to cover the top. I would go even farther than 1/2, more like 3/4. Then use undertank heat on a controller or use your ceramic emitter or radiant heat panel.

Too much ventilation is often the culprit, especially in Winter. In Summer, ventilation is usually no big deal.

The little 5 gal tank I have here - see how much of the screen is covered? In Winter, I even put a strip of foam in that open space. And this is for a colubrid.