How Alpaca Holds Warmth — and Why Its Fibre Structure Feels Different from Cashmere
May 21, 2026 — Mercer St.
Alpaca and cashmere are both warm relative to their weight. They are also both soft, both used in fine knitwear, and both priced above commercial wool. But they insulate through different mechanisms, and they feel different against the skin for reasons that go beyond fibre diameter.
This note explains how alpaca holds warmth, how that differs from cashmere, and what the practical differences are for a garment you intend to wear.
In this note
- What alpaca is — the fibre, the animal, the grades
- How alpaca holds warmth — the role of medullation
- How this differs from cashmere’s insulation mechanism
- What the practical differences are — for warmth, weight, feel, and care
- What alpaca labels tell you — and do not
What alpaca is
Alpaca fibre comes from the alpaca (Vicugna pacos), a camelid native to the Andean highlands of South America. Two breeds are used commercially: the Huacaya, which produces a dense, crimped fleece, and the Suri, which produces long, silky, straight fibre.
Huacaya alpaca is the more widely produced of the two and is used in most commercial alpaca knitwear. Suri alpaca is rarer, has a distinctive drape, and is less common in everyday garments.
Alpaca is graded primarily by fibre diameter, measured in microns (μm):
- Royal alpaca: below approximately 19μm
- Baby alpaca: approximately 19–22μm (despite the name, this is not necessarily from young animals — it refers to the first shearing in some cases, or to diameter grade alone in others)
- Fine alpaca: approximately 22–26μm
- Medium alpaca: approximately 26–29μm
Mercer Note
"Baby alpaca" is a grade name, not a guarantee that the fibre comes from young animals. The term is used inconsistently across the market. Fibre diameter in microns is the more reliable indicator. [VERIFY: confirm alpaca grade and sourcing with supplier before publishing product-specific claims]
How alpaca holds warmth — medullation
Alpaca fibre has a structure that differs from cashmere and wool at the microscopic level. Many alpaca fibres contain a medulla — an air-filled channel running through the centre of the fibre. This medullation is present to varying degrees depending on the grade and the individual fibre.
The air trapped within the medullated fibres contributes to the insulating character of alpaca. Insulation works by trapping air and reducing heat transfer; a fibre that contains air within its own structure adds to this effect beyond the air trapped between fibres in the yarn and fabric.
It is important not to overstate this: medullation is one factor among several, and its contribution to warmth in a finished garment depends on the proportion of medullated fibres, their diameter, the yarn construction, and the knit structure. Alpaca is not categorically warmer than cashmere. It is warm relative to its weight — which is what makes it useful for lightweight knitwear.
How cashmere insulates differently
Cashmere insulates primarily through extreme fineness. A cashmere fibre at 14–15 microns creates a very dense network of fine fibres in the yarn and fabric, trapping a large amount of air in the spaces between them. It does not rely on medullation — most cashmere fibres do not have a significant medullated core.
The practical result is that both materials are warm relative to their weight, but through different mechanisms. Cashmere’s warmth comes primarily from fibre fineness and yarn density. Alpaca’s warmth comes from a combination of fibre structure (medullation) and the light, airy yarn construction that fine alpaca enables.
Cashmere insulates through fineness. Alpaca insulates partly through air held inside the fibre itself. Neither is simply warmer than the other.
Practical differences in a garment
| Property | Fine alpaca | Fine cashmere |
|---|---|---|
| Warmth to weight | High | High |
| Softness | Soft (slightly different hand from cashmere) | Very soft (smoother, closer) |
| Surface character | Halo — slight fuzz; warm texture | Smooth, quiet drape; less halo |
| Pilling | Less prone than cashmere (longer fibres in quality grades) | More prone (depends on grade and construction) |
| Care | Hand wash cold or dry clean | Hand wash cold or dry clean |
| Weight | Light to medium | Light |
What alpaca labels tell you
"100% alpaca" confirms fibre content but says nothing about grade, diameter, origin, or yarn construction. "Baby alpaca" is a grade name that is used inconsistently. The most reliable way to evaluate alpaca — as with cashmere — is to handle the fabric. Fine alpaca has a characteristic hand: warm, soft, with a slight halo and a lightness that is recognisable once you have felt it.
What to look for
- Is the alpaca grade stated? If so, is it by diameter in microns or by marketing grade name?
- Is the origin noted?
- Is it Huacaya or Suri?
- Is the yarn construction and gauge described?
What this means for Mercer St.
Alpaca selection at Mercer St. is based on physical sampling and fibre specification. Specific grade and production details are confirmed with the supplier and noted on the product page. [VERIFY: confirm alpaca grade, source, and specifications with supplier before publishing product-specific claims]