Sounds like a nice project. My first thought is you may need more insulation. At least that would be my plan going in. I'd have walls, floor and ceiling at least 12 inches thick with insulation. I've heard the blow in cellulose is good and cheap. You could build another wall outside your existing walls and fill it with insulation.

Only problem with more insulation is it might take too long to cool down. Can you explain if there is anyway designed in to quickly cool the fruit? Can you blow in cold air thru ducts?

My other thought is that those rain gutter cables that are designed to keep rain gutters from freezing could be strung around on the inside walls. This would be better than heat lamps, because the heat is more diffuse. The cables are sturdy, cheap, and come with a thermostat.

More heat mass inside might also help. Barrels of water for instance. Any of that included?

I like the ground for insulation although that isn't always practical. My storage room is my well house that was built into the side of a hill. Dirt insulates all but the entry side.

I've read that garbage cans buried in the ground can function well for apple storage, where you bring it out a half bushel at a time, placing a bale of hay or something similar over the top. Creates it's own low oxygen environment close to the virtues of CA storage. You are obviously looking at a much more sophisticated set-up, but I mention this for others drawn to the topic operating on a smaller scale.

Fruitnut, I like the idea of the gutter cables. I will look into those. I need to compare the energy draw of those to heat lamps.

Based on the Coolbot website, I'm hoping that two layers of 2" blue board will be adequate insulation, at least for cooling season. Apparently, when the gizmo is fitted to the AC, it cools down very well. I don't know about heating during the winter.