Key Takeaways
- A single lab metric like total heat loss (THL) does not, on its own, predict how well a multilayer fabric system releases heat in real conditions.
- In a study of firefighter turnout gear, THL, evaporative resistance (Ref), and thermal protective performance (TPP) did not correlate with one another — each measures something different.
- THL predicted heat strain only under mild lab conditions; Ref only mattered under hot, stressful conditions. Neither covers the full picture alone.
- A construction that looked better on the standard THL test did not deliver a real heat-loss advantage once conditions got hotter.
- The practical lesson: judge any performance-fabric claim by the conditions the test was run under, and be cautious when a decision rests on one number.
When someone hands you a single test number to prove a fabric "breathes well," it is worth asking what that number actually measures, and under what conditions. A metric can be accurate and still be misleading if it only describes performance inside a narrow band of temperature and humidity. Research on firefighter protective clothing makes this concrete, and while that context is specialized, the reasoning behind it applies to how any performance fabric claim should be read.
The Problem: Two Jobs That Pull in Opposite Directions
Firefighter turnout gear has to solve two conflicting requirements at the same time. It must let body heat escape so the wearer does not overheat — heat strain is a documented cause of firefighter injury and cardiac events — while also blocking extreme external heat during a fire. The first job wants the system to release heat freely; the second job wants it to resist heat transfer. A turnout system is a compromise between those two demands, built from a layered construction: an outer shell, a moisture barrier, and a thermal liner.
For years, the industry leaned heavily on one number to judge how well a system sheds heat: total heat loss (THL). THL is measured under mild laboratory conditions — 25°C and 65% relative humidity. The question a group of researchers at NC State University's Textile Protection and Comfort Center set out to answer was simple: is that one number enough?
What the Study Actually Tested
The team — Gao, Deaton, Barker, Fang, and Watson — assembled different combinations of moisture-barrier and thermal-liner layers and evaluated them on a physiological (sweating) manikin. Crucially, they ran the tests under two very different environments: the mild standard condition, and a hot, physically stressful condition closer to what a working firefighter experiences.
Alongside THL, they measured a second breathability metric, evaporative resistance (Ref), taken at 35°C and 40% relative humidity. They then compared both against thermal protective performance (TPP), the measure of how well the system blocks external heat. The goal was to see whether any single one of these metrics could stand in for the others, and whether either breathability number actually predicted real heat strain.
This work was done on firefighter turnout systems — protective clothing — not on activewear or base layers. The specific numbers below describe that protective context. The broader lesson about how to read a fabric claim is what carries over.
What They Found
The results undercut the idea that one number can carry the whole judgment.
The three metrics did not track each other. THL and Ref did not correlate. Neither of them correlated with TPP. Each metric measures a genuinely different property, so a good score on one tells you very little about the others.
A construction that looked better on paper did not hold up. A two-component moisture barrier showed an apparent advantage on the standard mild-condition THL test. That advantage did not translate into a real heat-loss benefit once the system was tested under hotter, more stressful conditions.
Some differences that seemed like they should matter didn't. Single-layer and double-layer spunlace thermal liners produced no measurable difference in heat strain outcomes, in either the mild or the hot condition.
Each metric was only valid in its own range. THL predicted heat strain under mild conditions but not hot ones. Ref was relevant under hot conditions but not mild ones. Neither metric alone described performance across the full range a firefighter actually encounters.
| Metric | Measured at | Predicts heat strain in |
|---|---|---|
| Total heat loss (THL) | 25°C, 65% RH | Mild conditions only |
| Evaporative resistance (Ref) | 35°C, 40% RH | Hot conditions only |
The authors concluded that relying on a single metric risks underestimating real heat-strain risk. They recommended evaluating systems with both Ref and THL together, and suggested that the NFPA 1971 standard for firefighter turnout gear should require both rather than leaning on one.
Why This Matters Beyond Turnout Gear
The specific findings belong to protective clothing, but the underlying principle is general: a lab metric describes performance under the exact conditions it was measured in, and those conditions may not match how the product is used. A number generated at 25°C and 65% humidity answers a question about 25°C and 65% humidity. It does not automatically answer what happens when the wearer is hot, sweating, and working hard.
That is worth remembering whenever a fabric is sold on a single breathability figure. The honest questions are: what conditions was this tested under, does that range match the intended use, and is there a second metric that describes the parts this one leaves out? A claim built on one number in one environment is not wrong, but it is partial — and partial information about thermal comfort can lead to a real gap between the spec sheet and how a garment feels in use.
How FJORATEX Can Support This
FJORATEX develops and sources knit fabrics for performance apparel, which is a different application than firefighter turnout systems. We have not run this study or tested these findings on our own constructions, and we would not claim otherwise. What research like this does is shape how we read technical data with sourcing teams — treating a single test result as one input rather than a verdict, asking what conditions a number was generated under, and pushing for the additional measurements that describe real-use performance. When a fabric decision rests on a breathability or comfort claim, we would rather help a client interpret the data carefully than let one figure carry more weight than it can support.
Research source: Gao, Deaton, Barker, Fang, and Watson, "Effects of the moisture barrier and thermal liner components on the heat strain and thermal protective performance of firefighter turnout systems," Textile Research Journal, 2022.