I tried to express to my father-in-law when he built it that he was building a refrigerator. He did not see my POV. he delivered a box. And it breathed readily. I sealed it better with some Elmers wood glue in the seams. Also, I put in fiberglass insulation, and then lined it with 1/4 inch SC. Basically framed it like a house on the inside. i.e., the studs were 2 pieces of half inch pink foam insulation cut in to tiny strips (like 2x4s in the wall) and then between them was the fluffy insulation., then what would be the sheetrock, was actual 1/4 SC. It was VERY time consuming... But the result is a humidor which holds both it's temperature and humidity like a rock.
So from the outside, it’s ½ walnut, 3/8 SC, 1+ inch insulation, ¼ SC.
This was overboard, yes. But in the end, I feel it was worth it.