Every knitted garment begins as a decision about fibre, and that decision shapes almost everything that follows — how the piece feels, how warm it is, how it handles moisture, how long it lasts and how it should be cleaned. Natural and synthetic fibres each bring real strengths, and the best answer for a given garment often involves a considered blend rather than a purist stance either way. What matters is understanding the trade-offs well enough to read a composition label with a clear eye.
What natural fibres do well
Natural fibres — wool and its fine forms like merino, cashmere and other animal fibres, plus plant fibres such as cotton and linen — earn their reputation through behaviour. Wool and cashmere trap air to insulate efficiently at low weight, manage moisture by absorbing it without feeling wet, and resist odour, so they can be worn several times between washes. Merino regulates temperature across a surprising range. Cotton and linen breathe well and take repeated washing, making them the natural choice for warm weather. Crucially, good natural fibres also tend to age gracefully, softening and developing character rather than degrading.
The trade-offs are honest ones: natural fibres usually cost more, ask for more careful cleaning, and — in the case of some wools — can feel scratchy if the fibre is coarse. They are covered in depth in our comparison of cashmere, merino and cotton.
What synthetic fibres do well
Synthetic fibres — acrylic, polyester, nylon (polyamide), elastane and others — are engineered, and that lets them add specific properties. Acrylic can mimic the warmth and softness of wool at lower cost and is easy to machine-wash. Nylon adds abrasion resistance and strength, which is why a small percentage often appears in socks, cuffs and high-wear areas. Elastane adds stretch and recovery, helping close-fitting knits keep their shape. Synthetics are also typically cheaper, quick-drying and low-maintenance.
Their weaknesses are equally real: they breathe less well, can trap odour, are more prone to a "warm and clammy" feel, and — a growing concern — shed microplastics in the wash. A high proportion of cheap synthetic is also often a sign that a garment has been built to a price rather than to last.
Where blends genuinely help
Blends are where the argument stops being either/or. A well-judged blend uses each fibre for what it does best: a touch of nylon to strengthen a fine cashmere so it pills less and lasts longer; a little elastane in a ribbed cuff for recovery; cotton blended with a cellulosic fibre for drape. The presence of a synthetic is not automatically a red flag — the question is whether it is there to improve the garment or simply to cheapen it. A 95% cashmere / 5% nylon knit is usually a considered choice; a 20% wool / 80% acrylic knit sold as a "wool sweater" is usually a cost decision.
A third category sits slightly apart: regenerated cellulosic fibres such as viscose, modal and lyocell. Made from plant sources but chemically processed, they are technically neither fully natural nor conventionally synthetic. They add fluid drape and a cool hand, which makes them useful in summer knitwear, though quality and environmental credentials vary by production method.
How to read a composition label
Check the percentages, not just the headline fibre. Note the named fibres and their proportions, consider whether each addition improves or cheapens the piece, and match the result to how you'll wear and wash it. When a label is vague or a "luxury" price sits on a mostly-synthetic composition, be sceptical.
The sustainability picture
Neither category is automatically greener. Natural fibres are renewable and biodegradable, but farming — particularly of animal fibres — carries land, water and welfare considerations, which is why certifications such as the Responsible Wool Standard exist. Synthetics are petroleum-derived and shed microplastics, but can carry a lower per-garment footprint in some measures and are increasingly available in recycled forms. The most meaningful sustainability lever is usually longevity: a well-made garment you keep for a decade beats a disposable one in almost any fibre. We discuss this in the sustainable knitwear guide, and note that some makers — Frett among them — lean toward natural, and where possible renewable and biodegradable, fibres alongside certified materials.
A practical rule
Buy the fibre that suits the job, judge the whole garment rather than the fibre alone, and treat a synthetic as a tool rather than a sin. For versatile everyday layers, natural fibres or considered blends usually reward you with comfort and longevity. For high-wear or performance-specific pieces, a smart synthetic content earns its place. Either way, the construction and finishing signals in how to identify quality knitwear matter at least as much as the fibre on the label.
Related reading