How tall is the enclosure? What is the room temp?

The rhp alone should be enough to adequetly heat the cage but we'll need to know this first.

I would always use a t-stat for any heat source, including an rhp, an not just a dimmer. I don't think many dimmers can keep it from getting too hot - especially if the room temps fluxuate. And I want to know just what the unit itself is set at and what the probe is reading.