Air Permeability

Air permeability is the volume of air that passes through a fabric under a specified pressure difference, in a specified time, across a known area. It is reported in litres per square metre per minute (l/m²/min) or in m³/(m²·min), with the test pressure always stated alongside.

It is a property of the fabric as a physical filter. How that translates into comfort is a related but separate question — see the distinction below, which is where most confusion happens.

The standard method

ISO 3795 is the common method. A specimen is clamped in a holder, a pressure difference is applied across it, and the volume of air passing per unit time is measured. The pressure is part of the specification:

A figure without its pressure is not a measurement. A fabric that passes 300 l/m²/min at 100 Pa may pass a very different number at 1,000 Pa, and suppliers quote the pressure inconsistently. Always ask for both.

What drives the number

Air permeability is mostly about how open the structure is, and it is the clearest example of a property you can change without changing the fibre:

Note the tension with weight: a lightweight open weave can be highly permeable while a lightweight tight weave is not, and a heavily finished cloth can be nearly impermeable whatever its base structure. Air permeability is a function of the finished cloth, not the greige.

Air permeability is not "breathability"

These get conflated, and they are different measurements:

A fabric can be highly air-permeable and still perform poorly as a comfort fabric if vapour transport is poor — for instance an open weave in a thick, heavy yarn that moves air but does not wick moisture off the skin. If comfort is the goal, MVTR is the relevant test, not air permeability. Air permeability matters more for filtration, technical applications, and the air exchange people mean when they say a fabric "breathes".

Where it is used

• Technical and industrial textiles — filtration, venting, dust bags, geotextiles
• Apparel — lining and shell fabrics, especially where wind resistance is specified
• Medical — breathability claims and barrier-layer design
• Packaging — materials for air flow and ventilation panels
• Furnishings — where airflow through a curtain or blind affects drape and comfort

For a woven mill this is one of the few specs where weave construction is the whole answer: a buyer asking for a permeability target is really asking you to weave a structure that produces it, and the figure follows from ends-and-picks, yarn, and finish.

What to ask

1. What pressure — 100 Pa, 200 Pa, or something else?
2. Which method — ISO 3795, and which edition?
3. Greige or finished? Finished is almost always the relevant answer, and finishing can dominate the result.
4. Warp and weft separately, or an average?
5. Conditioned or as-received? See the conditioning note in four-point — it matters here too.
6. Is there also an MVTR requirement? If the end use is comfort, ask for both.

Weaving to a target

The practical point for a mill: permeability is designed into the loom, not measured after the fact. Ends and picks per inch, yarn size, and the finishing route together set the figure. That is why it belongs in the specification conversation at the point of choosing the construction — asking a mill to hit a permeability target on a construction already chosen to a different brief is a contradiction, not a request.

Weaverine does not publish air permeability figures on this page. The value belongs to a specific construction, weight, finish and test pressure, and a generic number would describe cloth nobody is buying. Ask for the figure against your own construction and test pressure, and check that the supplier states the pressure every time.