You could fill a library with reports and books describing the importance of energy-efficient heating systems and home insulation. However, not a word has been said or written about the energy savings potential of clothing, even though there has been a lot of progress in this area too. Modern thermal underclothing offers the possibility to turn the thermostat much lower without sacrificing comfort or sex appeal. The potential energy savings are huge; the costs are almost nil.

Photo credit: Woolpower.

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This article has been translated into French and Spanish.

Over the last decades, both the insulation of homes and the energy efficiency of heating appliances have improved considerably. These efforts have led to substantial energy savings in fuel consumption. In spite of population growth, higher comfort levels, and a trend towards building larger homes, total energy consumption for space heating by American households came down from 5.32 quadrillion Btu in 1993 to 4.30 quadrillion Btu in 2005 (source). Similar trends can be seen in other industrialized countries.

Nevertheless, space heating still consumes a huge amount of energy, which comes almost exclusively from fossil fuels. Moreover, these figures do not take into account the energy that was spent to demolish old buildings and build new, more energy-efficient homes. Research (source - pdf) indicates that it can take 35 to 50 years before this embodied energy is recovered. This means that if a new, efficient building does not last that long, the result is more energy consumption, not less - though it will show up otherwise in statistics.

Further improvements in energy-efficient buildings and heating systems can be expected, but apart from the embodied energy required to make the housing stock more efficient, there is an additional problem that prevents a fast and steep reduction in energy consumption: cost. Home insulation and energy-efficient heating appliances are expensive, which means that many people simply cannot afford the investment. There is also the problem of "split incentives": the owner of a rented house has no incentive to improve efficiency if the tenant is paying the heating bills.

Room temperature

There is another way to reduce energy consumption for space heating that does not have any of these disadvantages: lowering the thermostat and putting on more clothes. Although room temperature is hardly ever mentioned as a factor in energy use, it is a decisive factor in the energy consumption of heating systems - just as the speed of an automobile is a decisive factor in the energy use of an engine. Precisely how much energy can be saved by lowering the thermostat depends on the outdoor temperature. In temperate climates, lowering the thermostat just 1° C (or about 2°F) yields an energy savings of about 9 to 10 percent (sources: 1 & 2 - p20, pdf).



As far as I was able to find out, nobody has published a research report on the evolution of the average room temperature in winter throughout recent history. Today, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends an indoor winter temperature between 21 and 23 degrees Celsius (70 to 73.5°F). A Dutch report (.pdf, in dutch) mentions a rise in average winter indoor temperature from 20° C in 1984 to 21° C in 1992. David MacKay mentions an average room temperature of 13° Celsius (55°F) in the UK in 1970.

While these fragmentary data are far from sufficient to prove a rise in indoor temperatures, we can also count on the experience of many people who are old enough to remember that watching television in a t-shirt during winter is a relatively recent phenomenon. There seems to be no doubt that our comfort level has gradually risen because of better heating systems.

Note that the reduction in energy use for space heating thanks to more efficient homes was less than 20 percent from 1993 to 2005. Lowering the thermostat by 2° C (or 4°F) would thus result in energy reduction comparable to that. Turning down the thermostat from 22° to 18° C would initiate an energy savings of at least 35 percent. And, as we shall see, significantly lower indoor temperatures are perfectly possible without sacrificing comfort.

The body as a heating system

When discussing space heating, we overlook the fact that our own bodies are heating appliances too. The human body's core temperature is 37° C (98.6°F), and much of the skin's temperature is around 33° to 34° C (92°F). Because the temperature of the environment is often lower than that, our body constantly emits heat into the atmosphere. A small percentage of this heat is lost through breathing, but the largest part of heat loss occurs via the skin. To limit this heat transfer from the skin to the environment, most mammals (and birds) are covered with hair (or feathers). Humans do not have this protection and this is why we have resorted to clothing ever since we left our origins in Africa (where it was hot enough to survive without additional layers of clothing).

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Insulation of the body is much more energy efficient than insulation of the space in which this body finds itself.

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Clothing does not produce heating by itself - it only prevents body heat from escaping into the environment. This happens by warming the layer of air between skin and clothing. Air is a relatively bad conductor of heat and therefore a good insulator. Exactly the same technique is applied when we insulate a home. The only difference is that in the case of a building we can use more rigid and bulky materials because a building does not have to move or feel comfortable. Naturally, insulation of the body is much more energy efficient than insulation of the space in which this body finds itself. Insulating the body only requires a small layer of air to be heated, while a heating system has to warm all the air in a room to achieve the same result.

Thermal properties of clothing: the "clo" unit



The insulating properties of clothing can be expressed in "clo"-units, where one "clo" equals the thermal insulation required to keep a resting person (for instance, a couch potato) indefinitely comfortable at a temperature of 21° Celsius (70° Fahrenheit). The "clo", which is of course derived from the word "clothes", is not a standard international unit (the standard international unit of thermal resistance is m²K/W, where 1 clo corresponds to 0.155 m²K/W), but it has the advantage of being easily understood: one "clo" is equal to a man dressed in a three-piece business suit (shirt, trousers, suit jacket) and light underclothes.

Burton, who defined the clo-unit, wrote in 1946:

"We found that we could explain even to a General or Admiral, without a course in physics for which he had neither the time nor patience, that his uniform had about one clo-unit of thermal insulation, his greatcoat another one clo-unit, and that together they provided him with a total of two clo-units."

In Europe, a similar value was developed called the "tog" (British slang for clothes), which equals 0.645 clo. Both values can be compared to the R-value of building insulation materials, where 1 clo equals 0.88 R (or 1 R-value equals 1.137 clo). The clo is more commonly used than the tog, so we will stick to the American unit here. Clo-values are calculated by means of a thermal manikin.

Maintaining thermal comfort

The clo is an interesting unit because it allows us to precisely calculate which clothes we have to wear to feel comfortable at any given indoor temperature. According to the "Encyclopedia of occupational health and safety", the required clo-value to maintain a neutral thermal sensation rises to about 2.7 at an indoor temperature of 10° Celsius (50°F). When the indoor temperature drops to 0° C (32°F), the required thermal insulation rises to 4 clo. As a rule of thumb, each change of 0.18 clo units compensates for a 1° C change in air temperature (according to the American Society of Heating, Refrigeration and Air-Conditioning Engineers - ASHREA).

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One "clo" equals the thermal insulation required to keep a couch potato wearing a typical business suit indefinitely comfortable at a temperature of 21° Celsius (70° Fahrenheit).

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Alternatively, we can calculate the clo-value of any given piece of clothing and of any given clothing ensemble. The ASHREA, the ISO and some other research teams have compiled overviews that list hundreds of individual clothing pieces with their corresponding clo-values (see sources). A t-shirt with short sleeves has a value of about 0.10 clo, while a sleeveless undershirt offers about 0.06 clo. Knickers add about 0.20 clo. A short-sleeved shirt has a clo-value of about 0.15 to 0.25, while a long-sleeved shirt offers about 0.20 to 0.30 clo. Long-sleeved sweaters offer 0.20 to 0.40 clo, trousers offer 0.25 to 0.35 clo, and a long skirt or robe 0.22 to 0.77 clo. Briefs add a thermal insulation of 0.05 clo, while socks add 0.04 to 0.10 clo. Long underwear offers 0.20 to 0.35 clo for the upper as well as the lower part. All these values can simply be added to calculate the total clo-value of a clothing ensemble. An alternative method is to measure the thickness of a clothing ensemble: every layer of 2 centimetres results in an approximate 1.6 clo-value.

Saving energy costs



From these data, it can easily be demonstrated how even slight changes in clothing insulation can have a profound impact on heating costs and energy use. A person wearing briefs (0.05 clo), light socks (0.05 clo), a t-shirt (0.10 clo), a heavy shirt with long sleeves (0.25 clo), a sweater (0.30 clo) and long pants (0.30 clo) is protected by a total thermal insulation of 1 clo, meaning that this person will remain comfortable hanging out in front of the television at a temperature of 21° Celcius (70°F).

However, without the heavy shirt and sweater, this value drops to 0.55 clo. This means that watching television wearing just a t-shirt requires an air temperature of 24° Celcius (75°F) in order to maintain thermal comfort. This would lead to a rise in energy consumption of 20 to 30 percent.

On the other hand, if this person combines his original ensemble (including heavy shirt and sweater) with a full set of long underwear, the clo-value rises up to 1.7, allowing the temperature to drop to about 17° C (63°F), saving 30 to 40 percent on heating costs and energy use compared to the normal winter outfit, and saving 50 to 70 percent on heating costs and energy use compared to the outfit with only a t-shirt on the upper body.

How many articles of clothing can you wear?



When we are talking about common clothing, raising the clo-value of an ensemble basically comes down to adding more weight in clothes. A general rule of thumb is that the clo-value equals 0.15 times the clothing weight in pounds. Wearing 6.6 pounds (3 kg) of garments thus equals 1 clo. The relationship between thermal comfort and clothing weight explains why we tend to prefer a higher air temperature to adding more clothing. If we would like to stay comfortably warm at an indoor temperature of 0° Celsius (4 clo), we would have to wear 26 pounds (12 kg) of clothes. The US Army found in the 1960s that a maximum of 4 to 5 clo-units could be worn for a man to remain mobile and dexterous enough for military tasks. Additional clothing weight thus limits our freedom of movement, and even couch potatoes have to get up from time to time.

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One layer of thermal long underwear allows you to turn down the thermostat with at least 4° C, saving up to 40% on space heating energy.

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High-tech long underwear

However, things have changed. The military, space agencies and the sports clothing industry have considerably improved the warmth/weight ratio of clothing over the last decades. This has resulted in a very diverse and fashionable line of lightweight clothes with high clo-values. A great deal of this progress is due to the use of new, synthetic materials. While these have been used for all kinds of garments (sweaters, pants, jackets), their importance for indoor use is especially relevant in the case of long underwear. This clothing layer (which is actually worn in combination with briefs) has the largest potential to substitute a heating system.

Pumping coefficient

Because it fits tightly around the body, long underwear has an optimal "pumping coefficient". The pumping coefficient is another factor that defines clothing insulation, in addition to the clo-value. It refers to the motion of air produced by wearer movement. Even couch potatoes move from time to time, and this activity can disturb the insulating air layer around the body, degrading thermal comfort at least temporarily.

Because the pumping coefficient is much better for long underwear than for loose-fitting garments such as ponchos, wide pants, or thick knitted sweaters, long underwear offers more thermal comfort even when clo-values are similar. Another factor is the chimney effect: even without wearer movement, loosely hanging clothes ventilate the trapped air layers, thus reducing insulation.

Long underwear has more advantages over other clothing options. It does not hide your body shape and can maintain sex-appeal, a common concern for both men and women. It can easily be worn underneath normal clothing. And, last but not least, it can be worn in layers, further improving upon the insulation value: more air is trapped using several thin layers than by a single, bulkier layer. According to the US Air Force Survival Book, one layer of long underwear (long pants + long-sleeved t-shirt) equals a clo-value of 0.6, while two layers of long underwear add a clo-value of 1.5. In other words, the clo-value more than doubles by using only two layers. Combine this outfit with a typical business suit (or a similar, more comfortable clothing ensemble), and thermal insulation rises to 2.5 clo, enough to keep a couch potato indefinitely comfortable at a temperature of only 12.7° Celcius (55°F) - far below the common indoor temperatures of today. This clothing ensemble could yield a reduction in energy use for space heating of up to 80 percent.

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With two layers of thermal underwear, the insulation value more than doubles. It is perfectly possible to maintain thermal comfort at temperatures around or even below 10°C (50°F)



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Unfortunately, the clo-values of modern thermal underwear have not been listed in well-documented overviews, as is the case for more common clothing options. Nevertheless, fragmentary information points to considerably higher clo-values than those for traditional long underwear. Calculations by well-informed amateur hikers (see for instance here) show clo-values that are at least double those of the long underwear mentioned by the US Air Force (for instance, 0.66 clo for the upper piece alone). This would mean that the same thermal comfort could be achieved using only one layer of long underwear plus the equivalent of a winter business suit (2.5 clo at 12.7°C), or that using two layers plus the suit could bring the comfort level for a resting person down to a temperature of 0°C (wearing 4 clo of clothing).

Another indication for the additional energy savings potential of high-tech long underwear are the clo-values of different materials. According to the "Handbook of technical textiles", the warmth/weight ratios of pile fabrics like polyester and acrylic are 2.5 to 8 times higher than those of woven and knitted fabrics like wool or cotton (materials used for traditional long underwear). Quilt battings like Thinsulate offer warmth/weight ratios that are 13 to 17 times those of cotton and wool.

Synthetic or natural materials?



It may sound strange to promote the use of synthetic clothing on a blog like Low-tech Magazine. However, both natural and synthetic materials have their advantages and drawbacks, and both can be a sustainable choice - even though synthetic clothes are made from fossil fuels. This is especially true when the clothing is used as a substitute for a heating system; the energy saved by lowering the thermostat is much larger than the energy required to manufacture the garments. Indeed, these high-insulating garments demonstrate how valuable fossil fuels are as a material, and how foolish we are to simply burn them.

Synthetic long underwear not only has a higher insulation value than natural materials, it also lasts much longer, it is easy on the skin (many people can't tolerate wool) and it can be very cheap. The main drawbacks of synthetic underclothing are their high fire susceptibility and their tendency to attract dirt. Synthetic thermal underwear should be washed regularly - a process that consumes energy. This is less an issue for indoor use than for outdoor sports, because couch potatoes don't produce sweat. Moreover, synthetic clothes dry very easily, which means that there is no need to put them in a dryer after washing. Of course, clothes can also be washed in a pedal-powered washing machine, and the hot water could come from a solar boiler, eliminating fossil energy use altogether (and keeping you more than warm enough while doing the laundry).

That being said, synthetic clothes are not a necessity. Even the use of long underwear made from natural materials like cotton and wool has the potential for considerable energy savings. Cotton might have a relatively low insulation value, but a full layer of cotton long underwear will still add at least 0.4 clo to your thermal comfort - enough to lower the indoor temperature by 2.5° C and save more than 20 percent on heating bills. Using wool can more than double this potential to about 1 clo for a full layer of long underwear (allowing for an indoor temperature reduction of more than 6° C). Wool made a comeback as a material used for hiking and mountaineering clothes in the mid-1990s, at which point Icebreaker was the first manufacturer to position itself in the market with woollen thermic underwear.

The company uses wool from the merino sheep in New Zealand, which produce some of the finest and softest wools available. Patagonia also offers a series of merino wool underwear, and several European manufacturers (Mammut, Woolpower and Helly Hansen) mix merino wool with synthetic materials. This leads to more durable clothing - wool wears out much faster then synthetic materials. An important advantage of wool over synthetic (and over other natural) materials is that it maintains a good smell for a very long time. Regular washing is not required. The largest disadvantage of merino-wool is the price: you won't find a full set of long underwear for less than 200 euro. But again: this investment will quickly payfor itself if it allows you to lower the thermostat.

Thermal comfort: more than clothes & air temperature



Thermal comfort is not just dependent on air temperature and the thermal insulation properties of clothing alone. In fact, more than a dozen other factors - both personal and environmental - play a role. However, environmental factors are of much less relevance for indoor thermal comfort than for outdoor use. Indoor clothing does not have to be windproof, waterproof or be able to wick away perspiration.

After air temperature, the environmental factors influencing thermal comfort are the mean radiant temperature, relative humidity and air movement. The last two are included in the clo-value, which is defined in an environment with a relative humidity of less than 50 percent and an air speed of 6 metres per minute (stagnant air). Wind has a profound influence on the thermal insulation of clothing when we are outdoors because it disturbs the insulating air layer between skin and clothing. Indoors, air movement is mostly a negligible factor, although it should be kept in mind that any draft can lower the thermal comfort of a clothing ensemble.

Radiant heat is another major influence on thermal comfort when we are outdoors. The radiant heat of the sun can make you feel hot even when air temperature is low. Indoors the influence of radiant heat is much less. Nevertheless, it can have a positive influence on indoor thermal comfort, because sunlight that enters the room will be absorbed by walls and furniture, and gradually released. This is especially so in passive houses and in homes heated by a tile stove, where radiant heat is an important factor in thermal comfort.

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Environmental factors are of much less relevance for indoor thermal comfort than for outdoor use. Indoor clothing does not have to be windproof, waterproof or be able to wick away perspiration.

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Along with the clo-value and the pumping coefficient, the third factor defining the thermal insulation of clothing is the "permeability index". The thermal properties of clothing drastically degrade when they become wet, either by sweating or by external moisture. This can be very dangerous if you are physically active in a cold outdoor climate because during a resting period your body can quickly lose heat, possibly leading to hypothermia and death. But of course, the permeability index is not of any importance for indoor couch potatoes: they don't sweat. Indoors, rain is not a concern either.

Human activity indoors

The most significant factor influencing thermal comfort - even more important than air temperature and clothing - is human activity or body heat production (the metabolic rate). For instance, while it takes 12 clo-units to keep a resting person warm at an extremely low temperature of minus 40° C, this comes down to only 4 clo when this person is walking, and to only 1.25 clo when this person is running at 16 km/h. One of the most obvious reasons why our forefathers could bear lower indoor temperatures, was that they were more physically active than many of us.

It is telling that one defence mechanism of the body against cold is to increase its heat production. This happens first by muscle tensing and ultimately by shivering, which can increase body heat production by up to five times (from 100 watts at rest to about 500 watts). The metabolic rate also has a profound influence at non-extreme temperatures. While a resting person (like a couch potato) requires a thermal insulation of 2.7 clo at an indoor temperature of 10° Celsius (50°F), this is lowered to only 1.7 clo when this person is engaged in very light activity (like typing, drawing or having an animated conversation). In this case, the combination of long underwear with the equivalent of a typical business suit suffices to keep him or her warm. As a general rule of thumb, each increase of 30 watts in heat production allows the comfort temperature to go down by about 1.7°C. On the other hand, when sleeping instead of just resting, the required thermal insulation approximately doubles - for instance to 2 clo at a temperature of 20° Celsius. This is why sleeping bags can have thermal insulation of more than 10 clo-units.

Personal factors other than clothing or activity can also contribute to thermal comfort. Men seem to tolerate lower temperatures than women, and both small children and the elderly need higher temperatures to sustain their thermal comfort. Research has shown that - even regardless of age and gender - different people prefer slightly different ideal temperatures. Furthermore, people also get used to prevailing temperatures, leading to clearly observable cultural differences. The clo-values given for different indoor air temperatures are thus not more than guidelines - personal differences will occur.

Hands and feet

The clo-value refers to the whole body surface and thus includes heat transfer by exposed body parts (head and hands, in some cases also arms, legs, feet or torso). Both the clothing insulation and the skin coverage are important determinants of heat loss. In real life the two are correlated in the sense that winter clothing not only insulates better, but also covers a larger proportion of the body than summer wear.

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The most significant factor influencing thermal comfort - even more important than air temperature and clothing - is body heat production

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Hands and feet are always the first victims when thermal discomfort sets in. Together with the head and the neck, they lose more heat than other parts of the body. However, it is important to note that if the body as a whole is kept warm enough, hands and feet will not be greatly affected by lower indoor temperatures. Cooling down the extremities is yet another defence reaction of the body if the core temperature falls. This thermoregulatory mechanism - "vasoconstriction" - reduces the blood flow to the skin, improving skin insulation and thus limiting heat loss. It happens all over the body, but due to their small mass and large surface area, vasoconstriction has the most profound effect on the hands and the feet.

At extreme cold temperatures, vasoconstriction can save your life - though it might cost you some fingers and toes, or worse. In order to maintain body core temperature (which is vital for survival), the body will sacrifice hands, feet and nose first, followed by the limbs. Because vasoconstriction only occurs when the core body temperature falls, it won't happen if you're dressed warmly enough. While insulating your neck and feet will greatly improve your thermal comfort, there is no need to wear gloves or caps indoors. In fact, it doesn't matter very much which parts of your body you choose to insulate - the important thing is to limit total heat loss so that the body core temperature remains stable. For instance, if you prefer to wear a high-insulating cap indoors, you can pretty much forget about all the rest and be comfortable in relatively light clothes at low temperatures.

Life without heating?



Of course, this article is not a plea to get rid of heating systems altogether, although in some climates this is clearly possible - saving not only heating costs but also the instalment of a heating system and other investments. However, for many of us, a heating system remains a necessity, if only because temperatures regularly drop below freezing point (water pipes would freeze over, and keeping full thermal comfort by clothing alone will become difficult). But even then, thermal underclothing could lead to an important reduction in energy consumption by making it possible to lower the average indoor temperature a few degrees and to shorten the heating season by a couple of months.

The energy savings potential of clothing is so large that it cannot be ignored - though in fact this is exactly what is happening now. This does not mean that home insulation and efficient heating systems should not be encouraged. All three paths should be pursued, but improving clothing insulation is obviously the cheapest, easiest and fastest way. One final disadvantage is that visitors not wearing thermal underclothing will feel uncomfortable at your place, even if you and your family are feeling all right. Offering casual visitors an extra layer of thermal underwear might not always be an option.

Kris De Decker (edited by Rachel Meyer)

Pictures of people wearing thermal long underwear: Icebreaker.

Pictures of thermal clothing: Patagonia, Woolpower and Helly Hansen.

Last picture: a "union suit", the predecessor of thermal underwear.

Thanks to Tim Joye.

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