Quick Answer: A 6-mil closed-cell vapor barrier and minimum R-19 (walls) to R-30 (ceiling) insulation are absolute requirements for any wine cellar. Without a vapor barrier, warm ambient moisture will migrate through the walls, causing catastrophic internal condensation and cooling system failure.
The physical construction of a wine cellar is the primary defense against thermal load. A wine cellar is technically a 'refrigerated envelope.' Because the air inside is held at 55°F, it holds less moisture than the warmer air outside the room. Physics dictates that moisture will aggressively migrate from the warm side to the cold side to equalize pressure. If a vapor barrier is omitted, this moisture penetrates the drywall and condenses directly inside the wall cavity.
Closed-cell spray foam is the gold standard because it acts as both a high R-value insulator and a seamless vapor barrier. Foil-faced rigid foam can also work, but every single seam must be meticulously taped with HVAC foil tape. Any breach in the envelope forces the cooling unit to run continuously, burning out the compressor in a fraction of its lifespan.
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