Thermal Comfort
Insulation is measured in still air and worn in wind. A permeable fill can lose half its rating before the wind is even strong.
— tog
Still-air rating degraded for wind and permeability
The penetration coefficient is empirical and must be fitted to your own measurements — it is not a universal constant, and the model is a comparison tool between constructions rather than a predictor of absolute field performance. Cold-weather and survival clothing is decision-support only here: validate on a thermal manikin under the intended exposure.
Wind-Degraded Thermal Resistance (Tog & Clo) Matrix — free, with the formula and a worked example, at Textile School.