Terms and Definitions
Breathability does not, as the term might imply, relate to an exchange of air. Instead it is the ability of a fabric to allow moisture vapour to pass through it. A fabric may carry this out in numerous different ways, and exchange of air is only one possible avenue. Moisture vapour permeability (MVP) and moisture vapour transmission (MVT) are perhaps better, and certainly more technical, terms to use than breathability. Neither ASTM nor the Textile Institute define breathability, but MVP and MVT are both defined in test standards.
According to EN 31092:1993, water-vapour permeability is “a characteristic of a textile material or composite depending on water-vapour resistance and temperature. Water-vapour permeability is expressed in grams per square metre hour pascal.” According to BS 3546:2001, water-vapour permeability is the “ability of a coated fabric to transmit water vapour above a specified level whilst maintaining a high degree of water penetration resistance”.
The water-vapour permeability index is the ratio of thermal and water-vapour resistances, in accordance with the definition in standard EN 31092:1993.
Water vapour resistance, Ret, can be thought of as the ‘opposite’ to breathability and is, according to BS 3546:2001 the “water-vapour pressure difference between the two faces of a material divided by the resultant evaporative heat flux per unit area in the direction of the gradient.”
Air permeability is intrinsically linked to breathability. Air permeability is “the velocity of an air flow passing perpendicularly through a test specimen under specified conditions of test area, pressure drop, and time” (according to BS EN ISO 9237: 1995). All air permeable fabrics are breathable to some extent, though not all breathable fabrics are air permeable.