When one sets out to build a custom home they should be limited solely to their dreams and budget constraints. If they want a wine cellar so be it, and if the neighbor wants a conference room to host AA meetings that is thier choice. I once did the plumbing in a house that had four bedrooms but 6 full baths and three 1/2 baths and only two people live in the house. Personally i thought it was a bit too much, but far be it from me to tell the next man or woman how to build their dreamhouse. What truely amazes me is the number of people who are designing their dream house, then post on the forum asking the opinions of others as to whether they should have a certain feature or not? The whole idea of a custom home is to create an enviroment that befits your personal lifestyle and interests. In my personal opinion if a guest drops by and don't like your choices they have the option of going back to their little corner of the world where things are done their way. If a wine cellar complete with a tasting bar befits your social habits then by all means that is what you should have, just don't forget an adequate storage facility for the cheese to be served with the wine.

R 13 isn't really great insulation. In addition, the Whisperkool unit isn't designed to vent to the outside. Cooling units are designed to operate within a general range of temperatures. So a fridge, an AC, and the Whisperkool unit are all designed to operate within their own unique parameters. An AC unit usually goes 25 degrees or so and expects the top temp to be somewhere in the 90s. So you get cooling down to the mid sixties or so. They don't get things quite as cold when the exterior temps are in the high 90s or 100s. A fridge is designed to cool down to the mid 30s or so, and to operate in an ambient temp of around 70. That's way past the condensation point, so you have really tight seals and super insulation. The Whisperkool is designed to cool down to about 55 or so from an ambient temp of around 70, not the 90s you may experience outside. It's not rocket science tho. If you want to get cooler, you need a larger evaporation coil and a more powerful condenser and you help the machine with tighter seals and better insulation. Thus, you can leave your door slightly ajar with a window AC unit and you'll still get fairly cool, but you wouldn't leave the fridge door slightly open. To your main question - the one degree fluctuation is not all that bad. Don't sweat it. To your larger question that you didn't ask - you're possibly overtaxing the Whisperkool unit. If it dies on you, you can replace it with a regular window AC unit. Just seriously oversize it, then tweak the thermostat - either replace / bypass it, or tape a resistor onto the probe so that it thinks the air is actually warmer than it is and it cools a bit more as a result. It's how I cool my wine cellar.

You are going to have a lot of problems. You're sealing the concrete wall with the insulation? "I started with 1 layer of the 4” XPS over the ceiling and then covered all the walls with 1 layer of 4” XPS foam leaving 1’ of bare concrete at the floor on the exterior North and East walls)." What's on the other side of the concrete? Dirt? Or is it an attached house like a row house? If it's a detached house and it's just dirt on the other side, which seems to be the case because you said it extends 7 ft below grade, you will have a great deal of condensation and eventually mold. The concrete walls are not completely dry and sealing them the way you did allows no place for the moisture to go. Don't seal those walls off. "As for venting, no. It is a fully inclosed concrete box. Monitored humidity is a constant 70%." So the ceiling is concrete too and your entry stairs are over that? If so, no wonder you have condensation. On the other hand, you have the makings of a great wine cellar. I wish I had that. Depending on where you are, the soil temperature can vary quite a bit between winter and summer but you're in a cool area so you should be fine. If you have 70 percent humidity in the concrete room, why are you insulating with a vapor-proof material and then sealing off all possible avenues for the moisture to escape? You can't do that. What happens to the temperatures if you don't seal the ceiling and concrete walls and just insulate the walls exposed to the basement? I think you'd be far better off with that approach. If the concrete walls give you a stable temp, why insulate them? Also - the door is R10 and the rest of the insulation is R20? If you really want R20, put insulation on the door too. Otherwise, you have a huge section of your wall that has a lower R value than everything else and you will effectively have insulation of R10. To start, I would take down the insulation from the concrete and monitor the resulting temp in the room. It seems like you can actually get a pretty good temperature and perfect humidity with what you have, doing minimal work. Good luck.

IMO we need a bit of info . 1st what do you need in way of storage like how many bottles for sure. Why do you need custom , there are 100s of beautiful wine racks to be found that are not custom. Are you having a tasting space in the wine cellar. What is your style ?