Moisture regain is the percentage of water a bone-dry fibre absorbs from standard atmosphere (20°C, 65% RH), per ISO 6741-1. It determines how much moisture a fabric can absorb before feeling damp. Merino wool and viscose both have 13-15% moisture regain — they absorb sweat vapour before it reaches the skin surface. Synthetics like polyester (0.4%) absorb almost nothing, leading to surface wetting. High moisture regain correlates with comfort in both hot-humid and cold-dry conditions.