How Is Air Permeability Measured in Knitted Fabric?

Air permeability is measured by conditioning a representative specimen, clamping it over a known test aperture, establishing a specified pressure difference between its two faces, and measuring the volume of air flowing through the exposed area. The result is normalized to area and reported with the named method, pressure differential, test-head area, units, airflow direction, specimen state, and replicate data. Values are comparable only when those conditions are compatible.

Air permeability is airflow through a controlled area

In a knitted fabric, air can travel through spaces between yarns, through openings created by the loop architecture, and through deliberately transferred or missed regions. Air permeability quantifies that passage under a laboratory pressure difference. It does not identify one route through the structure and it does not, by itself, explain why two fabrics produce different values. The result must be read beside the construction.

ISO 9237:1995 describes a method for determining the permeability of fabrics to air and remains current after confirmation in 2023. ASTM D737-18(2023) covers a broad range that expressly includes knitted, layered, pile, treated, and coated fabrics. Both standards make the test conditions part of the result. A number detached from the method, pressure, area, unit, and specimen state is therefore incomplete.

Air permeability is often compressed into the consumer word breathability, but the terms are not interchangeable. Airflow, water-vapour transfer, liquid-moisture transport, and dry heat transfer are different mechanisms. A fabric may admit very little moving air yet offer relatively low resistance to water vapour; an open fabric may pass air readily while retaining liquid because of fibre chemistry or finish. Thermal and water-vapour resistance require separate measurements.

The apparatus balances pressure and measured flow

Testing begins with a sampling plan. Specimens should represent the width, length, colourway, and construction zones under review without repeatedly sampling the same wale or local defect. Engineered knitwear may require separate specimens for plain ground, mesh, rib, tuck field, jacquard, placket, or plated area because one panel can contain several air paths. Course and wale orientation should be retained in the record even when the final value is not directional.

The specimens are brought into the agreed standard atmosphere before measurement. ISO 139 defines standard atmospheres for conditioning and textile testing. This step matters because moisture regain, recent steaming, compaction, and relaxation can alter loop dimensions, fabric thickness, and the relationship between yarns. A development swatch taken directly from the machine cannot be assumed to represent the washed or finished production state.

For the measurement, the specimen is laid flat over the test head and clamped so the seal prevents air bypass around its perimeter. The instrument draws or forces air through the exposed area until the prescribed pressure difference is stable. It then measures the volumetric flow required to maintain that difference. The apparatus or calculation converts flow to a rate per unit area. Multiple locations are tested because loop structures, finishing, and handling introduce real local variation.

Clamping must secure the specimen without opening, compressing, or distorting it beyond the selected method. Stretching a relaxed knit over the head enlarges the pores and can create an artificially high result; allowing leakage at the edge does the same. Thick ribs, relief structures, seams, and very open constructions may also require a laboratory to confirm that the aperture, pressure range, and seal are suitable before the data are used for approval.

ASTM D737 notes that surface texture can make the direction of airflow significant. A raised, napped, coated, or asymmetric structure should therefore be tested from the specified face and the direction recorded. The reporting unit must also be explicit. Laboratories may express air permeability in different SI or customary unit forms; a shared label such as “airflow” does not make unmatched reports directly comparable.

Loop architecture controls open area and flow resistance

Stitch length changes loop dimensions and generally changes fabric cover, but its effect cannot be separated from yarn count, machine gauge, take-down, and relaxation. Longer loops can produce more open area, yet a bulky or hairy yarn may still obstruct the passages. Shorter loops can compact the surface, but excessive take-down or finishing pressure can alter thickness and pore geometry in a different way. The measured fabric state is more informative than a stitch-length setting in isolation.

The relationship between courses and wales describes stitch density and dimensional balance. Tuck stitches hold yarn in a needle for more than one knitting cycle and add yarn accumulation; miss stitches create floats and can redistribute open area; transfers can form eyelets or move loops between beds. Rib and interlock structures contain two-bed pathways unlike single jersey. Plating may place a different yarn or filament on each face, adding cover or changing surface resistance without changing the face design visibly.

Yarn variables include linear density, ply, twist, filament number, texturing, hairiness, bulk, and compressibility. Finishes can bind surface fibres, fill interstices, form a film, or alter wet relaxation. Brushing may increase apparent thickness and tortuosity. Washing may open one structure and compact another. Coatings and laminates can dominate the result even when the knitted substrate remains visible.

Fabric mass in grams per square metre is not a substitute for permeability. Two fabrics with similar GSM can differ in thickness, fibre packing, loop scale, open area, and finish; a heavier, lofty construction may contain more connected air space than a lighter, tightly compacted surface. GSM is useful production evidence, but it cannot predict airflow on its own.

A useful specification defines purpose before target

Higher air permeability is not automatically better. A summer layer intended to release moving air may benefit from a more open value. A wind-resistant outer layer, a compact collar, or a protective insert may require less flow. The relevant question is whether the tested construction meets the intended function while preserving appearance, handle, stretch, recovery, dimensional stability, and manufacturing tolerance.

During development, compare controlled variants rather than changing several inputs at once. Hold yarn and finish constant while adjusting loop length or stitch architecture, then hold the selected structure constant while assessing yarn or finishing changes. Test the approved care state, because laundering, steaming, pressing, and tumble or flat drying can move the loop geometry. If the garment contains materially different zones, set a sampling map instead of averaging them into one untraceable figure.

A production-ready report should identify the standard and edition, apparatus where required, test-head area, pressure differential, airflow direction, unit, number and location of specimens, course and wale orientation, conditioning atmosphere, pretreatment, finished or washed state, individual values, mean, and relevant variability. When laboratories disagree, comparative testing on homogeneous material is more useful than comparing isolated certificates with missing conditions.

The development process translates this evidence into loop specification, panel mapping, yarn and finish controls, sampling instructions, and factory approval criteria. Selected structural applications appear in Work. Anne's scope is outlined in About; a project-specific review can be requested through the contact form.

Common questions about air permeability

Airflow becomes useful when the structure and method remain visible

An air-permeability value is most effective as comparative development evidence. When it is tied to a named procedure, exact specimen state, loop architecture, yarn, finish, and end use, it can guide a precise adjustment. Detached from those conditions, it becomes an attractive but weak number.

Airflow should also be separated from liquid behaviour. Compare how liquid wicking is measured with how drying rate is measured to distinguish pressure-driven airflow, capillary transport, and evaporation.