Air Permeability of Knit Fabric: Testing Methods and Sportswear Performance Implications

When sourcing activewear fabrics, breathability claims are common — but without a measurable, reportable parameter, they are difficult to verify before bulk production. Fabric air permeability is the specific physical property that quantifies how freely air passes through a fabric under defined conditions. For apparel brands developing sportswear, training kits, or performance basics, knowing how to read and request air permeability data from a supplier is a practical step that reduces sampling revisions and limits post-production performance complaints.

This guide explains how air permeability is tested, how knit structure affects the result, and which verification steps are worth including in your pre-sampling checklist.

Why Fabric Air Permeability Matters in Sportswear Sourcing

Air permeability measures how much air passes through a fabric per unit area per unit time under a fixed pressure differential. It is expressed in mm/s or L/m²/s (ISO system) or cfm/ft² (ASTM system). This is a distinct metric from water vapour transmission — the two together define what is loosely called "breathability", but they are tested separately and behave differently depending on fabric construction.

From a sourcing perspective, air permeability matters for two practical reasons:

• Thermoregulation performance: During exercise, convective heat loss through the fabric is partly governed by how freely air circulates across the skin surface. Fabrics with low air permeability may trap heat, which can cause discomfort during high-intensity activity.
• Specification alignment: If your tech pack does not specify a minimum air permeability value, the supplier has no testable target to meet. Performance can vary significantly across rolls and colourways without a documented reference.

Brands that treat air permeability as a measurable, reportable spec — rather than a qualitative description — are better positioned to manage bulk consistency and resolve quality disputes with supporting data.

How Fabric Air Permeability Is Tested — ISO 9237 and ASTM D737 Explained for Buyers

Two test standards dominate the industry. Understanding what each measures helps buyers specify which report to request and how to read the numbers.

ISO 9237

ISO 9237:1995 is a commonly requested standard for fabric air permeability reports in international textile sourcing. It measures the permeability of fabrics to air under defined laboratory conditions. ASTM D737 is also widely used, especially when suppliers or buyers follow North American testing requirements. Because report units, conditions, and lab settings may differ, buyers should compare results only when the test method and sample condition are clearly stated.

ASTM D737

ASTM D737 is the North American equivalent. It operates on a similar principle — measuring airflow through a fabric under controlled pressure — but uses cfm/ft² (cubic feet per minute per square foot) as its unit and may apply a slightly different pressure differential depending on the fabric category. Results between ISO 9237 and ASTM D737 cannot be directly compared without conversion, so it is important to align on the standard before requesting a test report.

What to request from your supplier:

• Specify the standard used in your target supply chain, such as ISO 9237 or ASTM D737, and avoid comparing reports produced under different methods.
• Confirm which face of the fabric was tested — some constructions show different values on the face vs the back side
• Check whether the test was conducted on finished or greige fabric, as dyeing, finishing, and coating treatments may affect the result
• Request the test area and number of specimens tested; single-specimen results are less reliable than averages across five or more samples

Note: Air permeability values may vary depending on yarn count, GSM, fabric construction, and finishing treatment. Reported values should be treated as reference data for a specific sample, not as absolute guarantees for bulk production.

How Knit Structure Affects Fabric Air Permeability — Mesh, Bird's Eye, and Pique Compared

Knit structure is one of the primary determinants of air permeability. Open-hole constructions allow significantly higher airflow than compact solid-knit structures, though the relationship is also influenced by yarn type, GSM, and finishing. The three structures most relevant to sportswear sourcing — knitted mesh, bird's eye fabric, and pique knit fabric — differ meaningfully in their air permeability characteristics.

Fabric Structure	Air Permeability Level	Structural Feature	Typical Application
Knitted Mesh	High	Intentional open holes formed in knitting; high porosity	High-intensity training panels, jersey shells, uniform inserts
Bird's Eye Knit	Moderate–High	Small regular openings in a stable knit base	Polo shirts, training tops, performance uniforms
Pique Knit	Moderate	Raised surface texture over a tighter knit base	Polo shirts, leisure sportswear, casual performance tops

Knitted mesh achieves the highest air permeability by design — the open-hole structure allows unobstructed airflow and is well suited for garment panels that require maximum ventilation, such as the underarm sections of training tops or the body of football jerseys. Bird's eye fabric offers a balance between structure and ventilation; its small, regular openings provide meaningful airflow while maintaining enough surface integrity for uniform printing and panel assembly. Pique knit, with its characteristic raised texture, sits at the moderate end of this comparison — the raised loops create a slightly more open surface than a plain jersey, but the knit base remains relatively compact.

Construction Variables That Influence Air Permeability

Beyond fabric type, several construction variables can shift air permeability readings — sometimes significantly. Understanding these variables helps buyers interpret supplier test reports and anticipate how a fabric may perform across a range of specifications.

GSM (grams per square metre)

Higher GSM typically corresponds to more material per unit area, which can reduce air permeability as pores become denser or yarn coverage increases. However, this relationship is not absolute — a heavyweight open-structure mesh may still outperform a lighter compact jersey. GSM should be considered alongside construction type, not in isolation.

Yarn count

Finer yarns tend to enable tighter knit construction, which may reduce air permeability. Coarser yarns, by contrast, often produce a more open structure that allows greater airflow. The specific effect depends on machine gauge, stitch density, and finishing — these variables interact, and no single factor determines the outcome predictably without sample testing.

Finishing treatments

Post-knitting treatments can materially change air permeability. Water-repellent (DWR) finishes, for example, tend to partially block fabric pores and can reduce measured air permeability even without altering the underlying knit structure. Anti-pilling treatments, peaching, and calendering may each affect the result to varying degrees. Buyers sourcing treated fabrics should request test reports conducted on the finished fabric, not the greige.

Spandex/elastane content and stretch state

When a knit fabric containing elastane is stretched, the pore structure may open and change the measured airflow. The effect depends on stretch direction, knit density, elastane ratio, and finishing. Most standard reports reflect the selected laboratory condition, so buyers should not treat an unstretched test result as a full prediction of in-wear breathability.

What to Ask Your Supplier Before Bulk

Fabric air permeability is a straightforward parameter to request at the sampling stage, but the value of the data depends on how the test was conducted and whether it reflects the production-intent specification. Before approving a sample for bulk, consider including the following checks in your evaluation:

• Request a formal test report: Ask the supplier for an ISO 9237 or ASTM D737 test report attached to the sample. Verbal claims of breathability without documentation are not verifiable.
• Specify the test standard relevant to your target market: ISO 9237 is standard in European and most Asian markets; ASTM D737 applies primarily to North American supply chains. Mismatched standards make supplier comparison difficult.
• Clarify the test conditions: Confirm whether the report reflects finished fabric (post-dyeing and post-finishing) and which side of the fabric was tested. Some two-face constructions show notably different values depending on face orientation.
• Align on acceptable tolerance: Air permeability can vary roll-to-roll even within the same production run, depending on yarn lot and machine settings. Discuss what variance range the supplier can reliably hold across bulk and ask for supporting data from previous production if available.
• Cross-reference with GSM and construction: If a supplier increases GSM or changes finishing between sampling and bulk — both of which can happen when managing yarn availability — ask whether air permeability has been re-tested on the revised construction.

If specific air permeability test data for Runtang's knitted mesh, bird's eye, or pique fabrics are required for your tech pack, please request samples and test reports directly.

Frequently Asked Questions

What ISO standard is used to test fabric air permeability?

ISO 9237:1995 is the main ISO standard used for fabric air permeability testing. It defines a method for measuring how air passes through permeable textile fabrics under laboratory conditions. ASTM D737 is another common method, especially in North American testing contexts. Buyers should confirm the exact method, sample condition, and reporting unit before comparing supplier reports.

What air permeability value is typical for sportswear knit fabric?

Air permeability varies considerably by fabric type and construction. Air permeability values vary too widely to define one reliable “typical” number for sportswear knits. An open knitted mesh will usually test higher than compact pique or jersey, but the final result depends on hole size, GSM, yarn count, finishing, and whether the fabric is tested after dyeing or coating. For bulk sourcing, buyers should request ISO 9237 data for the exact sampled fabric instead of comparing general benchmark ranges.

How does the knit fabric structure affect air permeability?

Knit structure affects air permeability primarily through pore size, pore density, and yarn coverage. Open-hole constructions such as knitted mesh create intentional air channels that allow significantly higher airflow. Bird's eye and pique fabrics introduce smaller, more controlled openings within a more structured knit base, resulting in moderate air permeability. Compact single or double jersey structures with high yarn coverage tend to produce the lowest readings within the knit category. Finishing treatments, GSM, and yarn count further modify these baseline characteristics.

Ready to Request Samples?

Runtang Textile Co., Ltd. supplies knitted mesh, bird's eye, and pique knit fabrics for sportswear and performance apparel brands across Europe, North America, and Australia. With over 300 brand clients and a production capacity of 20 million yards per year, we provide fabric samples with technical specifications to support your development process. Request a sample or get a quote →