The methods of measuring the thermal insulation and water vapor permeability of fabrics

Introduction

Measuring the thermal insulation and water vapor permeability of fabrics is essential for evaluating their comfort, functionality, and suitability for various applications, ranging from winter clothing to sportswear and industrial textiles.

Thermal insulation determines a fabric’s ability to resist heat transfer, a critical attribute for retaining warmth in cold environments. Methods such as the Guarded Hot Plate, Thermal Manikin Testing, and advanced techniques like the TCi Thermal Conductivity Analyzer provide precise assessments of a fabric’s insulating properties.

Water vapor permeability, on the other hand, evaluates a fabric’s capacity to allow moisture to pass through, ensuring breathability and comfort. Techniques like the Cup Method and Sweating Guarded Hot Plate are widely used to measure this property, enabling the development of performance fabrics that balance protection and comfort.

This article explores the principles, procedures, and applications of these measurement methods, offering a comprehensive understanding of how they contribute to fabric innovation and optimization.

1. Thermal Insulation Measurement:

Thermal insulation evaluates a fabric’s ability to resist heat transfer, which is crucial for applications like winter clothing, sportswear, and industrial protective garments. 

There are two fundamental ways to determine the thermal characteristics of textile fabrics; one is the steady-state method, whereas the other is the transient method. The steady-state measurement determines thermal conductivity, insulation, and interfacial thermal conductance by measuring the temperature differential (∆) under the steady-state heat flow (q) through the sample. Heat sources employed in transient techniques are delivered either periodically or as a pulse, leading to periodic (phase signal output) or transient (amplitude signal output) temperature changes in the sample. Depending on the thermal characteristics, sample setup, and measurement temperature, each approach has unique benefits and drawbacks and is only appropriate for a certain subset of materials.

A. Guarded Hot Plate Method:

• Principle: The Guarded Hot Plate Method (ISO 11092) measures thermal resistance by placing a fabric sample between a heated plate and a cooler environment. The heat flow through the fabric is measured, allowing calculation of the thermal resistance (R-value), which indicates the fabric’s insulating properties.

Figure 1. Schematic of guarded hot plate apparatus

• Procedure:
  1. A fabric sample is placed between a heated plate and a cooler environment.
  2. The system measures the energy required to maintain a constant temperature difference.
  3. Thermal resistance (R-value) is calculated using heat flow and temperature data.
• Applications: Widely used for testing thermal insulation in textiles.

B. Thermal Manikin Testing:

• Principle: The Thermal Manikin Test (ASTM F1291) evaluates the thermal insulation of garments by placing them on a heated mannequin. The manikin, equipped with sensors, measures heat loss in a controlled environment, comprehensively assessing the garment’s insulation properties, fit, and design.

Figure 2: Thermal Manikin Block Diagram 

• Procedure:
  1. The mannequin is dressed in the fabric and placed in a controlled environment.
  2. The energy required to maintain the mannequin’s surface temperature is measured.
  3. Insulation is expressed as clo value (1 clo = insulation of typical business clothing).
• Applications: Ideal for testing complete garments or ensembles.

C. Alambeta Tester:

• Principle: The Alambeta tester measures the thermal properties of textiles by simulating skin contact. It involves placing a fabric sample on a heated measuring head and recording the heat flow and temperature difference across the fabric. This method calculates the textile’s thermal resistance and thermal conductivity, providing quick and accurate results without damaging the sample.
• Procedure:
  1. A sensor plate at a known temperature is pressed against the fabric.
  2. Heat flow through the fabric is recorded.
  3. Thermal conductivity, resistance, and absorptivity are calculated.
• Applications: Quick laboratory testing of fabrics.

Figure 3: Schematic of Alambeta instrument: 1. Measuring head with a cool plate at an ambient temperature of 21°C; 2. Metal block; 3. Heating element; 4. Heat flow sensor with movable heated plate at 32°C; 5. Tested sample; 6. Instrument base; 7. Bottom Heat flow sensor; 8. Thermometer; 9. The initial distance (h) between the device’s upper and lower plates

D. Tog Meter:

• Principle: The Tog Meter (ISO 5085-1) measures the thermal resistance of textiles, indicating their insulating properties. It works by placing a fabric sample between a heated plate and a cooler environment and measuring the heat flow through the sample to determine its thermal resistance (R-value). Measures the thermal resistance of fabrics in terms of “Tog” (1 Tog = 0.1 m²·K/W).
• Procedure:
  1. The fabric sample is placed between a heated plate and a cold plate.
  2. Heat flow through the fabric is measured while maintaining a steady temperature gradient.
  3. Thermal resistance is calculated and expressed in Tog units.
• Applications: Commonly used for bedding, insulating materials, and apparel testing.

Figure 4. Togmeter: Two-plate method

Figure 5: C-Therm (TCi) thermal conductivity analyser

The methodology is derived from the Transient Plane Source (TPS) technology but incorporates key modifications:

• Unlike the traditional TPS method, which uses a double-sided interface, the MTPS method simplifies the process with a single-sided interface, making it faster and more versatile.

2. Water Vapor Permeability Measurement:

Water vapor permeability assesses a fabric’s ability to allow moisture to pass through, which is crucial for comfort in clothing, sportswear, and medical textiles. There are several methods of measuring water vapor permeability. The Cup Method (ISO 15496), Sweating Guarded Hot Plate (ISO 11092), and Upright Cup Method are common methods for assessing the water vapor permeability of textile materials.

A. Cup Method:

• Principle: The Cup Method (ASTM E96, ISO 15496) measures water vapor permeability by sealing a fabric sample over a cup containing either water or desiccant. The weight change over time is recorded to determine the water vapor transmission rate (WVTR), indicating how easily moisture vapor passes through the fabric.
• Procedure:
  1. A fabric sample is sealed over a water-filled cup.
  2. The system is placed in a controlled environment with known temperature and humidity.
  3. The weight loss of the cup is measured over time, indicating water vapor transmission.
• Applications: Commonly used for fabric comparison and research.

Figure 6: The principle of ASTM E96 water vapor transmission test.

B. Sweating Guarded Hot Plate (ISO 11092):

• Principle: The Sweating Guarded Hot Plate (ISO 11092) measures both thermal resistance and water vapor permeability. It uses a heated plate with moisture simulation to assess how well a fabric resists heat transfer and allows moisture vapor to pass through, providing a comprehensive evaluation of comfort properties.
• Procedure:
  1. A heated plate simulates human skin sweating.
  2. The fabric is tested for its ability to transmit water vapor while maintaining thermal balance.
  3. Water vapor resistance (Ret value) is calculated, where lower Ret values indicate higher permeability.
• Applications: Ideal for performance testing of breathable and waterproof textiles.

Figure 7. Schematic diagram of sweating guarded hot plate

C. Upright Cup Method:

• Principle: Similar to the cup method, it uses an upright orientation. The cup is in a vertical or upright position.
• Procedure:
  1. Water or a desiccant is placed in the cup.
  2. The sample is sealed, and water vapor transfer is recorded over time.
  3. Water vapor permeability is calculated using mass change.
• Applications: Alternative to the traditional cup method, especially for thin fabrics.

Key Factors Influencing Measurements:

• Fabric Structure: Thickness, porosity, and weave type impact both thermal and vapor performance.
• Environmental Conditions: Temperature, humidity, and air movement must be controlled.
• Fabric Treatments: Coatings, laminations, or finishes may alter thermal and vapor properties.

Conclusion:

Understanding and measuring the thermal insulation and water vapor permeability of fabrics is fundamental to developing textiles that deliver both functionality and comfort. These properties are crucial for various applications, from keeping individuals warm in cold climates to ensuring breathability in high-performance garments.

The diverse range of methods—such as the Guarded Hot Plate, Thermal Manikin Testing, TCi Thermal Conductivity Analyzer, and Cup Method—provides tailored solutions for evaluating fabrics based on their intended use. Each technique has its unique advantages, allowing researchers and manufacturers to optimize fabric performance to meet specific requirements.

By leveraging these advanced testing methods, the textile industry can design materials that not only enhance user comfort but also address the growing demand for innovative, high-performing, and sustainable fabrics. Whether for outdoor gear, athletic wear, or medical textiles, these evaluations are key to creating products that excel in both utility and user satisfaction.