Thermal Technologies –
From Fibre to Smart Textiles

The human body works best at temperatures around 36.5 – 37.5°C. However, the ambient temperatures differ from what is optimal for the human body and warmth migrates to colder areas. Two strategies are available to design warm clothing:

The R_ct value

The R_ct value defines the thermal resistance of a material. It is measured with the sweating  guarded hotplate (SGHP). The higher the R_ct value, the better the insulation properties. For example, to maintain the body temperature and prevent freezing, materials with higher R_ct values are necessary at temperatures around 10°C than at temperatures around 20°C.

R_ct = m² * K/W

The R_ct value can be converted to the clo value: 1 clo = R_ct/0.155

The Clo Value

The clo value defines the thermal performance of materials and/or clothing. A clo value of 1 means an average adult can maintain thermal equilibrium in an environment at 21 °C.

The thermal performance of the insulation increases with the clo value meaning the higher the clo value, the better the insulation. Fundamentally, it is possible to sum up the clo values of

various layers to a total clo value of the clothing system. Below numbers are estimated clo values to be considered for the specific end use.

The R_c value

Thermal insulation can be measured on a single material (R_ct), but also on a finished garment (R_c) considering ventilation within the clothing and, using movable manikins, the effect of the movements by the wearer on the insulation can be taken into account.

The determination of the R_ct value is less expensive because the testing is performed on unfabricated materials. For the R_c value, fabricated materials are required when using the manikins for measurements. Thus, the resulting values are more realistic.

The R_et value

The R_et is a material-specific value and describes to what extent moisture vapor is transmitted from the inside to the outside of the material while being worn. It is measured with the sweating guarded hotplate (SGHP), too.

The lower the R_et value, the better the breathability. The breathability is also important for warming and insulating materials to keep the micro-climate dry.

To see the chart in full size, please download the PDF-file

I. Fibre Cross Sections

An air cushion is a good layer of insulation to (better) retain warmth. When this cushion is found in the fibres (see example below), it is called a hollow fibre system. Hollow fibres can occur naturally or can be constructed using funnel shaped molds in the manufacture of man-made fibres, e.g., polyesters.

I.II Textured Yarns

To increase the insulating effect, air cushions can be generated within the yarn. The product is then called a textured yarn. Textured fibres are not only used in knitted and woven fabrics, but also in paddings/non-wovens. Thanks to the natural structure of wool, it is already composed of 85% air.

II. Mechanical constructions in and on the fabric

Greater warmth insulation can be achieved by brushing the fabric surface and/or the bonding of several coated layers. This method results in technically engineered air cushions or reflective heat barriers. Selected examples:

III. Chemical additives e.g., activated carbon particles

Various carbon compounds, mostly diverse activated carbon particles from raw materials that contain carbon, can also increase the thermal performance of a textile:

I. Phase Change Materials (PCM)

If a large amount of body heat is generated, PCMs absorb it and return the excess warmth as required by the body when the ambient temperature drops. The secret is in the microscopically small encapsulated materials such as paraffin, which become liquid or solidify within a defined temperature range. Heat is stored or released during changes in the aggregate state of the paraffin, which affects the skin‘s microclimate. A pleasant “side effect”: Sweating is significantly reduced thanks to active temperature control. PCMs are added to the synthetic fibres as micro particles during the spinning process and can be coated on the fabric.

II. Exothermic reaction in wool and (EKS) fibres

If merino wool absorbs moisture, it results in an exothermic (chemical) reaction: The water molecules collide with the polar molecule groups of the wool. The result is a release of energy in the form of absorption heat. The merino wool fibres can simultaneously absorb moisture while the fibre surface repels water. The effect is that wool can warm even when wet – unlike cotton, which becomes soaked with water. Since the human body perspires, even when not obviously sweating, these particularly sensitive EKS-fibres function without sweat. EKS-fibres are hydrophilic, so they absorb enough moisture for the reaction and work according to the same principle: The collision of water molecules with the polar molecular groups of the yarn releases heat.

III. Electronic Heating Systems: Smart Textiles

Active heat generation is becoming increasingly smart. Extremely thin conductive metallic threads or microscopically small carbon nanotubes are integrated into the carrier fabric. The metallic threads may run entirely or partially through the weave or knitted fabric; or, completely or partially as a kind of overlay weave. The metal alloys used, do not oxidize and can be combined with natural fibres. The fabric is linked to an external power source (e.g., a 5V powerbank via USB-connection). The heat supply can be regulated by means of either an operating device or an app. Various temperature settings or a desired end temperature ensure that just the right warmth is achieved. The wearer selects the heat level or additional sensors may assist to determine the actual state and adjust the thermal performance.

• It seems obvious that all insulation materials are based on volume/loft: In fibres, this can be achieved by hollow parts. In yarns and fabrics, it can be achieved by texturization or by brushing (see above).
• Certain additives in the fibres enlarge the surface and create more volume/loft that retains the warmth (see above).
• Non-wovens/paddings show the highest thermal weight efficiency in the chart (Side B), but PERFORMANCE DAYS did not include down feathers or test values in the review.

Rule of thumb for paddings

Different paddings and their values were visualized as the example below. The result: It looks like each product (all with the same composition) has a “sweet point” (see diagram below) for conserving warmth.

A heavy/thick fabric or padding can store the warmth better. But where is the tradeoff between weight and performance? To compare different weights calculate the thermal weight efficency which is = clo value : gram x 1000

The graphs above show: The clo value is not linear relative to weight. It reaches a saturation. The best clo/gram ratio does not come automatically along with the highest weight.
Possibly there is a maximum for storable air in textiles that is defined by material and volume/loft.

• Metals like aluminum or titanium are a barrier for IR (= infrared)-warmth. They reflect warmth back to the body (see above).
• By combining several layers, more warmth can be stored. Example: baselayer + padding + lightweight.

All of these passive technologies use the natural body warmth and depend on it. Most do not differentiate between what the body already has got and what is needed. They do not create warmth and they don’t release superfluous warmth. They just store warmth. To release warmth, the layer must be opened or taken away. This is the classical layering system.

PCMs can also store the warmth. However, they do not return it immediately, but at different times, when needed.

Other systems like EKS use exothermic processes: Water/sweat collides with the molecules of the fibre. The collision sets energy free in form of warmth.

Physical/chemical reactions can’t be actively controlled by the wearer. 

Smart Textiles are one step ahead. The electrical resistance, e.g., in copper threads emits energy as heat. By switching the system on and off the temperature can be controlled. Some systems can be programmed on a wellbeing temperature, so that the system will switch on and off automatically to keep the desired temperature. Weight remains an issue when it comes to the battery, but technology is getting better: lighter and more durable. However, Smart Technology stays more expensive than the older conventional passive systems.

• Clo and R_ct values (see above) are the most relevant values and are easy to compare.
• The clo / R_ct values compared were not tested under dry and wet conditions, after production, or after several washings for their durability.
• The clo / R_ct values determine only the warmth efficiency and do not take into consideration the wind pressure from outside, moisture-management, etc.
• Women tend to feel cold faster, not because they are women, but because genetically they have less muscle tissue (more muscles create more blood circulation. Circulation = energy = warmth).
• Generally, cold spots in a garment can be avoided by designing intelligent patterns and seams engineering. However, under some conditions it should be possible to lose heat to prevent overheating.