RHP will heat them from both sides, they get heat they pull in from the surface, and the radiant heat from above. At the same time, the RHP will heat up a cage to proper air temps as long as other factors allow it. Such as....Room temp, cage is on a inside wall, size of cage, cage is black or white, PVC door or glass, objects in the cage. All these things can be factors on heating a cage with only a RHP. I myself use only a RHP in mine. I can reach temps within the average range pretty easy. You have to properly check ambient air temps by hanging a probe and blocking it from the RHP with paper or something. Your setup will be different than other peoples. Many people do use only a RHP from what I understand. Just make sure it's regulated with the thermostat.