Not arguing, just asking. I’m in southwest Louisiana, we don’t see temps below 80 degrees at this time of year, I’m not sure the concrete will be that cool, do you? I’ve never checked it.
A point of view after a few decades in Houston;
* A non controlled area sitting on a concrete slab will be close to ambient, for me to get 60F I put up a foam wall shed with window AC. Do not expect 50F on the ground.
* I use my under the stairs for storage. I like the spot but for conditioned space insulating will be more work. A refrigerator has about 1.5 inch foam in place, this is good enough. Yes my sample shed was 4” but it sat outside on a slab.
* your number one need is to moderate temperature swings, a constant 75F will be better than swings from 70 at night to 90 in the day. You could reduce the effect with a tight, low mg oxygen per year bottle seal. ie Off the shelf, natural corks breath creating oxidation.
* with green or brown glass bottles one can mostly ignore light. With clear glass the UV from grocery store lights will have tasteable effects over a year.
* Your local AC vendor will have good ideas. They may not have done wine but temperature is an issue on a lot of food products.
My question is how important is this hobby? Are you willing to give up a room? In the ideal you would also run a controlled temp fermentation area. At the minimum size up only long term storage cases with something looking like a fridge.
Part of a wine cellar is display. This means 3’ aisles.
Oxidation will happen even in a freezer. Looking at how to use antioxidants like tannins and free SO
2 is a good use of energy. Also head space, oxygen exposure in racking, ,,, “total package oxygen “.
