Thinking of building or finding a good deal on something like this near a lake and golf resort....
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They did it off grid but you don't have to, you can make it as modern as you like with electric, running water, plumbing, A.C. etc. As with all earth, concrete, brick walls there is no R rating, it's transference, hot to cold, cold to hot. As the walls warm up the heat is slowely transferred inwards so when the temp drops at night the walls are radiating stored heat inward. Conversely through the night the walls slowly cool ultimately radiating cold inward during the day. Being where you are you can supplement with a swamp cooler for the heat of summer and a wood or pellet stove for the winter.