Development of a Numerical model to predict the thermal insulating characteristics of ultra-low density non-woven fabrics used in extreme cold weather clothing

Ultra-lightweight, high-loft nonwoven fabrics made from hollow polyester fibers are promising thermal insulation materials for extreme cold-weather clothing owing to their low mass, flexibility, and excellent insulation performance. Their highly porous fibrous structure traps large volumes of still air, thereby reducing solid-phase heat conduction. Beyond apparel, such materials also have potential applications in aerospace systems, emergency shelters, and other thermal protection systems requiring lightweight and efficient insulation under severe environmental conditions. In this study, a comprehensive continuum-scale steady-state numerical model was developed to predict the thermal insulation performance of ultra-low-density, high-bulk nonwoven fabrics produced from blends of hollow polyester fibers. The fibrous assembly was represented as a homogeneous porous medium using effective transport properties without resolving individual fibers or pore-scale flow fields. The model incorporates coupled conduction, radiation, and convection within the porous structure. Six nonwoven specimens (S1–S6), each with an areal density of 100 gsm, were fabricated using blends of three hollow polyester fibers with different outer diameters (1.8H, 4H, and 7H). Conduction was modeled using Maxwell–Eucken-based formulations adapted for hollow fibers, while radiative transport was evaluated using Mie-theory-based optical extinction coefficients dependent on fiber diameter, wavelength, and refractive characteristics. Convective transport was estimated from experimentally measured fabric permeability and thickness using an equivalent porous-medium approach. Model predictions were validated against Sweating Guarded Hot Plate (SGHP) measurements conducted in accordance with ISO 11092, showing good agreement for all six investigated fabric configurations. The results from the developed model demonstrate that radiative heat transfer remains important even under room-temperature conditions, in ultra-low-density nonwovens, despite being commonly neglected in textile insulation analysis. For ultra-high porosity fabrics (porosity ≥ 99 %), heat conduction is governed primarily by stagnant air, whereas radiation contributes approximately 25%–35% of the total heat loss due to reduced optical thickness and increased radiative mean free paths. Simply increasing porosity to suppress conductive heat transfer may therefore not improve insulation, as the accompanying increase in radiative transmission can offset the conductive benefit. Increasing fiber diameter reduced insulation performance by 45%–50%, whereas increasing fiber hollowness improved insulation by 40%–45% through stronger optical attenuation. Natural convection was found to be negligible because the microfibrous network provides high specific surface area and strong viscous resistance to entrapped air. Lower ambient temperatures also improved insulation performance by 9%–10% owing to reduced radiative conductivity. The proposed model is validated for highly porous hollow polyester nonwoven insulation materials under standardized dry thermal conditions and can be used to analyze the influence of fiber and fabric structural parameters on thermal insulation performance, thereby supporting the design of lightweight insulation layers for extreme cold-weather clothing.