Wet Insulation Collapse: A Bound on the Insulation Common Fills Retain as They Take Up Water, Ranked by Insulation Retained per Dollar When Wet

Garments are rated dry, but they are worn in rain, sweat, and river crossings, and the fills inside them do not respond to water equally. This paper originates a bound on wet insulation collapse for six common fills — cotton batting, down, wool batting, polyester wadding, kapok, and dried plant fibre — modelled with thermal conductivity rising as water replaces air in the pore space and with loft falling by a stated fill-specific fraction. The collapse is severe and it begins early. Merely damp, down retains 48.5 percent of its dry insulation and cotton 55.7 percent. In rain-level wetting, down retains 14.6 percent, cotton 21.0 percent, wool 24.8 percent, and plant fibre 28.7 percent, while kapok retains 48.2 percent and polyester 47.9 percent. Soaked, the failure is near-total for the prestige and staple fills: down retains 2.8 percent of its dry value (0.14 clo against a dry 5.16 clo), cotton 7.5 percent, wool 11.4 percent, and plant fibre 12.6 percent; polyester, the best retainer, still keeps only 27.8 percent. Ranked by insulation retained per dollar when soaked, plant fibre leads at 0.0987 clo per dollar because it is cheap, followed by polyester at 0.0787 and kapok at 0.0561, while down falls to last place at 0.0053 clo per dollar — about one-nineteenth of the leader. The popular belief that wool insulates well when wet is not supported as an absolute claim: wool retains more than cotton and far more than down, but in rain it still loses about three-quarters of its value. No fill studied survives soaking as effective insulation; the honest specification for wet conditions is a fill chosen for retention (polyester or kapok), a shell that keeps water out, and a dry reserve layer kept sealed until it is needed.