The thermal insulation provided by a fabric bears little relationship to the thermal insulation of the fibres within the fabric. This is due to the fact that the thermal insulation of textile fibres is actually much poorer than air. Indeed, the thermal conductivity of air is ten times lower than most textile fibres. (Fig 1)
Consequently, the thermal insulation of a fabric is provided not by the fibres themselves but by the air between the fibres that compose the fabric. Since the merino fibre is so much finer than normal wool and most other textiles, for a given weight, merino contains more air spaces, and hence provides greater insulation. (Fig 4)
In addition, unlike smooth synthetic fibres, the scales on the surface of the merino fibres help keep the fibres apart. This increases the surface area available to resist the passage of air through the fabric, and so helps explain its insulation capacity. (Fig 6)
Furthermore, studies have shown that the insulation provided by a fabric bears a direct relationship to the thickness of the fabric. (Fig 7)
If the surface of the fabric can be brushed up to increase its effective thickness, the thermal insulation is proportionately increased. The crimp of its fibres means the surface of a merino fabric can be readily brushed up to produce a significant increase in effective thickness with no increase in weight. (Fig 8)
The perception of merino as an effective insulator is therefore explained not by the thermal insulation of the fibre itself but by the way in which the fineness and scales of merino fibres combine to trap more of the air upon which all textiles rely for their insulation.
There are other ways in which merino provides warmth.
In 1858 Coulier was the first to observe that when dry wool was moved to a moist room, it produced a rise in temperature. This became known as the “heat of sorption”. This is explained by the fact when moisture vapour is absorbed into merino’s internal structure, it transforms from gas to liquid and the phase change produces the rise in temperature. (Fig 6)
And, the rise is significant. In fact, the heat of sorption from a kg of merino can be equivalent to the output from an electric blanket over eight hours. Merino produces significant heat of sorption because it has an amazing capacity to absorb moisture – up to 35% of its dry weight.
Synthetic fibres, on the other hand, produce negligible heat of sorption because they don’t have the same ability to absorb water vapour as merino. (Fig 7)
Merino has a further advantage over most synthetics in that it feels warm to the touch. Sensors in the tips of our fingers are able to detect small changes in temperature. Consequently, when we touch a smooth object with a high thermal conductivity like metal, heat flows readily from our body to the cooler object. When this happens, our fingers sense a drop in temperature and the object feels “cold”.
Conversely, if we touch a fabric with a bulkier surface that is largely composed of air - a poor thermal conductor - the heat transfer is much less, our fingers sense little change in temperature, and the object feels “warm”. This explains why merino, which at the microscopic level is bulky at the surface, “feels” warmer to the touch than a smooth synthetic, and why merino is much preferred when it comes to garments to be worn next to the skin in winter. (Fig 8)
A further perception that enhances merino’s reputation for warmth is that it feels warm even when wet. This can be explained in a number of ways.
Firstly, just as a merino fibre can absorb up to 35% of its weight in water, a merino garment can absorb up to 60% of its own weight before it feels wet to the touch. This is partly due to merino’s ability to bond moisture within the internal structure of its fibres, and partly due to the bulky surface nature of merino, which isolates the wetness from our touch.
It has also been shown that water evaporated from the inner side of a wet fabric is recondensed near the outer side. If the fabric is strongly wicking, like most synthetics, the water is drawn back more quickly to a position nearer the body where it must be re-evaporated. This requires more heat to be drawn from the body which results in a drop in temperature.
Untreated merino on the other hand is poorly wicking, and this cycle of evaporation, recondensation and re-evaporation doesn’t occur to the same extent, saving the body’s energy, and making us feel warmer.
The fineness and scales of merino fibres and its amazing capacity to absorb moisture combine to produce fabrics which offer superior insulation and higher levels of heat of sorption than synthetics, as well as feeling warm to the touch, even when wet.
Figures and tables are courtesy of CSIRO unless otherwise stated.