Thermal Conductivities of Materials: A Guided Tour of the Numbers

Touch a metal railing and a wooden bench on the same cold morning. Both are at the same temperature, yet the metal feels far colder, because it pulls heat out of your hand much faster. The property behind that difference is thermal conductivity, and the range it spans across ordinary materials is enormous. Air conducts heat about as poorly as anything you can hold in your hand without a vacuum. Copper conducts it more than ten thousand times better. Diamond, at room temperature, beats copper by a factor of about five.

Wikipedia maintains a long reference page called "List of thermal conductivities," which gathers measured values for hundreds of materials from handbooks, data series and research papers. A raw list is useful for looking things up, but it does not explain why the numbers are what they are, how they were measured, why sources disagree, or how to use them sensibly. This article is a guide to that list and to the wider subject: what thermal conductivity means, how scientists came to measure it, why some materials conduct heat brilliantly and others barely at all, how the values change with temperature and structure, and how engineers use them. Representative values are given in tables throughout, drawn from the Wikipedia list and related references. Worked examples are labeled as such.

What thermal conductivity is

The definition

Thermal conductivity, usually written k, λ or κ, is the constant of proportionality in Fourier's law of heat conduction. In one dimension the law says that the heat flux q (watts per square meter) through a material is proportional to the temperature gradient:

q = −k · dT/dx

The minus sign records that heat flows from hot to cold. A material with high k carries a large heat flux for a given gradient; a material with low k carries little.

Thermal conductivity is an intensive property, as the Wikipedia list's introduction notes: it does not depend on how much material you have. A thin copper wire and a thick copper bar have the same conductivity; what differs is how much heat each carries, which depends on cross-sectional area and length as well.

Units

The SI unit is the watt per meter per kelvin, W·m⁻¹·K⁻¹ (equivalently W/(m·K)). Some research papers use watts per centimeter per kelvin. In United States engineering practice, conductivity is often given in BTU per hour per foot per degree Fahrenheit. The conversion factor is about 1.73. The Wikipedia list gives 1 BTU·h⁻¹·ft⁻¹·°F⁻¹ = 1.728 W·m⁻¹·K⁻¹, while the main Wikipedia article on thermal conductivity gives 1.730735, a figure it describes as unduly precise. The small difference reflects different definitions of the British thermal unit; for engineering purposes either is fine.

Building insulation is often described not by conductivity but by R-value, the thermal resistance of a given thickness. For a single uniform layer, R equals thickness divided by conductivity. The SI unit of R is square meter kelvin per watt, sometimes written RSI. A United States R-value, in square foot degree Fahrenheit hour per BTU, is about 5.68 times the SI value.

Related quantities

Conductivity tells you about steady heat flow. Diffusivity, which also involves how much heat the material stores, tells you how quickly a temperature change travels through it. Both matter, and they rank materials differently. Water, for example, has a modest conductivity but a very low diffusivity because of its large heat capacity.

How heat is carried

The enormous range of conductivities comes from the different ways heat can travel through matter. The main Wikipedia thermal conductivity article summarizes the mechanisms.

Electrons in metals

In metals, free electrons carry most of the heat. The same electrons carry electric current, which is why good electrical conductors are usually good thermal conductors. This connection was noticed experimentally in 1853 by Gustav Wiedemann and Rudolph Franz, who reported that the ratio of thermal to electrical conductivity has approximately the same value for different metals at the same temperature. In 1872 Ludvig Lorenz found that this ratio is proportional to temperature. The resulting Wiedemann-Franz law is written

κ/σ = L·T

where σ is electrical conductivity, T is absolute temperature and L is the Lorenz number. Paul Drude's derivation around 1900 got close to the right value, though with errors that partly canceled. The modern theoretical value is L = (π²/3)(kB/e)², about 2.44 × 10⁻⁸ W·Ω·K⁻². Measured values differ somewhat by metal; Charles Kittel's tables give 2.23 × 10⁻⁸ for copper at 0 °C and 3.2 × 10⁻⁸ for tungsten at 100 °C. The law holds best at high temperatures and at very low temperatures of a few kelvins and can fail in between. In the purest silver at very low temperatures, the Lorenz number can fall by up to a factor of ten.

Phonons in nonmetals

In electrical insulators such as diamond, sapphire, glass and ceramics, there are no free electrons, and heat is carried by vibrations of the crystal lattice, described as quasiparticles called phonons. A stiff lattice of light atoms with strong bonds, like diamond, carries phonons very efficiently, which is why diamond outperforms every metal at room temperature even though it is an electrical insulator. Anything that scatters phonons, including impurities, isotopic mixtures, grain boundaries and disorder, lowers conductivity. Glasses, being disordered, are poor conductors. At high temperatures phonons scatter off each other, and the conductivity of many crystals falls as temperature rises.

Molecules in gases

In a gas, heat moves when fast-moving molecules collide with slower ones. The Wikipedia article notes that in a simple model of a dilute monatomic gas, conductivity is independent of density at fixed temperature, and rises with temperature (in practice faster than the square root of temperature the simplest theory predicts). Light molecules move faster at a given temperature, so hydrogen and helium are much better conductors than heavy gases such as argon.

The Wikipedia list adds an important practical caution: in ordinary conditions, heat in a gas moves mostly by advection, the bulk movement of the gas through convection or turbulence, rather than by conduction. The very low conductivity of still air is real, but air is rarely still. That is why insulating materials trap air in small cells or fibers.

Liquids

Liquids are the least well understood. The thermal conductivity article notes that the mechanisms of liquid conduction are poorly understood, and that a rough model by Percy Bridgman relates conductivity to the speed of sound. Water is an unusually good conductor for a liquid, at around 0.6 W·m⁻¹·K⁻¹.

A short history of measuring conductivity

Ingenhousz's wax-coated wires

One of the earliest recorded comparisons of how well metals conduct heat comes from the Dutch scientist Jan Ingenhousz. In a letter to Benjamin Franklin dated 5 December 1780, quoted in Wikipedia's thermal conductivity article, he described dipping wires of seven metals, coated in wax, into heated oil and watching how far along each wire the wax melted. He concluded that "Silver conducted heat far the best of all other metals," and ranked them from best to worst as silver, copper, gold, tin, iron, steel and lead. That ranking is still correct for the pure metals more than two centuries later.

Fourier's theory

The mathematical framework came from Joseph Fourier. He presented a memoir, "On the Propagation of Heat in Solid Bodies," to the Paris Institute on 21 December 1807, and published his full theory in 1822 as Théorie analytique de la chaleur. Fourier proposed the partial differential equation for heat diffusion, now called the heat equation, and treated heat flow between neighboring particles as proportional to their small temperature difference. Fourier's law, with its single coefficient k, is the definition every conductivity list relies on.

The transient methods

Early measurements were steady-state: hold one end of a sample hot and the other cold, wait for temperatures to settle, and measure the heat flow and gradient. Steady-state methods are conceptually simple but slow and vulnerable to heat losses. Transient methods, which analyze how temperatures change over time, came later.

The hot-wire method has deep roots. Joseph Priestley used a heated-wire experiment in 1781 to compare gases, and Sven Pyk and Bertil Stålhane proposed the first truly transient hot-wire method, for solids and powders, in 1931. In 1971, Davis and colleagues introduced thinner, shorter wires that allowed measurement times of 1 to 10 seconds, and Haarman introduced an electronic Wheatstone bridge; Healy and colleagues published the theory with corrections in 1976. Because a modern measurement takes about a second, convection barely has time to start, which makes the method especially useful for fluids.

The laser flash method, developed by Parker and colleagues in 1961, measures thermal diffusivity directly. A short pulse of energy heats one face of a thin plane-parallel sample, and a detector records the temperature rise on the other face. In the ideal one-dimensional adiabatic case, the diffusivity is

a = 0.1388 · d²/t½

where d is the sample thickness and t½ is the time for the rear face to reach half its maximum temperature rise. Cowan added heat losses by radiation and convection in 1963, and Cape and Lehman treated finite pulse effects the same year. Conductivity follows by multiplying diffusivity by density and specific heat. The Wikipedia list notes that laser flash analysis is a common way of measuring conductivity today.

The big data compilations

For much of the twentieth century, engineers relied on a few large compilations. The Wikipedia list draws heavily on the Thermophysical Properties Research Center (TPRC) data series from Purdue University and on the CRC Handbook of Chemistry and Physics, along with older handbooks, manufacturer data sheets and research papers. The list's editors flag that these sources contain numerous transcription errors and inconsistent values, so some numbers should be treated with caution.

How the Wikipedia list is organized

The list has two main parts.

The first is a sortable table of values for materials at atmospheric pressure and around 293 K (20 °C). This is the part most people use for quick comparisons.

The second is an analytical list giving values at various temperatures and pressures, together with the measurement method. Methods are coded as L for longitudinal heat flow (axial flow with minimal radial loss), R for radial heat flow, and P for periodic or transient heat flow. This part shows how much conductivity can change with temperature and preparation.

Air has its own section. For air in thin gaps or at reduced pressure, the list uses an approximation by C. J. Lasance, published in 2002 in the magazine Electronics Cooling, which reduces the effective conductivity as pressure falls or as the gap becomes comparable to the distance molecules travel between collisions. The constant in that approximation is given as about 7.6 × 10⁻⁵ m·K/N.

Representative values

The tables below collect values from the Wikipedia list and the main thermal conductivity article, at or near room temperature unless noted. Where a source gives a range, the range is shown. They are for orientation, not design; for any real calculation, use data for the specific grade, temperature and condition.

Metals

The ranking is the one Ingenhousz found in 1780 for the metals he tested, with silver first, copper second and gold third. Alloys conduct much less well than pure metals, because alloying atoms scatter the electrons. Stainless steel and titanium alloys are poor conductors for metals, which is useful when you want strength without heat leakage, as in cookware handles or cryogenic supports.

Crystals, carbon forms and ceramics

These materials are electrical insulators or semiconductors, so their heat is carried by phonons. The top of this table shows what light atoms, strong bonds and a clean lattice can do.

Building and everyday materials

Concrete and brick ranges are wide because these are composites whose density, moisture and aggregate vary. Wood's conductivity depends on species, density, moisture and grain direction; it conducts better along the grain than across it.

Insulators

Every good insulator in this table works by trapping gas in tiny pores or between fibers. The solid matrix is kept as thin and sparse as possible, and the pores are small enough to suppress convection. The result is a conductivity close to, and in the case of aerogel even slightly below, that of still air.

Liquids

Water's conductivity has an unusual maximum as temperature rises, according to the TPRC estimates in the list, unlike most liquids.

Gases

Hydrogen conducts heat about seven times as well as air, because its light molecules move so fast. Argon, a heavy gas, conducts about a third less than air, which is why argon is used to fill the gap in double-glazed windows. Air's conductivity also varies with temperature; the list's TPRC values rise from about 0.0092 at 100 K to about 0.087 at 1,500 K.

The ladder of magnitudes

Putting the classes side by side shows how wide the range really is. Each step in this ladder is roughly a factor of ten:

Hydrogen is the odd one out among gases, sitting a full step above air. Water is the odd one out among common liquids, sitting near the top of its class.

Air: the most-quoted number

Air appears in almost every heat transfer calculation, so it gets special treatment in the Wikipedia list. Three details stand out.

First, even "standard" air varies. The list gives typical values by location and season of about 0.0243 to 0.0266 W·m⁻¹·K⁻¹ at sea level at 30 degrees north latitude, and about 0.0229 at 60 degrees north in January, reflecting the temperature dependence of the gas.

Second, air in very thin gaps or at reduced pressure conducts less than bulk air. When the gap between two surfaces approaches the distance a molecule travels between collisions, molecules carry heat directly from wall to wall and the bulk value no longer applies. That is what the Lasance approximation in the list captures. It matters in electronics, where tiny air gaps between a component and a heat sink can dominate the thermal resistance, and in vacuum insulation, where lowering pressure suppresses gas conduction.

Third, the list includes historical measurements by Taylor, from 1920, on metal laminates with air between the layers. Under pressure, at about 358 K, these laminates showed effective conductivities of about 0.43 to 1.65 W·m⁻¹·K⁻¹, far below the metal and far above air: the contacts between layers, not the metal itself, controlled heat flow. The same principle governs bolted joints, stacked transformer laminations and any assembly where heat must cross an interface.

Moisture and porous materials

The list makes the effect of water on porous materials visible. Moist wood with 12 percent or more moisture is listed at about 0.0909 to 0.4 W·m⁻¹·K⁻¹, compared with about 0.089 to 0.102 for dry pine across the grain. Wallboard measured in 1929 sits at about 0.058 to 0.064. Lightweight concrete ranges from about 0.16 to 0.51 depending on density, compared with about 1.28 for ordinary concrete. In each case the solid matrix is the same kind of material, and the conductivity is set largely by how much of the pore space is filled with air, how much with water and how dense the solid is. The same physics, at much larger scale, governs the thermal behavior of soils and rocks.

Temperature dependence

The Wikipedia list notes that for most materials conductivity varies with temperature, usually nonlinearly. The analytical section shows just how dramatic that can be.

Pure metals at low temperature

Pure metals reach very high conductivities at cryogenic temperatures, because the lattice vibrations that scatter electrons die away, while impurity scattering remains small. The thermal conductivity article notes that many pure metals peak between 2 K and 10 K. Values from the list's TPRC and CRC data:

The peak values depend heavily on purity, so different samples of the "same" metal can differ by large factors at low temperature. At room temperature and above, the differences between samples are much smaller. Iron, according to the list, ranges from about 149 W·m⁻¹·K⁻¹ at 2 K to about 997 near 20 K and falls to about 30 by 900 K.

Alloys and nonmetals

In alloys, conductivity usually rises with temperature, according to the thermal conductivity article. For nonmetals, conductivity is roughly constant at high temperatures and decreases at temperatures well below the Debye temperature. Glass shows the gradual rise typical of disordered materials: the list's Pyrex 7740 values, from a 1966 National Bureau of Standards source, rise from 0.58 W·m⁻¹·K⁻¹ at 100 K to 1.89 at 800 K.

Ice

Ice becomes a better conductor as it gets colder. The list cites values of about 2.2 to 2.6 W·m⁻¹·K⁻¹ near 273 K, rising to about 3.9 near 173 K, depending on the model used.

Structure matters: isotopes, direction and form

Isotopically pure diamond

Natural carbon is about 98.9 percent carbon-12, with the rest mostly carbon-13. The mass difference scatters phonons. The thermal conductivity article reports that at about 100 K, natural type IIa diamond reaches about 10,000 W·m⁻¹·K⁻¹, 99.9 percent isotopically enriched synthetic diamond reaches about 41,000, and about 200,000 has been predicted for 99.999 percent carbon-12 at 80 K. The list's own diamond table is jumbled in places, so its figures should be checked against the cited sources, but the lesson is clear: tiny amounts of disorder matter enormously in the best crystals.

Anisotropy

Many materials conduct differently in different directions. Graphite is the extreme case: its layers conduct heat very well in-plane and poorly between layers. The thermal diffusivity table on Wikipedia gives pyrolytic graphite a diffusivity of 1,220 mm²/s parallel to the layers and only 3.6 normal to them, a ratio of more than 300. Wood conducts better along the grain than across. Sapphire differs by about 10 percent between its crystal axes. Laminates, cables, fiber-reinforced composites and cold-pressed metals are also anisotropic. For such materials, a single number in a list is incomplete; the direction must be stated.

Bulk versus individual

Carbon nanotubes illustrate how form affects performance. Individual single-wall tubes reach about 3,500 W·m⁻¹·K⁻¹ at 300 K and multi-wall tubes about 3,180, but bulk materials made of nanotubes range from about 2.5 for multi-wall mats to about 200 for aligned single-wall material. Contacts between tubes, not the tubes themselves, limit the bulk value.

Thermal diffusivity alongside conductivity

Because diffusivity determines how fast temperatures change, it is often the more useful number for transient problems. Wikipedia's thermal diffusivity table gives, in square millimeters per second:

Note that air, a terrible conductor, has a diffusivity close to that of iron. Air stores very little heat per unit volume, so the little heat it conducts changes its temperature quickly. Water, a reasonably good conductor for a nonmetal, has a very low diffusivity because it stores so much heat.

Worked examples

Example 1: laser flash timing (hypothetical sample sizes)

Using the laser flash formula t½ = 0.1388·d²/a and the diffusivities above, consider samples 2 millimeters thick:

The calculation shows why laser flash instruments need fast detectors for metals and why thin samples are preferred for good conductors: the signal for a 2 mm copper disc is over in milliseconds.

Example 2: heat loss through a wall layer (hypothetical)

Suppose a 10 m² panel has a steady 20 K temperature difference across it, and consider the conduction through the material layer alone, ignoring air films at the surfaces. Using conductivities within the listed ranges:

The first four rows show why insulation works: 100 mm of polystyrene carries about one eighteenth of the heat of 200 mm of concrete. The last row looks absurd, and it is. A single pane of glass does not really lose 45 kilowatts, because the still air films on each surface add far more resistance than the glass itself. This is a reminder of the R-value caveats noted in Wikipedia's article on that subject: material conductivity is only one part of real heat loss, alongside surface films, radiation, air leakage and parallel paths through framing.

Example 3: how much better is copper than air?

Taking 401 W·m⁻¹·K⁻¹ for copper and 0.025 for air, the ratio is about 16,000. The thermal conductivity article says, more cautiously, that copper's conductivity is over 10,000 times that of air under standard conditions, and that values across materials span at least four orders of magnitude. Natural diamond at about 2,200 is roughly 5.5 times copper.

Using the numbers: applications

Buildings

Insulation standards and energy codes are built on conductivity and R-values. Wikipedia's R-value article gives typical United States R-values per inch: vacuum insulated panels about R-45, aerogel about R-10 to R-30, polyurethane and phenolic foams about R-7, expanded polystyrene about R-4, fiberglass and rock wool roughly R-2.5 to R-4, snow about R-1 and brick about R-0.2. The same article cautions that installation, compression, temperature, aging and test conditions all change real performance; fiberglass batts nominally rated R-13 are reported at R-14 at −12 °C and R-12 at 43 °C.

Electronics cooling

A modern processor produces concentrated heat that must cross several layers to reach the air: silicon, a thermal interface material, a metal heat spreader, another interface, and a heat sink. The heat sink is aluminium or copper because of their high conductivity. The weak links are the interface materials. The list gives thermal greases at about 0.4 to 3.0 W·m⁻¹·K⁻¹ and conductive epoxies at 0.682 to 4.8, hundreds of times lower than the metals on either side. That is why interface layers are made as thin as possible and why research into materials such as graphene, boron arsenide and cubic boron nitride attracts so much interest.

Cryogenics

Designers of cryogenic equipment exploit both extremes of the low-temperature data. Supports and pipes that must carry little heat into a cold region use stainless steel or titanium alloys, whose conductivities stay low. Thermal links that must carry heat efficiently use high-purity copper or aluminium, whose conductivities soar at low temperature. Because those peak values depend so strongly on purity, cryogenic engineers rely on data for the specific grade rather than generic list values.

Cooking, clothing and everyday life

The same numbers explain everyday experience. Copper and aluminium pans spread heat evenly; stainless steel pans often have copper or aluminium cores for that reason. Wooden spoons and plastic handles stay cool. Wool and down keep us warm by trapping air, and lose much of their insulation when wet, because water conducts heat more than twenty times better than air. Snow insulates the ground below it, as its low conductivity in the list (as low as 0.05 for dry snow) suggests.

Earth science and soils

Rock and soil conductivities determine how heat flows out of the Earth, how deep permafrost extends and how ground-source heat pumps are sized. The list includes rock and soil data from sources such as Horai (1971) on rock-forming minerals, Robertson's formulas for rocks and soils, Birch and Clark on granite and gabbro, and Kersten's and Andersland's soil measurements via U.S. Army Cold Regions Research and Engineering Laboratory reports.

How to read a conductivity list wisely

The Wikipedia list is a compilation, and compilations carry the uncertainties of their sources. Some practical rules follow from the material above.

1. Check the temperature. The sortable list is for about 293 K. Values at other temperatures can differ greatly, especially for pure metals at low temperature.
2. Check the purity and form. "Copper" can mean oxygen-free high-conductivity copper, an alloy or a powder compact. "Diamond" can mean natural, synthetic, isotopically enriched or polycrystalline.
3. Check the direction. For graphite, wood, composites and many crystals, conductivity depends on direction.
4. Check the moisture. Porous materials such as wood, brick, concrete and soil conduct much better when wet.
5. Check for convection. Values for gases and liquids assume conduction only. In practice, moving fluids carry far more heat by advection, which the list explicitly warns about for gases.
6. Check the source. The list's editors flag transcription errors and inconsistencies in older handbooks and data series. Where a value matters, trace it back to the original measurement.
7. Expect ranges. A spread such as 0.8 to 2.5 for concrete is not sloppiness; it reflects real variation among materials sharing a name.

Common misconceptions

"Metal feels colder because it is colder"

On a cold day, metal and wood outdoors are at the same temperature. Metal feels colder because its high conductivity, and its higher effusivity, draws heat from your skin faster, cooling the skin surface more.

"Good electrical conductors are always good thermal conductors, and vice versa"

The Wiedemann-Franz law links the two for metals. But diamond, an excellent electrical insulator, is one of the best thermal conductors known, because phonons rather than electrons carry its heat. The link holds for metals, not for all materials.

"Air is a good insulator, so a big air gap is a good insulator"

Still air is a good insulator; moving air is not. In a large gap, convection currents carry heat across, which is why insulation divides air into tiny pockets.

"A material's conductivity is a fixed number"

Conductivity depends on temperature, purity, structure, direction and, for porous materials, moisture. A list value is a value under stated conditions.

"Higher conductivity always means faster heating"

How fast a material heats depends on its diffusivity, which includes heat capacity. Air has a low conductivity but heats quickly because it stores little heat; water has a higher conductivity but heats slowly.

Frequently asked questions

What is the best thermal conductor?

Among bulk materials at room temperature, diamond, with natural diamond at about 2,200 W·m⁻¹·K⁻¹. Graphene measured near room temperature reaches about 4,840 to 5,300 in-plane. Among pure metals, silver, at about 429.

What is the worst?

Among ordinary materials, gases and highly porous solids. Silica aerogel at about 0.02 W·m⁻¹·K⁻¹ and argon at about 0.016 to 0.018 are among the lowest values in the list. A vacuum has no conduction at all, though heat still crosses it by radiation.

Why does the list give ranges for so many materials?

Because materials sharing a name vary in purity, density, moisture, grade and structure, and because different measurements and sources disagree. The editors of the Wikipedia list also flag transcription errors in older sources.

How is thermal conductivity measured today?

By steady-state methods such as guarded hot plates and comparative bars, and by transient methods including the hot-wire and needle-probe methods and laser flash analysis. Laser flash measures diffusivity, from which conductivity is calculated using density and specific heat.

What is the Lorenz number?

The constant in the Wiedemann-Franz law relating thermal and electrical conductivity in metals. Its theoretical value is about 2.44 × 10⁻⁸ W·Ω·K⁻².

How do I convert BTU-based conductivities to SI?

Multiply BTU·h⁻¹·ft⁻¹·°F⁻¹ by about 1.73 to get W·m⁻¹·K⁻¹. A value of 230 W·m⁻¹·K⁻¹, for instance, is about 133 in BTU units.

Why is water such a good conductor compared with other liquids?

Water's conductivity of about 0.6 W·m⁻¹·K⁻¹ is high for a liquid, though the mechanisms of liquid conduction are less well understood than those in solids and gases. Its very high heat capacity, by contrast, gives it a low diffusivity.

Summary

Thermal conductivity is the single coefficient in Fourier's law that tells how readily a material carries heat. Across the materials in Wikipedia's list, it ranges over at least five orders of magnitude, from gases and aerogels near 0.02 W·m⁻¹·K⁻¹ to diamond and graphene in the thousands. The spread reflects how heat is carried: by electrons in metals, by lattice vibrations in crystals and by molecular collisions in gases. The values change with temperature, sometimes dramatically, and with purity, isotopic composition, direction, porosity and moisture. Measurement has evolved from Ingenhousz's wax-coated wires of 1780 through Fourier's theory of 1822 to transient hot-wire methods and laser flash analysis. A list of conductivities is an indispensable starting point for engineers, scientists and curious readers, provided it is read with attention to the conditions behind each number.

Sources

• List of thermal conductivities (Wikipedia)
• Thermal conductivity and resistivity (Wikipedia)
• Wiedemann–Franz law (Wikipedia)
• Laser flash analysis (Wikipedia)
• Transient hot wire method (Wikipedia)
• Thermal diffusivity (Wikipedia)
• R-value (insulation) (Wikipedia))
• Joseph Fourier (Wikipedia)
• Thermal conduction (Wikipedia)
• Dong, Y., McCartney, J.S. & Lu, N. (2015). Critical Review of Thermal Conductivity Models for Unsaturated Soils (eScholarship)