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Choosing the right Thermal Fibres

I thought it would be useful to give you a brief description of the main fibres used in my thermal underwear - as they have very different properties.  You can find a full description of the fabrics (ie the mix of fibres) used in my thermal underwear collection by clicking here.



Wool is a natural fibre which is ideal for thermal garments.  It is generally characterised by:
 - excellent insulation properties,
 - ability to "wick" moisture away from the skin - keeping you dry even if you perspire
 - maintaining its thermal properties even when wet - vital if you are exercising
 - keeps you warm when you are outside
 - keeps you cool when you are inside

There are many different types of wool, but the most commonly used ones in thermal underwear are:  Angora Wool (from the Angora rabbit !), Merino Wool (from the Merino Sheep) and sheep's wool.  Sheep's wool with coarse fibres is often characterised by "its itchy" feel.  This is not present in Angora or Merino wool, and is much reduced in sheep's wool made up of finer fibers. 

1.  Angora Wool.   Angora wool is a soft luxurious fibre.  The fibres are hollow, and it is one of the warmest natural fibres you can get - up to seven times warmer than sheep's wool - making ideal for use in thermal underwear.  Angora wool is the lightest natural fibre known and can only be produced by the Angora Rabbit (Angora Goats produce Mohair). The rabbits are not harmed for their wool, but are sheared regularly. The hollow structure of the angora fibre makes it an excellent insulator and it is incredibly fine and soft, measuring between 10-13 microns (finer than most wools).  Angora is often blended with fine wool to increase warmth and enhance softness. Similar to Mohair, Angora is prized for the soft "halo" effect it gives to the knitwear.  As it is so warm, and has a natural cushion it has beneficial therapeutic effects for back problems, Arthritis, Carpal Tunnel Syndrome or any other pain affecting your joints.  It is, however, a little more expensive than other fibres.

You can find out more about Angora Wool here

2.  Merino Wool.  Merino Wool is produced from the white fleeced Merino Sheep, and has become one of the most popular materials for thermal underwear, due to the balance of cost and performance.  It has a much finer fibre than sheep's wool, making it more comfortable (less itchy) to wear.  With many more fibres contained in the fabric it traps more tiny air pockets and locks in body heat. Merino wool is highly breathable and it wicks moisture away from your body, releasing it into the air. This helps to regulate your temperature keeping you warm in cold weather and cool in hot.  It is soft, with a cashmere like feel (in fact it blends very well with cashmere).

3.  Sheep's Wool.  Sheep's wool also has great thermal properties, "wicks" moisture away from the skin, and maintains its thermal insulating properties even when wet.  Coarse fibres have given sheep's wool a bit of a bad name in underwear because of the "itchy" feel, but using finer fibres completely removes this problem.  Sheep's wool is three times warmer than cotton.


Cotton is a very popular, low cost and comfortable fibre - this makes it common in all types of clothing. However, from a thermal perspective it is a third less insulating than sheep's wool.  When wet the thermal insulating properties reduce even further.  It also does not "wick" moisture away from the skin.  This means that if you are perspiring wearing cotton undergarments, the material will stay wet, leaving the skin feeling uncomfortable, with little thermal insulation.

As a result, 100% cotton materials tend to be for summer use - where keeping warm is not as important.  However, it is common to use as a blend with other fibres, as it can provide additional comfort and reduce the overall cost of the blended material.


Silk is a great natural insulator, but is one of the more natural expensive fibres.  It is light, soft, and smooth to wear - not slippery like many synthetic alternative.  It is often used to provide a luxurious feel when mixed with other fibres.


Polyamide is generally used to reinforce natural fibres, for greater durability.  It also improves the elasticity and pliability of the garments.


The unbranded term for Lycra.  This is a rubber like synthetic fibre.  It is elastic, but very resilient and helps garments mold to the body shape.  It is often used in low percentages to improve the general elasticity of a garment.


Acrylic is lightweight, soft and warm - with a wool like feel.  The fibre is relatively cheap, hard wearing, and machine washable - making it a popular mix with more delicate fibers.  It is often used as a cheaper alternative to cashmere in materials owing to its soft feel.  It has good thermal properties - but is not as warming as wool.