What Is Thermal Conductivity in Insulation?

Isotopically pure diamond, with its carbon-13 content reduced to trace levels, reaches a thermal conductivity of 3,320 W/(m-K). That is about 50 percent above the value for natural diamond, according to measurement data published by Linseis in 2025.

Air and aerogel sit around 0.01 to 0.04 W/(m-K), so a single property covers an enormous range. Anyone specifying insulation or heat sinks ends up weighing it against thermal resistance and specific heat capacity, which are related but describe storage and blocking rather than flow.

What is thermal conductivity?

Put one side of a material at a higher temperature than the other and heat flows across. How fast it flows is what thermal conductivity measures, and engineers write that number as k, sometimes lambda.

It’s related to electrical conductivity in that both describe a rate rather than a fixed amount.

A material with a high k value passes heat along quickly, while a low k value marks an insulator that resists the flow.

The property doesn’t measure how much thermal energy a material can hold, and it isn’t identical to thermal resistance either. It also isn’t a constant. Temperature and internal structure both shift it.

Thermal conductivity vs thermal resistance and thermal diffusivity

People swap these terms for each other more than they should. Each answers a different question about heat.

Thermal conductivity vs thermal resistance

Conductivity belongs to the material itself, while resistance belongs to a specific part, thickness included. Take two slabs of the same low-conductivity foam, one thick and one thin. Their k value is identical, but the thick one resists heat flow far more.

In construction that resistance gets expressed as a material’s R-value, which climbs as thickness increases even though the underlying conductivity never changes.

Higher resistance always means less heat loss through a given assembly.

Thermal conductivity vs thermal diffusivity

Diffusivity is about timing. It tells you how quickly the temperature inside a material evens out once heat starts moving through it.

Copper has both high conductivity and high diffusivity, so it heats up and cools down fast. A material can conduct decently and still have low diffusivity if its density or heat capacity is high enough to slow the temperature response.

Thermal conductivity vs specific heat capacity

Specific heat capacity says nothing about speed. It measures how much energy a material stores for each degree its temperature rises.

A high-conductivity material moves heat through fast. A high specific heat material soaks up a lot of heat before its temperature climbs much. People confuse the two constantly.

Water is a decent example of the mismatch. It stores a lot of heat but conducts it only moderately compared to metals.

In buildings, that heat-storage behavior gets described through a material’s thermal mass, a separate concept built from density and specific heat rather than from k alone.

What is the formula for thermal conductivity?

Fourier’s law of heat conduction is where the property shows up. It governs how heat flows through a solid, and it says heat flux is proportional to the negative temperature gradient, scaled by the conductivity coefficient.

q = -k (dT/dx)

In that expression q is heat flux, the rate of heat transfer per unit area, and k is the thermal conductivity coefficient of the material. dT/dx stands for the temperature gradient across it. The negative sign reflects the fact that heat flows from hot to cold, opposite the gradient direction.

Joseph Fourier published the relationship in the early nineteenth century, and it still forms the basis of nearly every conduction calculation used today.

The equation assumes steady-state, one-dimensional flow. Real materials rarely sit still that neatly, so engineers add correction factors for multiple dimensions or changing temperatures over time.

How does heat conduction happen at the particle level?

Solids don’t all move heat the same way. Which carrier does most of the work depends on the material.

Heat transfer by phonons

Ceramics and most other nonmetals carry heat as lattice vibration, quantized into units called phonons.

A hot region of a crystal vibrates more intensely than a cold one. Those vibrations pass from atom to atom along the lattice, carrying energy without carrying charge.

Phonon transport explains why diamond, a nonmetal, still reaches extremely high conductivity. Its rigid, tightly bonded lattice moves vibrational energy with very little scattering.

Anything that scatters phonons lowers conductivity. Defects and impurities do it, and grain boundaries too, which is exactly why porous or amorphous materials insulate so well.

Heat transfer by free electrons

Metals conduct heat mostly through free electrons, the same particles responsible for electrical conductivity.

These electrons move fast and collide often, and they carry kinetic energy from hot regions to cold ones far more efficiently than lattice vibrations alone.

This shared mechanism is why the Wiedemann-Franz law links a metal’s thermal conductivity to its electrical conductivity through a nearly constant ratio.

The Debye model, built around phonon behavior, explains why conductivity in most solids drops sharply as temperature falls toward absolute zero.

What units is thermal conductivity measured in?

The units follow from what’s being measured, which is energy passing through an area over time for each degree of temperature difference. In SI that comes out as watts per meter-kelvin, written W/(m-K).

One BTU/(hr-ft-F) equals roughly 1.73 W/(m-K), so converting between the two systems is a straightforward multiplication.

The kelvin, part of the International System of Units, anchors the temperature term in the SI version of the formula.

Manufacturer spec sheets sometimes list conductivity without saying which system they use. That’s annoying, and worth checking twice before comparing two products.

What factors affect thermal conductivity?

A published k value is a starting point. Several things push it around once the material is in use.

Temperature dependence

Conductivity isn’t fixed. It shifts as a material heats up or cools down, and the direction of the shift depends on the material class.

Metals typically lose some conductivity as temperature rises, since electron collisions increase.

Insulation materials behave the opposite way. Research published in the Journal of Building Engineering in 2021 found that conductivity in common insulation products rises by roughly 20 to 30 percent across typical building temperature ranges.

That swing matters for anyone comparing a lab-rated k value against real attic or wall conditions.

Density and porosity

Lower density usually means lower conductivity, up to a point, because more of the material’s volume is trapped air rather than solid matter. Solid particles conduct heat far better than air pockets do, so packing more of them into the same space pushes conductivity up. Past a certain density the relationship can reverse for some fibrous materials.

Foams and fiber batts work on this principle, and so do porous ceramics. The air trapped inside the pores conducts far worse than the solid frame around it.

Moisture content

Wet materials conduct heat noticeably faster than dry ones, and the gap is large.

The same 2021 Journal of Building Engineering review found that water’s thermal conductivity runs about 20 times higher than still air, so moisture absorption always pushes a material’s effective k value up.

A soaked batt of insulation stops performing anywhere near its rated value. This is one reason vapor control and drainage details matter as much as the insulation material itself.

Crystal structure vs amorphous structure

Ordered, crystalline materials generally conduct heat better than disordered, amorphous ones.

Amorphous materials like glass conduct heat far worse than a crystal of similar chemical composition. Manufacturers sometimes introduce disorder deliberately, breaking up a material’s lattice to push conductivity down for insulation purposes.

Which materials have high thermal conductivity and which have low?

Across common materials, thermal conductivity spans roughly five orders of magnitude, from packed metal crystals down to gas-filled foams.

Metals sit near the top of that range. Ceramics, polymers, and liquids fill in most of what’s left, with gases and engineered insulation trailing at the very bottom.

Metals and high conductivity materials

Copper and aluminum dominate practical high-conductivity applications, with diamond in an extreme category of its own.

Natural single crystals of diamond reach up to about 2,200 W/(m-K), the highest of any common bulk material, and isotopically purified diamond can exceed 3,300 W/(m-K) (Materials, 2009; Linseis, 2025). Peer-reviewed testing puts copper at roughly 405 W/(m-K) and aluminum at roughly 243 W/(m-K) (Materials, 2025).

Diamond conducts through phonons rather than electrons, which is unusual among high-conductivity materials. It’s also expensive to use at scale, whatever the performance.

Copper stays the practical choice for wiring and heat sinks, and for cookware too, since it balances conductivity against cost and workability.

Insulators and low conductivity materials

At the opposite end, still air and engineered porous materials keep conductivity extremely low by trapping gas in tiny pockets.

Aerogel sits near the bottom of that range. Its rigid, nanoporous structure traps air so effectively that aerogel insulation shows up in aerospace and extreme-cold applications where thickness has to stay minimal.

More common products work on the same principle at a lower cost. Fiberglass traps still air inside a web of glass fibers, and that trapped air is what keeps fiberglass insulation effective in residential walls and attics.

Wood and most polymers land in the middle of the range, low enough to insulate reasonably well without needing an engineered pore structure.

How is thermal conductivity measured?

The basic idea is to send a known amount of heat through a sample, record what happens to its temperature, and work backward to the k value. Methods split into steady-state and transient, and the split mostly comes down to speed versus precision.

1. Prepare a flat, uniform specimen with known thickness
2. Apply a controlled heat source to one side, or a brief energy pulse
3. Record the resulting temperature response with calibrated sensors
4. Calculate k from the heat flow, geometry, and temperature data

Steady-state test methods

Guarded hot plate testing, standardized as ASTM C177, holds both sides of a sample at fixed temperatures until heat flow stabilizes. It covers materials from about 0.02 to 200 W/(m-K), which spans aerogels on one end and moderately conductive solids on the other.

C177 is an absolute method with higher precision, and that’s why it works as the reference standard. ASTM C518, the heat flow meter method, is comparative. It’s faster and cheaper and gets calibrated against C177 data.

ASHRAE and most building codes accept either method for rating insulation products, with ASTM C177 serving as the reference when results are disputed. If two numbers disagree, I’d go with the hot plate one.

Transient test methods

Transient methods skip the wait for steady state and read the temperature response as it happens.

Laser flash analysis fires a short energy pulse at one face and times the temperature rise on the other. Transient plane source instruments, often called by the brand name Hot Disk, place a sensor between two halves of the sample.

ASTM E1461 governs the laser flash method, applicable across a temperature range of roughly 75 to 2800 K, with diffusivity measurement uncertainty typically under 3 percent.

Instrument makers such as Netzsch build commercial laser flash systems, used on metals and ceramics as well as composites.

Why does thermal conductivity matter in building insulation?

Every insulation rating a builder specifies traces back to thermal conductivity. Lower k values translate directly into lower heat loss once that property gets built into an assembly.

A design team specifies a target U-value for a wall or roof assembly, then works backward to the k values and thicknesses needed to hit it.

ASHRAE Standard 90.1 sets minimum R-value and maximum U-value requirements by climate zone for commercial buildings, and most residential codes reference similar tables.

Hitting that number on paper doesn’t guarantee hitting it in the wall. Real-world performance drops for reasons the lab test doesn’t capture. Moisture raises the effective k value of the insulation layer, and gaps or compression cut coverage. Framing members and fasteners also create paths that bypass the insulation entirely.

That last problem has a name, thermal bridging, where a more conductive material cuts straight through an otherwise well-insulated assembly.

A wall can carry a strong rated R-value on paper and still underperform once bridging and installation gaps are accounted for.

Where is thermal conductivity applied in engineering and electronics?

Beyond construction, both high and low conductivity materials end up doing real engineering work. Picking the right one is as deliberate a decision as choosing a structural material.

Electronics cooling and heat sinks

Processors and power electronics generate heat that has to leave the chip fast, or performance and lifespan both suffer.

Heat sinks use high-conductivity metal, almost always aluminum or copper, to pull heat away from a hot component and spread it across a larger surface for the surrounding air to carry off. Aluminum is lighter and cheaper, and it machines into fin arrays more easily. Copper conducts better, but it weighs more and costs more.

Many designs use a copper base bonded to aluminum fins, taking the conduction advantage of one metal and the weight advantage of the other.

Heat exchangers and heat pipes

Heat exchangers move heat between two fluids without mixing them, relying on a conductive wall between the two flow paths.

Heat pipes use a sealed tube with a working fluid inside, and repeated evaporation and condensation move heat far faster than solid conduction alone could manage.

A well-designed heat pipe can move heat at an effective rate that plain copper conduction cannot match over the same distance, which is why laptops and dense server racks rely on them instead of solid metal blocks.

Thermoelectric materials

Thermoelectric devices turn a temperature difference directly into electrical current, and the demands on thermal conductivity are different here than in cooling.

Heat sink materials have high thermal and high electrical conductivity. Thermoelectric materials keep the high electrical conductivity but need low thermal conductivity, so the temperature difference across the device doesn’t collapse before it can be converted into current.

Graphene sits at the other extreme entirely. Balandin and colleagues measured suspended single-layer graphene at roughly 5,000 W/(m-K) at room temperature (Nano Letters, 2008), among the highest values ever recorded for any material.

That makes graphene good at spreading heat fast, the reverse of what thermoelectric design calls for. What counts as a good value depends entirely on the application.

When does thermal conductivity not apply, or where does the simple model break down?

Fourier’s law assumes a single, direction-independent k value and a material that isn’t changing state. Plenty of real situations break those assumptions, and the simple model gives wrong answers when applied to them.

Anisotropic materials

Some materials conduct heat differently depending on direction, which a single k value cannot capture.

Pyrolytic graphite is the clearest example. Researchers at UCLA measured its in-plane conductivity at roughly 2,054 W/(m-K), compared with only about 5.5 W/(m-K) across the layers (Review of Scientific Instruments, 2018). That is a difference of almost four hundred times between two directions in the same block of material.

Wood behaves similarly along the grain versus across it. In composite laminates, conductivity follows the fiber layup direction, and layered crystals conduct well within a layer but poorly between layers.

Applying one k value to an anisotropic material in the wrong direction can overstate or understate heat flow by an order of magnitude or more.

Phase change and moisture effects

The simple model assumes a material stays in one phase throughout the measurement, and that assumption fails the moment melting or freezing starts.

Phase change materials break it because their energy goes into melting or freezing rather than raising temperature. Saturated or wet materials break it differently, since k keeps climbing as water content rises.

Paraffin, a common phase change material, carries a thermal conductivity of only about 0.2 to 0.3 W/(m-K) in its solid state. That is low enough that heat absorption and release become the bottleneck rather than conduction. The range is consistently reported across phase-change-material literature.

A single lab-rated k value describes neither situation accurately, since both involve a property that is actively changing rather than holding steady.

Thin films and contact resistance

Bulk k values stop applying once a material gets thin enough that its boundaries matter more than its interior.

Interfacial thermal resistance, also called contact resistance, builds up wherever two surfaces meet, even when both materials individually conduct heat well.

This matters most in electronics packaging, where a thermal interface material has to fill microscopic gaps between a chip and its heat sink. Otherwise the bulk conductivity numbers on the datasheet become irrelevant.

Measurement methods like laser flash analysis account for this by measuring diffusivity through the whole stack rather than assuming a single uniform material.

FAQ on What Is Thermal Conductivity

What symbol is used for thermal conductivity?

Most engineering references use k, while physics texts often write it as lambda in Fourier’s law. Both labels describe the same coefficient, measured in W/(m-K), regardless of which field is writing the equation.

Is thermal conductivity a scalar or a tensor?

It depends on the material. In isotropic materials it’s a scalar, a single number describing heat flow in every direction. In anisotropic materials like wood or layered crystals it becomes a tensor, with different values along different crystal axes or grain directions.

Does thermal conductivity change with pressure?

Barely, for solids and liquids, since the atoms are already packed tightly. Gases are also largely unaffected across the normal range. Kinetic theory shows that as pressure rises, the larger number of molecules is offset by a shorter mean free path between collisions, so the two effects cancel and thermal conductivity stays roughly constant.

Close to a vacuum that changes. The mean free path grows large enough to matter, and conductivity drops sharply as pressure falls, an effect exploited in vacuum-gap insulation and Pirani gauges.

What is the thermal conductivity of a vacuum?

None. Conduction needs particles to carry heat between collisions, and a true vacuum has no particles. Heat still crosses a vacuum, but only by radiation, which is why thermos flasks and double-walled containers rely on a vacuum gap for insulation.

How do you convert thermal conductivity between units?

Multiply a BTU/(hr-ft-F) value by 1.73 to get W/(m-K), or divide by that factor to go the other way. Manufacturer datasheets from different countries often mix both systems, so checking the unit before comparing numbers matters.

Can thermal conductivity be negative?

No. In ordinary materials it’s always positive, since heat flows from hot to cold, never the reverse, under the second law of thermodynamics. A negative value would mean heat concentrating on its own, which contradicts that law.

What Should You Check First When Specifying Thermal Conductivity for a Project?

A published k value only helps once it’s matched to the conditions the material will actually see, and those rarely match the laboratory conditions a datasheet lists. Temperature and moisture both shift the number, and so does orientation.

Start with the test standard behind the figure, since ASTM C177 and C518 read differently. Layered or fibrous materials need an anisotropy check before any single value gets trusted. Then adjust for the moisture and temperature range the material will meet in service.

Working from service conditions instead of the headline number makes specification slower, but it means fewer field failures once the material has to perform outside a lab.

Comparing categories against project needs comes next, and the guide to types of insulation materials covers that once the physics is settled.