Why Polyester Knitted Fabric Develops Uneven Air Permeability Across the Roll: Stitch Density, Width Stretch & Finishing

A polyester knitted fabric can have the same:

• Colour.

• Yarn.

• Nominal GSM.

• Composition.

• Fabric name.

yet allow noticeably different amounts of air to pass through different parts of the same roll.

The centre may feel more breathable.

The edges may feel tighter.

Or the opposite may happen.

This is called uneven air permeability across the fabric width.

Air permeability is strongly controlled by the open spaces within the knitted structure.

Small changes in:

• Stitch density.

• Loop length.

• Width stretching.

• Fabric tension.

• Heat setting.

• Chemical finishing.

can alter those openings.

That means a fabric does not need a visible hole or obvious defect to show different breathability from one side to another.

This matters especially for:

• Sportswear.

• Activewear.

• Mesh jerseys.

• Running T-shirts.

• Training wear.

• Lightweight uniforms.

This article explains why polyester knitted fabric can have uneven air permeability across its width, how knitting and finishing create the variation, how it should be tested, and what buyers should check before bulk approval.

1. What Is Air Permeability?

Air permeability describes how easily air passes through a textile under a defined pressure difference.

In simple terms, it indicates how open the fabric structure is to airflow.

Higher air permeability generally means:

• Air passes more easily.

Lower air permeability means:

• The structure resists airflow more strongly.

It should not be confused directly with moisture-wicking or thermal comfort.

2. Why Is Air Permeability Important?

Airflow can influence how a garment feels during wear.

For sportswear, good airflow may help with:

• Ventilation.

• Heat dissipation.

• Perceived comfort.

For some technical products, however, very high permeability may be undesirable.

The correct value depends on end use.

3. What Controls Air Permeability in Knitted Polyester?

Important factors include:

• Yarn denier.

• Filament count.

• Loop length.

• Stitch density.

• Fabric thickness.

• Knit construction.

• Finished width.

• GSM.

• Finishing chemicals.

The open spaces between yarns are particularly important.

4. Why Can Air Permeability Vary Across One Roll?

The left edge, centre and right edge may not have exactly identical structure.

Possible reasons include:

• Uneven knitting tension.

• Uneven course or wale density.

• Different finishing stretch.

• Edge over-tension.

• Uneven heat setting.

Even small structural differences can change airflow.

5. What Is Stitch Density?

Stitch density describes the number of knitted loops present within a defined fabric area.

It is influenced by:

• Courses per unit length.

• Wales per unit width.

• Loop length.

Higher stitch density generally reduces open space.

This can lower air permeability.

6. Does Higher Stitch Density Always Mean Lower Airflow?

Often, yes, if other variables remain comparable.

More loops packed into the same area reduce pore size.

But the relationship is not perfectly simple because:

• Yarn thickness.

• Loop shape.

• Fabric thickness.

also matter.

7. How Does Loop Length Affect Air Permeability?

Longer loops generally create a more open knit.

This can increase:

• Pore size.

• Airflow.

Shorter loops generally create a more compact structure.

But very loose fabric may also become unstable.

8. Can the Edge Have Different Loop Geometry From the Centre?

Yes.

Edge zones can experience different mechanical conditions during:

• Knitting.

• Dyeing.

• Stentering.

This can alter loop shape and density.

The result may be different airflow near the selvage or edge region.

9. How Does Width Stretch Affect Air Permeability?

When knitted fabric is stretched wider during finishing, its loops spread apart.

This can increase the size of gaps between yarns.

As a result, air permeability may rise.

If the fabric is not stretched uniformly, permeability may vary across the width.

10. Can Over-Stentering Increase Air Permeability?

Yes.

Excessive width stretching can open the knitted structure.

Possible consequences include:

• Larger pores.

• Lower GSM in some cases.

• Higher air permeability.

This can also reduce bursting strength or alter stretch.

11. Can Under-Stentering Reduce Air Permeability?

Potentially.

A more relaxed, compact structure may have:

• Smaller openings.

• Higher local density.

This can reduce airflow.

12. Why Can Edge Tension Differ on a Stenter?

Fabric edges are controlled by:

• Pins.

• Clips.

• Chains.

These regions experience different mechanical restraint compared with the centre.

If tension is not balanced, edge structure may change.

13. Can Heat Setting Affect Air Permeability?

Yes.

Heat setting stabilizes the fabric at a particular geometry.

If the fabric is set while highly stretched, the open structure may become permanently stabilized.

If it is set more relaxed, the knit may remain denser.

14. Does Temperature Alone Control Permeability?

No.

Heat-setting temperature matters, but the important combination is:

temperature + width + overfeed + tension.

A high temperature on relaxed fabric does not produce the same result as the same temperature on heavily stretched fabric.

15. How Does GSM Relate to Air Permeability?

Higher GSM often corresponds to more material per area.

This can reduce airflow.

But GSM alone is not enough to predict permeability.

Two 180 GSM fabrics may have very different:

• Loop structures.

• Thickness.

• Mesh openness.

16. Can Same-GSM Fabrics Have Different Air Permeability?

Absolutely.

For example:

Fabric A

• 180 GSM.

• Compact single jersey.

Fabric B

• 180 GSM.

• Open mesh.

Their airflow can be dramatically different.

GSM measures mass, not pore geometry.

17. Can GSM Vary Across the Width and Affect Airflow?

Yes.

If one side of the roll has lower local GSM because it has been stretched more, it may also have:

• Larger openings.

• Higher air permeability.

However, GSM and permeability should both be measured rather than inferred.

18. How Does Yarn Denier Affect Airflow?

Coarser yarn occupies more physical space.

At the same stitch density, thicker yarn may reduce pore area.

Finer yarn may create more open space.

However, the knitting machine can compensate through structure.

19. Does Filament Count Matter?

Yes.

At equal denier, higher filament count creates finer individual filaments.

This can influence:

• Yarn bulk.

• Compressibility.

• Surface area.

These may affect fabric thickness and airflow indirectly.

20. How Does DTY Affect Air Permeability?

DTY is bulky and textured.

Its crimp fills more space inside the fabric.

A highly bulky DTY construction may reduce airflow compared with a smoother yarn arrangement.

However, knit design remains decisive.

21. How Does FDY Differ?

FDY is smoother and generally less bulky.

Depending on construction, this may produce:

• Thinner fabric.

• Different pore geometry.

But FDY does not automatically mean higher airflow.

22. Why Is Mesh Fabric Highly Sensitive?

Mesh intentionally contains larger openings.

A small change in:

• Stretch.

• Loop length.

• Finished width.

can noticeably alter those openings.

Therefore, mesh permeability can change significantly across batches or widths if finishing is not consistent.

23. Can Piqué Show Permeability Variation?

Yes.

Piqué contains a textured knit pattern.

Raised and recessed sections create non-uniform local porosity.

If finishing tension changes across the roll, the effect can become more pronounced.

24. Can Polyester-Lycra Show Uneven Airflow?

Yes.

Elastane changes loop recovery.

If stretch or heat setting varies across the width, one region may recover more strongly than another.

This changes:

• Pore size.

• Density.

• Air permeability.

25. Can Softener Affect Air Permeability?

Potentially.

Most normal softener applications do not physically close pores dramatically.

But heavy finishing can alter:

• Yarn packing.

• Surface film formation.

• Fabric compression.

Certain coatings or resin systems can reduce airflow significantly.

26. Can Silicone Reduce Airflow?

Usually, a normal silicone softener has a relatively small direct effect compared with structural variables.

But excessive application or film formation can alter fabric behaviour.

It should not automatically be assumed that silicone always reduces permeability.

27. Can Resin Finishing Reduce Permeability?

Yes.

A resin or binder can partially occupy spaces between fibres and yarns.

Heavy add-on may reduce:

• Pore size.

• Airflow.

This effect is especially important in coated or functional fabrics.

28. Can Uneven Chemical Pickup Cause Widthwise Variation?

Yes.

If one region receives more finish than another, its permeability may change.

Possible causes include:

• Uneven padding pressure.

• Chemical migration.

• Drying variation.

This is less common than structural variation but still possible.

29. Can Fabric Thickness Affect Airflow?

Yes.

Air has to travel through the full fabric thickness.

A thicker or more tortuous pathway generally creates more resistance.

That is why two fabrics with similar surface openness can still have different air permeability.

30. How Is Air Permeability Tested?

A typical test involves:

1. Clamping a defined fabric area.

2. Applying a specified pressure difference.

3. Measuring the volume or rate of air passing through.

Results are only meaningful when the test method and pressure conditions are stated.

31. Which Standards Are Commonly Used?

Commonly referenced methods include:

• ISO 9237 for textile fabric air permeability.

• ASTM D737 for textile fabric air permeability.

The buyer and supplier should agree on one method.

Results from different methods or pressure settings should not be compared blindly.

32. Why Should Multiple Width Positions Be Tested?

Testing only the centre may miss edge variation.

For quality-critical fabrics, samples may be taken from:

• Left side.

• Centre.

• Right side.

This helps determine whether airflow is uniform across the roll.

33. How Can Mills Diagnose Uneven Permeability?

Check:

1. Finished width.

2. GSM across width.

3. Stitch density.

4. Loop length.

5. Stenter tension.

6. Overfeed.

7. Heat-setting conditions.

8. Finishing pickup.

Comparing these measurements can reveal the likely cause.

34. Can Uneven Air Permeability Be Visible?

Sometimes.

If one area has visibly larger pores, the difference may be obvious.

But many permeability differences are too subtle to detect visually.

Instrument testing is more reliable.

35. How Can Mills Reduce Widthwise Variation?

Key controls include:

• Maintain uniform knitting tension.

• Control loop length.

• Avoid excessive width stretching.

• Balance stenter chains.

• Maintain uniform overfeed.

• Control heat-setting geometry.

• Monitor GSM across width.

• Test air permeability at several positions.

36. Common Misconceptions About Air Permeability

Misconception 1: Higher GSM Always Means Lower Air Permeability

False.

Construction matters greatly.

Misconception 2: Same GSM Means Same Breathability

False.

Pore geometry may be completely different.

Misconception 3: Mesh Always Has Uniform Airflow

False.

Mesh openings can vary with stretch and finishing.

Misconception 4: Air Permeability and Moisture Wicking Are the Same

False.

One measures airflow; the other describes liquid transport.

Misconception 5: Only Knitting Controls Airflow

False.

Finishing can substantially change structure.

37. Frequently Asked Questions

Why does one side of polyester fabric feel more breathable?

The local stitch density or finished stretch may be different.

Can stenter width affect air permeability?

Yes.

Can same-GSM polyester fabrics have different airflow?

Yes.

Does DTY affect airflow?

Yes, through bulk and structure.

Can finishing chemicals reduce permeability?

Heavy resin or coating systems can.

Should air permeability be tested at multiple positions?

Yes, when widthwise consistency matters.

Is high air permeability always better?

No. It depends on the garment application.

38. What Should Bulk Buyers Ask?

For breathable sportswear fabrics, ask:

1. What is the air-permeability result?

2. Which test method was used?

3. Was the fabric tested after final finishing?

4. Were multiple width positions checked?

5. Does the finished DIA match the approved sample?

6. Is GSM uniform across the width?

39. Why Is This Important for Sportswear?

A sports garment should perform consistently across all panels.

If one part of the fabric is much more open than another, garment panels can differ in:

• Airflow.

• Feel.

• Transparency.

• Stretch.

This can affect comfort and product consistency.

40. Final Takeaway

Air permeability is fundamentally controlled by the geometry of the fabric's open spaces.

For polyester knits, those spaces are influenced by:

loop length + stitch density + yarn bulk + finished width + stenter stretch + finishing.

That means even one roll can have different airflow at the edge and centre if its structure is not uniform.

For knitting mills, consistent loop formation is essential.

For finishing mills, width and tension control are equally important.

For buyers, the key lesson is:

Breathability cannot be judged from GSM alone—and one centre sample does not always represent the entire fabric width.

At Walrus – Yes India Can, we supply polyester knitted fabrics for sportswear, T-shirts, polos, uniforms, activewear and garment manufacturing.

Website: https://www.yesindiacan.in/

Contact: +91 90420 77722