Not surprised it's working so well. What was the highest temperature outside when the shed was holding an interior temp. of 85*?
If you do have problems in the winter you may need to start with the floor. I have heard of guys taking 1.5" pink or blue polystyrene insulation and glueing it down to the plywood floor. Then place another layer of plywood over the insulation and screw through that and into the lower layer of plywood. Evidently it makes for a fairly sturdy floor. You could add framing to make it more sturdy but I gather it's not necessary.
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Currently keeping:
6.10 Gonyosoma oxycephalum (Javan, mixed colors)
1.1 Philodryas baroni
1.1 Lampropeltis triangulum multistriata
1.0 Rhodesian Ridgeback