Thermophysiological Comfort: Ret, Rct and the Sweating Hot Plate

The physics of comfort: heat balance

To hold the body core near ~37 °C, the metabolic heat produced must be dissipated to the environment. This heat crosses the fabric layer two ways: the dry route (sensible heat driven by a temperature difference via conduction, convection and radiation) and the wet route (latent heat carried off by sweat evaporation; at skin temperature water's latent heat of vaporisation is roughly 2.4 MJ/kg and can move many times a resting person's metabolic heat). Thermophysiological comfort is when the fabric's resistance to these two flows matches the wearer's activity level. A fabric is therefore defined by two separate resistances; a single "breathability" figure tells only half the physics.

Two numbers: Rct and Ret

Thermal resistance Rct is the temperature difference between the two faces of the specimen divided by the dry heat flux per unit area, expressed in m²·K/W; the higher it is, the more insulating (warmer) the fabric. Evaporative resistance Ret is the water-vapour pressure difference between the two faces divided by the evaporative heat flux per unit area, expressed in m²·Pa/W; the lower it is, the more easily vapour (sweat) passes, i.e. the more the fabric "breathes". A winter insulation layer wants high Rct, while a high-effort garment needs low Ret; the two often conflict, turning engineering into a balancing problem.

The sweating guarded hot plate (skin model) — ISO 11092

Both resistances are measured on the sweating guarded hot plate ("skin model") defined in ISO 11092; the apparatus mimics the heat and moisture transfer of human skin, and the method draws heavily on hot-plate work developed at Hohenstein. The measuring plate, with a guard ring that prevents edge losses, is held at 35 °C ± 0.1 °C to represent skin temperature, and a controlled airflow of ~1 m/s passes over it. For Rct the chamber is set to 20 °C / 65% RH (a temperature gradient exists across the specimen); for Ret the chamber is brought to 35 °C / 40% RH (an isothermal condition where only the water-vapour pressure difference drives the flow). The plate feed is wetted through a porous membrane to simulate sweating.

The US counterpart is ASTM F1868, which uses the same sweating hot plate principle to measure thermal resistance, evaporative resistance and total heat loss (THL). The method's typical range is roughly 0.002–0.5 m²·K/W for intrinsic thermal resistance and roughly 0–1000 m²·Pa/W (0–1.0 kPa·m²/W) for intrinsic evaporative resistance, spanning everything from a thin knit tee to a thick laminated shell.

Breathability classes by Ret

A widely cited scheme attributed to Hohenstein relates Ret to wear comfort. These thresholds are frequently referenced in product specifications; separately, the protective-clothing standard EN 343 defines its own (differently set) Ret-based water-vapour resistance classes for rainwear. Low Ret = staying cool and dry; high Ret = a wet, suffocating feel at high effort.

Units: clo, tog and the vapour permeability index imt

Thermal insulation is also expressed in clo and tog. By definition 1 clo = 0.155 m²·K/W (the insulation that keeps a resting person in thermal balance in a lightly ventilated 21 °C room — roughly a business suit) and 1 tog = 0.1 m²·K/W (common in textiles and bedding). It follows that 1 clo ≈ 1.55 tog; divide a fabric's Rct by 0.155 for clo, or by 0.1 for tog.

The quantity that collapses thermal and evaporative resistance into a single ratio is the water-vapour permeability index imt. ISO 11092 defines it as imt = (Rct / Ret) × S, where S ≈ 60 Pa/K. It is dimensionless and ranges from 0 to 1; imt = 0 means fully vapour-impermeable, while imt = 1 means the material behaves exactly like an air layer of the same thickness. In practice comfortable fabrics usually fall around 0.2–0.4 — they retain some heat while releasing vapour proportionally. Because imt weighs Rct and Ret together, it is a more holistic comparison than Ret alone.

How structure, fibre, weight and finish change Rct/Ret

In low-density fabrics Rct rises roughly linearly with thickness (with the still air trapped inside); it is not the fibre itself but the motionless air between fibres that insulates. That is why brushed fleece, with its high loft, gives high Rct yet can keep Ret low because its porous structure passes vapour straight through. Fine filaments and microfilaments trap more air and so can give higher Rct at the same weight. Knit architecture matters too: open piqué and mesh lower Ret through high porosity, while dense interlock trends to higher Rct and Ret. As weight (g/m²) increases, thickness and resistance usually rise, but the relationship depends on structure.

Finishes can shift the balance either way. Moisture-management/wicking finishes spread sweat and ease latent-heat removal, raising perceived comfort; conversely, film-forming coatings or non-breathable laminates push Ret up markedly. A durable water repellent (DWR), applied correctly, ideally barely affects Ret because it does not block the pores, whereas microporous and hydrophilic membranes are designed to bar rain while allowing controlled vapour passage.

Dry number, wet reality: what happens when you sweat

Standard tests run at steady state under limited moisture load; in real sweating the fabric's water content climbs. Because water is roughly 25 times more conductive than air, a fabric's thermal conductivity rises as it wets, so effective Rct falls — a wet layer suddenly insulates far less and produces the "after-chill" felt post-exercise. Comfort therefore depends not only on dry Rct but on how quickly the fabric moves water away and dries (moisture management). The ideal system: low Ret to release vapour, fast drying to limit Rct loss when wet, and an architecture that preserves its loft.