Air Permeability VS Water-Vapor Permeability

As a textile fabric manufacturer, or a textile lab R&D researcher, air permeability, water-vapor permeability, and breathability sometimes cause confusion. This article brings your attention to their key features and differences.

“Air permeability” refers to the permeability of gases through polymeric materials such as films, coatings, and fabrics. “Water vapor permeability” refers to the ability of water vapor to pass through fabrics. Comfortable sensations are achieved only when both air permeability and water vapor permeability are met.

water vapor vs air

Fabric Air Permeability 

Refers to the ability of a fabric to allow air to pass through when there is a pressure difference on both sides of the fabric. Different products have different requirements for fabric air permeability, such as airplane parachutes, automobile safety airbags, and masks.

Windproofness: When low air permeability is desired in a product, “windproofness” is usually used to describe it. Although the terms differ, the testing methods are the same.

Air Permeability Test Method

Under specified pressure difference conditions, measure the airflow passing vertically through a given area of the sample within a certain time.

Fabric Water-Vapor Permeability 

Refers to the ability of a fabric to adsorb and diffuse water vapor. It is one of the indicators that reflect the body’s ability to maintain thermal balance during heat dissipation and sweating. Water-vapor permeability can be characterized by moisture permeability rate or moisture resistance. If moisture permeability is poor, sweat on the body’s surface is not easily dispersed, making people feel hot and uncomfortable. Sportswear, outdoor activity wear, and bedding all have high requirements for fabric water-vapor permeability.

Water-Vapor Permeability Test Method

Common textile water-vapor permeability testing methods mainly include:

  • the desiccant moisture absorption method
  • the water evaporation method
  • the evaporative heat plate method 

The moisture permeability rate refers to the mass of water vapor passing through a unit area of the sample vertically under specified temperature and humidity conditions within a specified time, with units of g/(m² · 24 h). The larger the moisture permeability rate test result, the better the product’s moisture permeability.

Moisture resistance refers to the ratio of the water vapor pressure difference on both sides of the sample to the unit area of the sample’s evaporative heat flux passing vertically, with units of m²·Pa/W, tested by the evaporative heat plate method. The smaller the moisture resistance test result of a product, the better its moisture permeability.

The desiccant moisture absorption method and the evaporation method test the moisture permeability rate of sheet-like fabrics with a thickness ≤10 mm, while the evaporative heat plate method tests the thermal resistance of various fabrics and their products.

Water Vapor Permeability VS Breathability

The concepts of water vapor permeability and breathability are essentially the same, both referring to moisture permeability. Breathability comes from the description of outdoor functional fabrics. Waterproof and breathable fabrics use the unit (g/m²/24hour) to express the weight of water vapor passing through a unit area within 24 hours, which matches the requirements of functional fabrics in actual use: outdoor sports mainly require waterproofness while allowing sweat to be quickly expelled, avoiding a feeling of stuffiness. It is important to note that the unit of measurement for breathability in this context (Breathability) and our conventional understanding of air permeability tested in mm/s are different. Air permeability primarily tests air, not water vapor, so the breathability in waterproof and breathable fabrics refers to moisture permeability, which is the rate of water vapor transmission.

Related Resources

An Ultimate Guide to Air Permeability of Fabric

Air Permeability Tester

What is Textile Air Permeability and how to test

Textile Water Vapor Rate Tester DR290G

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