Determination of Thermal Insulation Performance of Clothing: Testing Principles, Standards, and Meth

Clothing thermal insulation testing quantifies how effectively fabrics, garments, and clothing ensembles resist dry heat loss, using laboratory instruments such as the sweating guarded-hot plate and the thermal manikin. By measuring parameters like thermal resistance (Rct) and insulation value (clo) in accordance with ISO, ASTM, EN, and GB standards, manufacturers can validate warmth claims, optimize material selection, and ensure wearer comfort and safety. This article explains the technical principles, sample requirements, test items, standard methods, and practical applications of thermal insulation measurement for clothing.

Technical Overview

Thermal insulation performance is one of the most critical functional properties of clothing, particularly for cold-weather apparel, outdoor equipment, occupational workwear, and protective clothing systems. The human body continuously generates metabolic heat, and clothing acts as a barrier that slows the transfer of this heat to the cooler surrounding environment. Dry heat transfer through a clothing system occurs through three mechanisms: conduction across fibers and trapped air layers, convection within and across air gaps, and radiation between surfaces. The ability of a garment or fabric to retard this heat flow is expressed quantitatively as thermal resistance.

The internationally accepted physical unit for thermal resistance is Rct, expressed in square meters kelvin per watt (m²·K/W). A complementary physiological unit, the clo, was defined to relate laboratory results to human comfort: one clo (0.155 m²·K/W) represents the insulation required to keep a seated, resting person comfortable at approximately 21°C with low air movement. Higher values indicate warmer garments. Reference insulation values for typical clothing ensembles are compiled in ISO 9920 (Ergonomics of the thermal environment — Estimation of the thermal environment — Estimation of thermal insulation and water vapour resistance of clothing ensembles).

Standardized determination of thermal insulation is carried out with two principal devices: the sweating guarded-hot plate (skin model) for flat materials, per ISO 11092, ASTM F1868, and GB/T 11048; and the thermal manikin for whole garments and ensembles, per ISO 15831, ASTM F1291, GB/T 18398, and EN 342. Testing is typically performed by accredited third-party laboratories or in-house physical testing facilities equipped with climate-controlled chambers.

Testing Samples

Flat Fabrics and Material Assemblies

Woven, knitted, nonwoven, quilted, coated, laminated, and multilayer fabric assemblies used in garment construction can be evaluated on the guarded-hot plate. Specimens are cut large enough to fully cover the measuring plate and guard ring, typically 50 cm × 50 cm, without wrinkles or tension. Prior to testing, samples are conditioned to equilibrium in a standard atmosphere, for example 20 ± 2°C and 65 ± 4% relative humidity per ISO 139, or 21 ± 1°C and 65 ± 2% per ASTM D1776.

Finished Garments and Ensembles

Complete garments—jackets, parkas, trousers, coveralls, coats, and full layered ensembles—are tested on the thermal manikin. Garment size is selected to fit the manikin body dimensions appropriately, since fit governs the thickness of trapped air layers and therefore the measured insulation. Ensemble tests may combine base layers, mid-layers, outer shells, headwear, gloves, and footwear, and can be conducted under both stationary and walking conditions.

Sleeping Bags and Quilted Products

Sleeping bags, comforters, quilts, insulated pads, and related bedding products are evaluated using a thermal manikin positioned inside the product on a standardized support board. ISO 23537-1 (formerly EN 13537) specifies the requirements and defines comfort, limit, and extreme temperature ratings derived from the measured insulation.

Protective and Specialty Clothing

Cold-protective workwear, emergency responder garments, military clothing systems, and components of heated garments are tested to verify compliance with performance specifications and to support certification under PPE-type standards such as EN 342 and EN 14058.

Testing Items

The principal measurands reported in clothing thermal insulation testing include:

• Thermal resistance (Rct), in m²·K/W, the dry heat resistance of a fabric under steady-state conditions
• Thermal insulation in clo units (Icl, It), where clo = Rct / 0.155, optionally including the ambient air boundary layer
• Water-vapour resistance (Ret), in m²·Pa/W, determined on the sweating plate to characterize breathability alongside insulation
• Thermal transmittance or heat transfer coefficient (U, W/m²·K), the reciprocal-based expression of overall heat flow
• Heat retention percentage (warmth-keeping rate), a legacy index still reported under the historical version of GB/T 11048
• Resultant insulation under walking or moving-air conditions (Itr, Icler), per EN 342, reflecting the effect of pumping and ventilation
• Comfort, limit, and extreme temperature ratings for sleeping bags, per ISO 23537-1
• Permeability index (im), the dimensionless ratio linking dry and evaporative resistance for overall comfort assessment

Testing Methods

Sweating Guarded-Hot Plate Method

The guarded-hot plate, also called the skin model, simulates human skin with an electrically heated metal plate maintained at 35°C. A guard ring and bottom heater eliminate lateral and downward heat losses so that all supplied power flows axially through the specimen. For Rct determination under ISO 11092, EN ISO 11092, ASTM F1868, and GB/T 11048, the specimen is placed on the plate in a climatic chamber at 20°C and 65% relative humidity with parallel air flow of about 1 m/s. The electrical power required to hold the plate at constant temperature is recorded at steady state; subtracting the bare-plate resistance yields the thermal resistance of the material. For Ret determination, distilled water is supplied to a porous plate surface and both plate and air are held at 35°C with 40% relative humidity, allowing evaporative resistance to be calculated from the measured energy balance.

Thermal Manikin Method

The thermal manikin is an adult-sized, anthropometric body form divided into independently controlled heating zones with surface temperatures maintained at 33–35°C to simulate skin. The test garment is dressed onto the manikin inside a climate chamber, and the power required to sustain each zone temperature is used to compute total heat loss. Total insulation (It) includes the still air layer surrounding the clothed body; basic clothing insulation (Icl) is obtained by subtracting the boundary air layer resistance measured on the nude manikin. Principal standards include ISO 15831, ASTM F1291, GB/T 18398, and EN 342, the latter specifying walking and moving-air test modes for cold-protective ensembles. ASTM F2732 provides a practice for translating manikin-measured insulation into ambient temperature ratings for cold-weather clothing.

Related and Supplementary Methods

ASTM D1518 determines the thermal transmittance of batting systems using a hot plate, ISO 5085 addresses thermal resistance of textiles, and BS 4745 describes a two-plate Togmeter procedure. These results are frequently interpreted together with air permeability (ISO 9237, ASTM D737), moisture management, and thickness or compressional recovery tests, since loft, wind penetration, and moisture strongly influence real-world insulation.

Applications

• Material comparison and selection during product development of insulated jackets, mid-layers, and thermal underwear
• Quality control of incoming fabrics and verification of batch-to-batch consistency of insulation fillings
• Support for performance labeling, clo-value claims, and temperature-rating marketing statements
• Certification and conformity assessment of cold-protective clothing under EN 342 and EN 14058
• Comfort temperature ratings for sleeping bags and camping equipment under ISO 23537
• Human thermal physiology studies and cold-stress modeling, including required clothing insulation (IREQ) calculations under ISO 11079 using ISO 9920 ensemble data
• Benchmarking of new technologies such as aerogel fills, reflective linings, phase-change materials, and lightweight synthetic insulations

FAQ

What is the difference between Rct and clo?

Rct is the physical thermal resistance expressed in m²·K/W, while clo is a physiological unit defined so that one clo equals 0.155 m²·K/W. Both quantify the same property; conversion is performed by dividing Rct by 0.155. For example, a fabric with Rct of 0.155 m²·K/W has an insulation of 1.0 clo.

What clo value should insulated winter clothing provide?

Typical reference values are approximately 0.25–0.4 clo for a lightweight sweater, around 1.0 clo for a business suit, roughly 2 clo for a heavy winter parka, and 3 clo or more for extreme-cold expedition clothing. Sleeping bags generally range from about 4 to 10 clo depending on their intended temperature rating.

Why is water-vapour resistance (Ret) usually measured together with Rct?

High thermal insulation is often achieved at the expense of breathability. wearer comfort depends on both dry heat exchange and evaporative heat dissipation, so pairing Rct with Ret—and indices derived from their ratio—provides a complete picture of the thermal comfort performance of a fabric or garment system.

How many specimens are required and how should they be prepared?

Hot-plate testing normally requires at least three specimens per sample, conditioned to moisture equilibrium in a standard atmosphere for a minimum of 24 hours. Manikin testing typically requires one or two complete garments and involves repeated dressing and undressing cycles to account for variability in garment fit and layer arrangement.

Do laboratory results reflect real-world warmth?

Laboratory values represent steady-state conditions and therefore exclude some field variables such as wind compression, rain, body posture, and physiological moisture generation. Walking manikin procedures, moving-air options, and temperature-rating models such as ASTM F2732 were developed specifically to bring laboratory measurements closer to end-use conditions, and test reports should always state the standard and test mode applied.

How long does insulation testing take?

After conditioning, a guarded-hot plate test typically requires 45–90 minutes to reach steady state for each configuration, while full thermal manikin campaigns, including multiple dressing cycles and test modes, may take several hours to a full day. Accredited third-party laboratories generally issue final reports within five to ten working days of sample acceptance.