Why Are Several Thin Layers Warmer Than One Thick One?
― It's not the cloth that keeps you warm, but the air trapped between
When the seasons change, someone always says, "Layers are warmer." The amount of cloth is the same, or even less. So why does it work? The answer lies not in the cloth itself but in the thin layers of air between the pieces of cloth. And those layers have a clear rule of thumb: past a certain thickness, more doesn't help.
The mornings and evenings turn chilly, and it's time to dig long sleeves out of the back of the wardrobe. Have you ever wondered whether to pull on one thick sweater or wear a few thin shirts on top of each other?
Mountain climbers and outdoor workers usually choose the second. "Wear several thin layers, not one thick one" is almost a motto in the field.
It sounds odd. If warmth depended on the amount of cloth, the thick one should win. Yet in practice, using less cloth can keep you warmer.
There are just two reasons
The fibres of cloth actually conduct heat better than air does. When you wear layers, a still layer of air forms between them. That layer works as a better insulator than the cloth.
Each layer you add makes it harder for heat to escape, and the effects add together. So even thin cloth, stacked with air between the pieces, can beat one thick piece.
But it doesn't work without limit. If the air layer gets too thick, the air inside starts to circulate and carries the heat away instead. We'll calculate where that limit lies later on.
Air is the best insulator around you
How well a material passes heat is given by a number called thermal conductivity. The smaller the number, the worse the material is at passing heat.
For still air at 20℃, thermal conductivity is about 0.026. Wool and cotton fabrics, being packed with fibre, come to about 0.04. In other words, cloth passes heat more easily than air.
A fluffy sweater is warm for the same reason a down duvet is. What makes them warm is the large amount of air the fibres hold. Crush the down flat and it stops being warm, because the air has been pushed out.
Layering does the same thing, but "between one garment and the next." Look at Figure 1. Even with about the same amount of cloth, the version with air between the layers lets less heat through.
But keep the gap to 1 cm
So if you wear loose clothes and make the air layer thicker, will you get warmer without limit? No.
If the air stays perfectly still, heat travels through it only slowly. But once the layer gets thick, the warmed air at the bottom becomes lighter and starts to rise. When air begins to circulate, it carries the heat with it, and the heat pours out.
This tipping point comes at a gap of about 1 cm when the difference between skin and outside temperature is around 15℃. The calculation is in the fold-out at the end.
So the ideal gap is a few millimetres up to about 1 cm. If clothes cling tightly to the body, no air layer forms. If they are too baggy, the air circulates. Mountain clothing is cut "with a little room to spare" to aim for exactly this range.
Wind from outside blows away the precious air layers. However good the inner clothes are, if wind gets through, the effect drops sharply. A wind-resistant layer on the outside protects the layers beneath. This is why adding one outer layer often works better than making a middle layer thicker.
When sweat collects in the air layers, heat passes through those parts far more easily, because water conducts heat more than 20 times better than air. Take a layer off before you start moving, and put one on before you stop. That is the best way to make use of layers.
Summary
Layers are warm not because there is more cloth, but because there are more layers of still air between the pieces of cloth. The resistance to heat loss adds up layer by layer, but once the gap goes past 1 cm the air starts to circulate, and the gain levels off.
What keeps you warm is not the clothing itself,
but the air the clothing holds.
The same idea of trapping air shows up elsewhere. Why does a vacuum flask stay hot for hours? covers a method that removes the air altogether. Why do we get goosebumps when we're cold? is about the body trying to do the same thing with its hair.
- Fill two identical plastic bottles with the same amount of water at the same temperature.
- Wrap one tightly in three layers of towel. Wrap the other with the same towel loosely, leaving a gap about one finger wide.
- After 30 minutes, measure both water temperatures and compare. Even with the same amount of cloth, the loosely wrapped one should stay warmer.
If you widen the gap to about three fingers, the difference shrinks again, because the air starts to circulate inside.
For those who want to know more ― terms, formulas and links to textbooksShows clearly which level each topic belongs to, from middle school science to university courses
- Middle schoolCovered in middle school science
- High schoolCovered in high school "Physics Basics / Physics"
- High school+Advanced high school material, or treated as a textbook sidebar
- UniversityNot taught in high school; university specialist courses (heat transfer engineering, fluid mechanics)
- ResearchNot yet taught as settled fact even at university; questions researchers are studying now
Middle schoolTerms: this phenomenon has names
- Conduction: heat passing from the hotter side to the colder side without the material moving. This is mainly what happens inside layered clothing.
- Thermal conductivity: a number for how easily each material passes heat. Its unit is watts per metre per kelvin, and a smaller value means the material passes less heat.
- Thermal resistance: a quantity for how hard it is for heat to pass through a layer. It is found by dividing thickness by thermal conductivity, and it adds up when layers are stacked.
- Convection: heat carried by the movement of warmed air or water itself. It begins when the gap is too wide.
Middle schoolHigh schoolChecking with formulas: how many watts of difference?
Let's actually work out how many watts of heat leave the body with one thick layer, and with three thin layers.
| In symbols | Q ＝ A × ΔT ÷ R and R ＝ d ÷ λ , R total ＝ R1 ＋ R2 |
| In words | Heat lost ＝ area × temperature difference ÷ thermal resistance. Thermal resistance ＝ thickness ÷ thermal conductivity. When layers are lined up, their thermal resistances add. |
| Where it comes from | It comes from the law that heat flow is proportional to how fast temperature falls (Fourier's law). It has the same form as "current ＝ voltage ÷ resistance" in electricity, right down to resistances in series adding up. |
| Thermal conductivity of cloth λ | About 0.04 (unit: watts per metre per kelvin) |
| Thermal conductivity of still air λ | About 0.026 at 20℃ (same unit) |
| Adult body surface area A | About 1.7 (unit: square metres) |
| Skin-to-outside temperature difference ΔT | 15, the difference between 33℃ and 18℃ (unit: kelvin) |
| Thickness of one thick layer d | 0.008 (unit: metres) |
| Total cloth in three thin layers d | 0.006 (same unit) |
| Total air between them d | 0.004 (same unit) |
| Thermal resistance of one thick layer | 0.008 ÷ 0.04 ＝ 0.2 |
| Cloth part of the layered outfit | 0.006 ÷ 0.04 ＝ 0.15 |
| Air part of the layered outfit | 0.004 ÷ 0.026 ≒ 0.154 |
| Total thermal resistance of the layered outfit | 0.15 ＋ 0.154 ＝ 0.304 |
| Multiply area by temperature difference | 1.7 × 15 ＝ 25.5 |
| Heat lost through one thick layer | 25.5 ÷ 0.2 ＝ 127.5 |
| Heat lost through the layered outfit | 25.5 ÷ 0.304 ≒ 83.9 |
The unit is watts. Even though we cut the cloth by 2 mm, the heat lost dropped from 127.5 watts to 83.9 watts, a fall of nearly 40%. A person sitting quietly gives off about 100 watts. With one thick layer, more heat leaves than the body produces, so the body slowly cools. With layers, less heat leaves than the body produces, so by this calculation body temperature can be maintained.
High schoolHigh school+Why the gain stops at a 1 cm gap
High schoolMaking an air layer thicker raises its thermal resistance. But past a certain thickness, warmed air inside the layer rises and cooler air sinks, creating a loop of circulation. This is natural convection. Once it starts, heat is carried along with the air, and the benefit of the thicker layer disappears.
High school+Whether circulation starts depends on a contest between the force that lifts the air and the force of stickiness that holds it back. The dimensionless number for this ratio is called the Rayleigh number, and for a layer of air held between top and bottom plates, circulation is known to begin when the value goes above about 1708. Working backwards for air with a 15℃ temperature difference, the thickness that reaches this value is about 1 cm. The rule of thumb in the main text comes from this calculation.
UniversityHow clothing insulation is designed
In heat transfer engineering, problems like this are treated as a series circuit of thermal resistances. When convection is added, the ease of heat transfer including convection is expressed by the Nusselt number, and estimated from a relation with the Rayleigh number (the Rayleigh–Bénard convection correlation). In clothing science, thermal resistance is expressed in a special unit called the clo, and measured with a heated mannequin called a thermal manikin. International standards even set out a procedure for estimating the amount of clothing needed from this value.
📖 Derivations and further reading: Thermal conduction (Japanese Wikipedia) ／ Rayleigh number (Japanese Wikipedia)
ResearchWhat is still not well understood
- Clothing on a moving person can't yet be calculated well. With every step, clothing pumps air in and out like a bellows. How much this spoils the insulation depends heavily on body movement and the cut of the clothes, and there is no general formula.
- How sweat moves through cloth is a tough problem. Heat carried by water vapour and heat carried by absorbed water are tangled together, with condensation and re-evaporation across layers. Calculations that solve this combination correctly are still being improved.
- Manikin measurements and the warmth people feel don't match. Humans adjust their skin temperature by changing blood flow, so conditions differ from a manikin that holds its surface at a fixed temperature. How to close this gap is still a research topic.
In other words, the content of this article too is "an explanation within what is understood today." It is good enough as a rule of thumb when you are sitting still, but errors appear when you are moving around.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: everyday phenomena (how heat travels) | The difference between conduction and convection; air passes heat poorly |
| High school | Physics Basics: heat (heat transfer, quantity of heat) | Finding thermal resistance from thickness and conductivity, and calculating heat lost |
| High school+ | Physics, advanced (convection, dimensionless numbers) | Why the gain stops at a 1 cm gap; the Rayleigh number |
| University | Heat transfer engineering; building environmental engineering | Thermal resistances in series, the Nusselt number, the unit clo |
| Research | Clothing science; physiological anthropology | Evaluating clothing insulation including movement and sweat |
| ― | Links to daily life | Changing seasonal clothes, choosing clothing for hiking and outdoor work, choosing a duvet |
- National Astronomical Observatory of Japan (国立天文台), ed., Rika Nenpyo (理科年表, Chronological Scientific Tables) (thermal conductivity of air and various materials)
- Architectural Institute of Japan (日本建築学会), ed., Kenchiku Kankyo Kogaku Yo Kyozai: Kankyo-hen (建築環境工学用教材 環境編, Teaching Materials for Building Environmental Engineering: Environment) (series calculation of thermal resistance; thermal resistance of clothing)
- International standard ISO 9920 (estimation of thermal insulation and water vapour resistance of clothing)
- Teruko Tamura (田村照子), ed., Koromo Kankyo no Kagaku (衣環境の科学, The Science of the Clothing Environment), Kenpakusha (建帛社) (insulation of clothing and air layers)
*This article is a general science explainer. The figures given are rough guides to help you understand the mechanism. How warm real clothing is depends greatly on material, stitching, wind and moisture. In severe cold, follow weather information and the instructions of local authorities or organisers.