>>Yes 6ft high. I was thinking of heat from above and cooler below as thermal gradient.
I like this idea, especially for a species that is primarily arboreal. For a North American Ratnsnake, however, maybe not ideal. I'd consider a Yellow or Everglades Ratsnake which I believe are more arboreal than their obsoleta relatives. One of my favorite snakes ever was an Everglades Ratsnake that spent all of its time in the upper perches of a 2'x2'x3' cage. I don't know if they're all like that.
But there are many more qualified than I to address that.
>> The top I plan to use 100 watt ceramic heat emiter not far from top shelf area.
Regarding heating large cages, I highly recommend some sort of reflective insulation. Either foil-covered poly-iso insulation board or the various foil/bubble insulations that come in rolls.
You definately need a radiant heat blocker, you just can't rely on the natural R values of the wood your using or of "normal" insulation.
I had a 7'x7' garage snake room for a while that only had insulation in the attic above. The two exterior walls and the two walls interior to the rest of the garage were hollow. The floor was bare concrete which is horrible when using radiant heat.
At any rate, I covered the hollow-core door, walls and ceiling with Reflectix insulation. It has a very low R value but is excellent at blocking radiant heat. I was able to heat that 7'x7'x8' room to 85* with only 600 watts of power, even when it was twenty below outside.
Before I ramble on, give us more details about your cage and I'll recommend how to insulate it. For sure you'll need to pay special attention to the floors and ceiling.
Regarding how to heat it, I recommend from above. You could include some form of floor heat but don't do so at the expense of proper floor insulation.
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Current snakes:
0.2 Gonyosoma oxycephala - (Silver/Yellow)
3.3 Gonyosoma oxycephala - (Green)
2.1 Gonyosoma janseni - (Black)
3.3 Gonyosoma janseni - (Black & Tan)