Why does a dry-aging room sit near 80% humidity, not bone-dry?

Answer: Spoilage-shrink balance

Maximum surface drying — No. Bone-dry air would dry the surface faster, but maximum drying is not the goal. Too much drying turns sellable beef into trim loss before aging has time to improve flavor. A dry-aging room needs a controlled rind, not jerky: cold air, airflow, and moderate humidity let enzymes keep working while spoilage is restrained.

Spoilage-shrink balance ✓ — Right. Humidity is a balancing knob. Too high, and spoilage bacteria get a friendlier surface; too low, and the cut loses excessive weight before it can be sold. The useful comparison is cheese aging: a rind needs to dry and host the right biology, but the inside still has to remain edible. Around 75-85% relative humidity shows up repeatedly because it keeps that compromise workable.

Inward water movement — Not quite. Airflow and humidity do not pump water back into the center of the meat; the net movement is outward, which is why dry-aged beef shrinks. The room is kept humid enough to avoid runaway drying, not to reverse it. If water truly moved inward, dry aging would not produce the crust and yield loss that make it so expensive.