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Insulation: First the Body, Then the Home

Modern thermal underclothing offers the possibility to turn the thermostat much lower without sacrificing comfort or sex appeal.

Clothing insulation for different types of outfits. From Work Design, Stephan A. Konz, 1979.
Clothing insulation for different types of outfits. From Work Design, Stephan A. Konz, 1979.
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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.

This article explains through science and statistics how to maintain thermal comfort at any given indoor temperature using only clothes.

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).

Image: Thermal underclothing. Credit: Icebreaker.
Image: Thermal underclothing. Credit: Icebreaker.
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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).

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

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.

Clothing insulation for different types of outfits. From Work Design, Stephan A. Konz, 1979.
Clothing insulation for different types of outfits. From Work Design, Stephan A. Konz, 1979.
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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).

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).

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.

Image: Thermal underclothing. Credit: Icebreaker.
Image: Thermal underclothing. Credit: Icebreaker.
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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.

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.

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.

Image: Thermal underclothing. Credit: Patagonia.
Image: Thermal underclothing. Credit: Patagonia.
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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.

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)

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?

Synthetic clothes are made from fossil fuels. When the clothing is used as a substitute for a heating system, the energy save by lowering the thermostat is much larger than the energy required to manufacture the garments. Synthetic long underwear has a higher insulation value than many natural materials, it is more durable, and it can be very cheap. Nevertheless, there are disadvantages compared to natural materials: plastic fibre pollution, high fire susceptibility, and a tendency to attract dirt. Synthetic thermal underwear should be washed regularly - a process that takes energy.

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.

Image: Thermal underwear. Credit: Icebreaker.
Image: Thermal underwear. Credit: Icebreaker.
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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.

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 ancestors 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.

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

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.



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Kris De Decker

Roddy: this might be true, but the reality is that most of us live in houses that are ecological failures. And fixing that is much more costly and time-consuming than insulating your body.

Jonathan: there are indeed many forgotten technologies to cool your house without air-conditioning. I hope I can find the time to publish an article about them before the summer ends. However, most of these come down to architecture and it’s hard to apply them to the existing housing stock.

Chris H

Someone please show this to my wife. (Chuckling to self) I’ve been training her for a few years that we can feel awesome at 62F. Next year, I might try to push it to 60F. Nice to know that at one time this was more of a norm than it is today. Of course, probably then, the insulation was lacking and that was a relatively easily maintainable temp. Cheers!

Mr. S.

Sorry, I am not buying it. Not because you haven’t done good work on the article: you have. Also not because N.American homes are overheated, and the residents under-dressed: they are. I do not buy it because the Japanese live the way you suggest in the winter, and it is miserable, as described less seriously in my blog post:

You make a good point about the replacement and renovation of old buildings not necessarily being ecologically or financially justifiable. The way to make people conserve is to make them pay ALL of the externalities of what they consume, including the pollution, oil-wars and climate-change costs. Should triple the cost, and slash consumption.

I go this far. Reduce the heat to 19 or 20C, and wear long sleeves with a light sweater inside. I do not accept any temperature inside that makes me want gloves, or a hat for my balding pate. As for the building, go after the low-hanging fruit first: cheap and effective insulation such as blown insulation in the attic, weather-stripping drafts and heavy curtains for the windows at night.

Zvi Leve

This is all fine and good, but what about the vast majority of the planet’s residents who live in hot climates? Air-conditioning is what is driving the increase in home energy consumption in most of the world. There certainly are clothing technologies which could make a difference there too, and quite a bit can be done by designing structures to encourage natural air circulation.

But the fact remains that one of the principal reasons for the increase in ‘air conditioning’ in developing countries is the toxic air pollution in urban environments.


Regarding clothing: Its true natural fabrics often disguise unnatural levels of embodied energy to grow, treat water, harvest, clean, die, manufacture and then distribute globally to retail stores. But once you start wearing clothes, washing is the biggest energy hog. And again natural fabrics are often at a disadvantage here, as well. Maybe its not so much a matter of going au natural, but remaining dressed and a bit grungy..


A recommendation by the American Society of Heating, Refrigerating and Air-Conditioning Engineers as to what temperature people should set their thermostats makes about as much sense as the American Medical Association making recommendations on when to buy a new refrigerator.

Let’s examine why the ASHRAE would decide it is within the perview of their profession to make such a recommendation. The answer is obvious–it’s the same old “use it up, throw it away, buy a new one” mentality. When furnaces are running hard all winter to maintain a ridiculous 71+ degrees, they need servicing and replacement more often. Now I don’t believe many people would actually decide how warm to keep their house on the say-so of a specialized industry, what it means is that an entire industry, represented by this professional organization, has embraced consumerist waste because it benefits their members’ profits. Not to pick on this one Society; look around, nearly every industry and business profits from increased consumption and waste. Try to name one that does not.

Until EVERYONE rejects this kind of pervasive consumerist attitude and propoganda by corporations and special interests and instead commit to REAL solutions and not just band-aids, we’ll remain lemmings running for the cliff.

Alex Hallatt

I wholeheartedly agree with the idea, but it becomes problematic when you move from your home to work/other buildings which are heated. It’s hard to take off your merino longjohns/top when you do this. We have a stack of woolen blankets and do the nanna thing of having them over our laps when sitting at home on a cold winter’s day.

Steve B.

Keeping buildings warm (and cool) is the biggest source of CO2 pollution. Wouldn’t it be funny if we destroy civilization in order not to feel uncomfortable because we didn’t bother to put on appropriate clothing.

Another factoid: restaurants/movies etc.. typically keep the space colder in the summer than they do in the winter. This proves it’s not about normal human comfort, rather a psychological need (weakness) to “feel” warm or cold relative to how we perceive the season.


You forgot the most important piece of clothing, the hat. Most of the heat is lost throught the neck and head because the body can’t periodically shut down blood flow to the brain as is done with the other extremities. I worked outside construction for ten years and just putting on a hat that covered the head and back of the neck or even better a hooded sweatshirt or coat instantly boosted the comfort level.


I wish more people had a basic understanding of how temperature and power are related.

Calculating the power savings from lowering the thermostat setting is actually really simple. Power is proportional to temperature difference. For my home, I know I consume about 2000W of power to maintain the inside temperature 10 degrees C above the outside temperature. Therefore I know I consume about 200W for every degree C of temperature difference. Lowering the temperature inside by 1 degree thus reduces my power consumption by 200W. Over a 24 hour period that saves me 200*24 = 4800 Wh or 4.8 kWh which costs about $0.50 where I live. So for my home, I simply memorize this rule of thumb: It saves me $0.50 per day or $15 per month for every degree C I lower the thermostat.

I think every home owner should know what is the cost of 1 degree for their home.

Calculating the percentage savings from lowering the temperature is even easier. It depends only on temperature and not on any properties of your home. It’s simply the reduction in temperature divided by the initial temperature difference you were maintaining. Take the case of heating your home to 18 degrees C when it is 10 degrees C outside. The temperature difference you are maintaining is 8 degrees. Thus reducing the temperature by 2 degrees will save you 2/8 = 0.25 or 25%.

There are minor influences from geothermal heating, solar gain, and occupant heat output, but the above formulas generally hold true, especially for larger temperature differences.

Jeremy In Kansas

“You heat people, not rooms” is what I learned when I lived in the Colorado Rockies with only a wood-burning stove for heat.


Please tell me that you’re planning to do an article on how going naked inside the home can greatly reduce air conditioning costs! I’ve found that it’s comfortable up to about 85F without a fan, and it saves a lot of money on laundry.

Sorry, I don’t see how synthetic fabric can be considered “sustainable.”

(Why does it say, “We’re sorry, we cannot accept this data,” whenever I press the “Post” button? A more helpful comment in this dialogue might be to refresh your browser.)

Ross Couper

Going outside in the cold, but keeping warm because of some form of exercise, seems to make even a coldish inside temperature feel warm for up to an hour, despite shedding your outer layers as you come back inside.

This winter I’ve been keeping a kitchen at about 12°C and using clothing to keep comfortable and apart from some minor condensation problems, mostly inside a microwave oven, it has worked reasonably well. But after returning from a morning run 9°C feels OK for much longer than it takes for me to recover from the exercise.


Really interesting article :) I live in Sweden, and here it is definitely possible to get a set of long Merino underwear for less than 200 Euro, last week I bought one in 100% wool for about 70 Euro.

The norwegian company Slackline have created a garment called the “hyggepiece” ( ), a thicker hooded overall, meant to be used for example in the TV sofa. I guess that would be a great idea in this context?


Kris, in the UK there is data on average internal temperatures going back to the 70s:

Figure 27 Standards of comfort - mean internal and average winter external temperatures

Most of this trend is down to a rise in central heating ownership over the period from 30% to 90% (see Figure 21 Central heating ownership)

Joseph Bronson

Great article BUT. My wife has a condition called multiple chemical sensitivity. This condition is getting more and more press all the time as many more people world wide are diagnosed with MCS. My wife cannot wear synthetic materials because everything that we put on our skin has an effect on our bodies. Most people do not know that this effect is occurring because it usually takes many years for the effects to show up in people without MCS. These long term effects are usually not understood because very few researchers have connected the dots. My wife has had immediate adverse effects to the wearing of synthetic material. We now only wear natural fibers. Please do more research before promoting the use of petro-chemical based materials. They are part of the chemical stew that we must disengage from if we are ever going to become environmentally sane.

Yes, it is less destructive to the natural environment to produce petro-chemical clothes instead of energy. But, it is unbelievably detrimental to our internal environment to wear those clothes. Good health to you.


Interesting article but no mention of the crucial feet/head problem, nor of the great constriction people can feel wearing winter clothes indoors.

My solution:

The last 2 winters I have lived in couch potato mode in NE Europe winters at a measured 8-10 C in a largely unheated flat. Below 7 C, I would need fingerless gloves to type with. How do I manage to stay warm and comfortable and motionless at 8-10 C?

I wear clothes as follows:

  1. surplus British Army calf-length indoor slippers for only €6 with (a). drawstring around the calf on outer boot to stop cold air falling in from above (b)outer boot of padded cotton (c). inner removable boot of washable synthetic.

  2. Swedish second-hand army Arctic trousers or Milspec US army thermal trousers, no long underwear needed. I wear any sox that come to hand, not necessarily woollen.

  3. Trevira synthetic T-shirt under 300g/m2 synthetic fleece high-collar shirt under another fleece shirt of same specs. under a sleeveless padded cotton jacket. The collar is also padded, hence I have a total of 3 padded collars zipped shut at throat. So I need no scarf, inside or outside the apartment.

Body core is consequently protected by 4 layers, 3 of synthetic and 1 of padded cotton (synthetic filling). The sleeveless jacket, which permits great arm freedom and has no fitted shoulders, is crucial in imparting a feeling of not being constricted by all the clothing. I rarely need a woollen hat inside.

This clothes combination has the magnificent advantage that I need not change it it any way when going outside to -5 C: I wear the same clothes amount inside and out. I wear no cotton at all except for underwear below the waist. Hence any sweat generated while wearing this combination wicks away during and after a brisk walk of 45 mins at e.g. 0 C.


@Alex Hallatt Actually it is more a problem when you have bigger temperature differences. In winters I often start from apartment of +21C, go on street and wait a bus in -15C, ride a bus on +16C, walk again on -15C and finally get indoors on +20C. Dressing appropriately is a real challenge and lowering interior temperature would actually make my day easier and more comfortable.


I keep our house at 55 F. during the day, 45 F. at night. Have done so for the last 8 years. What works is layering with polar fleece and nylon. You need the nylon to trap water vapor; the fleece for insulation.

The nylon is your typical track warm up suit. Wear over polar fleece when it’s warmer, under polar fleece when colder.

You need a hat (berets are great), high collar for neck, shoes with thick rubber soles to keep your feet off the floor.

Everything you need to know is revealed in medieval paintings.

Look at the quilted clothes, capes, the high collars, the beret style hats. These people lived in cold buildings. Their clothes reflected it.


Thanks for the suggestion though it seems … a bit impractical. I doubt my family would appreciate me leaving a wet spot on the couch. :)

I realize that clothing probably isn’t the answer for cooling down. There’s only so many clothes you can take off. But I wonder if there are other low-tech solutions that we might have forgotten since the invention of air conditioners.

Roddy Pfeiffer

None of this is necessary. In 1978 the University of Saskatchewan was building houses that were heated with electric baseboard heat for $50 a year. They were using fiberglass insulation. Now we have high-R foams that are even better. We worry about wasting heat, but we live in thermal sieves.

Nobody has to wear outdoor clothes indoors. That is just an admission that your house is an ecological failure.

Israel Walker

I’ve been thinking about this for about 2 weeks now and I have a LOT to say.

One of the first is the declining use of radiant heat, such as hot water radiators and flame. In house with an average temperature of 55 degrees, you will get cold. It’s not bad when you go cozy up a 200 degree radiator or 450 degree fireplace. An old, cold house, rarely had the homogeneous temperature of a modern, forced air heated home. It makes a huge difference. Having a single space where you can warm up makes it possible to tolerate a much lower over all temperature overall.

Another is the increasing use of single story homes, and full rather than half storied attics. In an older home, the areas one was active like the kitchen were downstairs, and bedrooms were upstairs. In the absence of forced air, hot air stratifies to the top of the room. When the trapdoor is opened, the hot air rushes into the much smaller volume upstairs, warming it before bed.

Another is plasterwork being replaced with sheetrock in modern homes. This lowers the thermal mass of the house, and the heater has to cycle more frequently.

Yet another is the increasing volume (not size, thats’s next) of houses. It costs more to keep a basement at 55 then a crawlspace at 15. Though a temperature controlled basement might not increase the listed square footage of a house, it increases the heated volume. Higher ceiling have also become the norm, which further increases volume.

Also, square footage has ballooned enormously. In the quest for higher efficiency a lot of people seem to have forgotten that it takes more energy to heat 2000 square foot super insulated home than an totally uninsulated 8x12 cabin.

Yet another is the replacement of organic surfaces with inorganic surfaces. Unmentioned in your article was RELATIVE temperature. If oil finished wood floor and a tile floor are at exactly the same temperature the tile floor will feel far colder, because tile conducts heat from the body better. (Wood floors also breath, whereas tile or vinyl floors do not, building up a thin, and very thermally conductive layer of perspiration. A cold floor covered with good, old fashioned wool felt carpet or true linoleum (which is largely cork based) will feel much warmer than an identical floor covered with plastic based carpet or vinyl floor covering. Since we set our thermostats by the “feels like” temperature, this makes a large difference.

Another interesting point is the very modern expectation of daily bathing in winter. Our grandparents would have NEVER gotten their skin totally soaked in a 55 deg house. Baths were taken in the evening, when the house had it’s full thermal mass heated the hottest, not in the morning when the house was the coldest. If you are going to be naked and wet for a few minutes everyday, and the whole house has to be the same temperature, then need your whole house to be shower bathroom warm. (It was easier to keep some rooms boiling hot and some just under freezing with big old radiators than with forced air heat).

Nick Sturkenboom

What people often forget is that with the excellent insulation and double glass windows house in N-Europe are often too warm to live in during the summer, showing dramatic increases in the sales of air conditioning. Talking about irony…

Baksa Péter

“What people often forget is that with the excellent insulation and double glass windows house in N-Europe are often too warm to live in during the summer, showing dramatic increases in the sales of air conditioning. Talking about irony…” Posted by: Nick Sturkenboom | November 09, 2011 at 08:29 PM

This has nothing to do with double glass windows, maybe that they ventilate less when you don’t open them.

And no, that is not primarily because of the insulation. You have to build a house with lot of mass as a shield against heat. This shifts the time when the peak of the heat arrives at the inside of the walls. If that is at night, you can easily ventilate the room.

With more insulation however you don’t need as massive walls for winter - but you still need them for summer. You can build massive walls plus good insulation, that will still be good for summer.


I worked outside in Alaska for years. I had to find a solution IMMEDIATELY to eliminate rapid overheating when I went from -20F to an engineering meeting in a 70F (or uncontrolled) field trailer. I put a cotton turtle neck UNDER the wool union suit. The minute I walked into the office, the artic gear and wool shirt came off and the top of the union suit was unbottoned and taken off - arms tied around my waist. They all made fun of me because I had clothes on under my long underwear. That’s good. When the hollering started and everybody started overheating I was not that hot for about 20 minutes or so. Then when things got out of hand, it all went back on in about 10 seconds so I could get out of there. I still do that here in northern New Mexico. Stills works great.


Two years ago in Milan at an architecture expostition I found a company from Sardegna selling sheepswool insulation panels for the home.

I don’t think is was a bad idea, but I find the fact that wool is worth so little to us for clothing applications just sad.

Risa Bear

“I have my bearskin,” said the frontier dentist to Rooster Cogburn.

Louisa Kendricks

I can’t wear wool next to the skin either, but have found that wearing double-knit silk longjohn’s under my clothes, night and day makes all the difference. I haven’t bothered with heating or cooling for the last couple of years, not from choice but because due to ill-health I was no longer in a position to afford it. I live comfortably now when the temperature gets to 3-4 degrees centigrade overnight, often not rising above 10 degrees during the day. Curtains are closed when the sun goes down, but otherwise… Yes, I wear a beanie, and a couple or more layers, but it is entirely doable. I have to stress that it is the double-knit silk I use because it doesn’t ladder in the way that the single knit can do, (FYI, nail polish stops the ladders), and because after three years I haven’t had to make any repairs. That is good going.


Thanks for that interesting piece of text! To add something from my personal approach (still experimental, however): how about fluid heated clothing.

I’m gonna live in a well-insulated 4x4 truck soon, but to further save on heating costs I have obtained a “fluid heated overall” from ThermoFlash ( ). Now electrically heated clothing is quite common for motorbiking, but so far only ThermoFlash produces fluid-heated clothing, which is way more powerful (up to 3 kW per suit) and does not waste electricity on heat.

When living within the truck I can connect the fluid tubes to the independent vehicle heating, using the overall suit as a radiator. Not too practical in large homes however, as you the loooong tubes would obstruct your movement. However, ThermoFlash also has a mobile gas heating device that you can wear in a bag (approx. 2 kg).

I roughly calculated energy use for this idea, and it showed that it’s on par with a high-grade, modern “passive home” (the “controlled air exchange type”, plus heavy insulation). Just that heated clothing makes this possible not only in a limited-size home, but everywhere at once …


Nice piece.

I’ve long been a believer…but recently [for many reasons, some hurricane related] converted from forced air & baseboard convectors to ceiling-mounted radiant heat. My home is considerably more comfortable [blowing air to stay warm makes even less sense than heating space instead of people], & the fact that radiant warms objects instead of air means the heat energy re-radiates & has a more lasting effect, meaning greater efficiency & better temperature control.

The specific point here, however, is that people are objects, too, & can absorb radiant heat…provided they’re not too heavily insulated. My anecdotal experience clearly suggests that occupants can–in some circumstances–be more comfortable at lower temperatures by shedding clothes than by adding layers.

This is especially true when the radiant system is ceiling-mounted, rather than a heated floor. I won’t address here the endless debate about which system is better; they simply have different characteristics. For this discussion, it’s only necessary to imagine sitting on a sofa: with a radiant floor, direct exposure is blocked; with a radiant ceiling, occupants enjoy direct radiant heat absorption…provided they’re not clad in caps & heavy sweaters.

Radiant won’t be feasible or right for everyone, but its impact on the CLO concept must be considered.


Great article (imo)

Because of a skin/neurological condition that my wife has suffered from for the last 4 yrs, we set the thermostat in the Winter at 56-57F.

This has made me become very accustomed to the “layering” clothing effect and it’s benefits ;)

I normally wear a short-sleeve cotton t-shirt with a long-sleeve v-neck pull over and a sweat-shirt…along with 2 pairs of sweat pants lol

My feet would get cold at times even with 2 pair of socks…till I discovered “SmartWool” products. I ordered a pair of their heavy socks and they are the best feeling sock (like walking on a massaging cushion) and very warm. Much better than 2 pairs of cotton socks!

Our heating bill is quite low ;)

But I do have a full set of Airblaster microfleese in case of an emergency. heh

Warm Regards


Bruce Weir

Maybe you can solve a problem I’ve thought about but can’t resolve. I live in SW France and during the summer when the temperature outdoors rises to 30 - 35°C the temperature indoors is always 20°C when I can laze about in shorts and T-shirt. However, during the winter when the outdoor temperature may be 5°-10°C the indoor temperature is still 20°C but I cannot - noway - laze about in shorts and T-shirt. I need to be fully clothed. During winter and summer the indoor temperature is always a steady 20°C but my clothing is less in summer and much more in winter. Is there a radiation factor involved here?


Bruce Weir: I believe this is due to the difference in the temperature of the walls. In the summer, the walls of your house are often warmer than the air, so your body will lose less heat to them via infrared radiation. In the winter, they are colder than the room air, allowing them to absorb more, and your body to loose much more radiant heat. Here in the ‘states, I’ve noticed that the buildings that are most comfortable in the summer and least comfortable in the winter are those with solid masonry walls (which are probably much more common in France); the worst one I recall was a motel with a separate heater in each room that remained off when the room was unoccupied. After a night with the heat on, the walls still felt like blocks of ice and I felt cold no matter how hot the air got. Buildings with dry-lined, insulated masonry or wood-framed walls tend to feel more comfortable in the cold.


Daniel > The norwegian company Slackline have created a garment called the “hyggepiece” […], a thicker hooded overall, meant to be used for example in the TV sofa

In case someone else is curious to see what it looks like:

Markus Padourek

Also on the topic, what I forgot, instead of keeping the body warm through insulation there are other low to zero energy ways to keep the body warm:

One is just to be active once in a while, or as it is also called do a “micro-workout”, i.e. do a few minutes of body-weight exercises whenever you feel cold. For example push-ups, burpees, jumping jacks, planks, etc. or if you sit on a chair/sofa just put both of your hands beside you and push yourself up. I do that sometimes straight after getting up as that is when I often feel the coldest.

Another way along these lines is of course to create some of the energy yourself that you need for what you are doing, if possible e.g. by using hand or pedal powered tools.

Another thing one can do is to accustomise oneself more to cold temperatures. I was surprised to learn some years ago how much influence we have on this and what our body can get used to. Read e.g. Wim Hof’s “Becoming the Iceman”. In short, the biggest direct change I have taken from this is taking almost only cold showers and partially as a result of that I wear less clothing in colder temperatures. This discovery really had an influence on my life.

And lastly, at university I would often drink butter tea (buying only the highest quality butter I could find) as my breakfast in the winter and it would usually keep me warm and energised until the later afternoon, when I suddenly would start to feel a very noticable drop in percieved temperature (I often skipped lunch back then as I did not like the food we had at university). It may sound very questionworthy when you hear it the first time but you can read more about its benefits, history as well as the modern take on it here:

I am aware that most of these practices are not really for the masses, but maybe some people here find it interesting.


There is one problem - cold walls will get black mold in few years, because water from the air condensing on them and making them wet.