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Thermal conductivity is one of the most important physical properties used to evaluate a material’s ability to resist heat transfer. Based on fundamental heat-transfer principles and commonly reported experimental data, this article addresses a central question:
Which materials have the lowest thermal conductivity at room temperature?
First, the Short Answer
Among solid insulation materials at approximately room temperature and atmospheric pressure, silica aerogel is widely recognized as one of the materials with the lowest thermal conductivity. Depending on composition, density, pore structure, testing conditions, and formulation, high-performance silica aerogels can exhibit thermal conductivity values in the range of approximately 0.012–0.020 W/(m·K).
Commercial aerogel blankets and nanoporous composite insulation products typically show somewhat higher values, often around 0.015–0.025 W/(m·K) depending on the product and test conditions.
For comparison, still air near room temperature has a thermal conductivity of approximately 0.026 W/(m·K).
If the definition is broadened from a material itself to an engineered insulation system, vacuum insulation panels (VIPs) can achieve effective thermal conductivities of approximately 0.004–0.008 W/(m·K) by greatly reducing gas-phase heat transfer inside an evacuated core.
At the theoretical extreme, an ideal vacuum eliminates heat transfer by gas conduction and convection, although heat can still be transferred through radiation and solid structural connections.
This raises several interesting questions:
How can a solid material achieve a thermal conductivity lower than that of still air?
And why can gases such as argon improve the thermal performance of insulated glazing compared with ordinary air?
I. First, Clarify the Definitions and Boundary Conditions
Thermal conductivity, represented by λ, describes how readily a material conducts heat and is typically expressed in W/(m·K).
In general, the lower the thermal conductivity, the greater the material’s resistance to conductive heat transfer under comparable conditions.
Thermal conductivity appears as the proportionality coefficient in Fourier’s law of heat conduction, which relates heat flux to the temperature gradient.
However, several qualifications are essential.
1. Thermal Conductivity Is Temperature-Dependent
Thermal conductivity should not always be treated as a fixed constant.
For many insulation materials, its value changes with temperature. In numerous cases, thermal conductivity tends to increase as mean test temperature rises, although the exact relationship depends on the material.
Therefore, any reported λ-value should ideally be accompanied by its test temperature or mean temperature.
2. The Measured Value Depends on the Test Method
Thermal conductivity can also vary depending on sample preparation, density, thickness, moisture content, pressure, orientation, and measurement technique.
Commonly used international test methods include:
ASTM C177 — guarded hot plate method
ASTM C518 — heat flow meter method
ISO 8302 — guarded hot plate method
When comparing very low-conductivity materials, it is therefore important to ensure that the values were measured under comparable conditions.
3. Porous Materials Have Multiple Heat-Transfer Contributions
The effective thermal conductivity of a porous insulation material typically reflects several contributions, including:
Heat conduction through the solid framework
Heat conduction through the gas inside the pores
Possible convective effects in sufficiently large pores
Radiative heat transfer through the material
The design of ultra-low-conductivity insulation materials therefore involves suppressing several heat-transfer pathways simultaneously.
II. From Metals to Vacuum: Typical Thermal Conductivity Ranges
The following values are approximate and are intended primarily to illustrate differences in magnitude. Unless otherwise noted, they refer to conditions near room temperature.
Metals: Approximately 10–400+ W/(m·K)
Metals are generally excellent heat conductors.
Typical values include:
Silver: approximately 429 W/(m·K)
Copper: approximately 400 W/(m·K)
Aluminum: approximately 237 W/(m·K)
Their high thermal conductivity is largely associated with the efficient transport of energy by free electrons.
Common Solids and Liquids: Approximately 0.1–10 W/(m·K)
Examples include:
Glass: approximately 1 W/(m·K)
Liquid water: approximately 0.6 W/(m·K)
Ice: approximately 2.2 W/(m·K)
Wood across the grain: approximately 0.1–0.25 W/(m·K)
These values are already much lower than those of most metals, but they are still significantly higher than those of high-performance insulation materials.
Conventional Engineering Insulation: Approximately 0.025–0.050 W/(m·K)
Typical examples include:
EPS: approximately 0.033–0.040 W/(m·K)
XPS: approximately 0.028–0.040 W/(m·K)
Mineral wool: commonly around 0.030–0.045 W/(m·K)
Polyurethane insulation: often around 0.020–0.030 W/(m·K) depending on formulation and blowing gas
These materials achieve low thermal conductivity largely by incorporating large volumes of low-conductivity gas into their internal structure.
Still Air: Approximately 0.026 W/(m·K)
Still air is an important reference point in insulation science.
Many traditional fibrous and cellular insulation materials work primarily by trapping relatively motionless air and limiting convection.
However, the gas inside those pores still conducts heat.
This is one reason why simply increasing porosity does not indefinitely reduce thermal conductivity.
Low-Conductivity Gases
Certain gases have lower thermal conductivity than air.
Approximate values near room temperature include:
Carbon dioxide: approximately 0.016–0.017 W/(m·K)
Argon: approximately 0.017–0.018 W/(m·K)
Krypton: approximately 0.009–0.010 W/(m·K)
Xenon: approximately 0.005–0.006 W/(m·K)
This is why gases such as argon and krypton are sometimes used in high-performance insulated glazing.
However, gas thermal conductivity is influenced by several molecular properties and should not be explained solely by molecular weight.
Nanoporous Insulation: Approximately 0.012–0.025 W/(m·K)
High-performance nanoporous materials can reduce gas-phase heat conduction below that of unrestricted still air.
Examples include:
Silica aerogels
Nanoporous silica materials
Certain advanced flexible nanoporous insulation materials
Nanoporous composite insulation systems
Depending on formulation and test conditions, these materials may achieve thermal conductivity values in the range of approximately 0.012–0.025 W/(m·K).
Vacuum Insulation Systems: Approximately 0.004–0.008 W/(m·K)
Vacuum insulation panels can reach considerably lower effective thermal conductivity values because the internal vacuum strongly suppresses gas conduction and convection.
Typical VIP systems may achieve approximately:
0.004–0.008 W/(m·K)
under appropriate conditions.
High-vacuum multilayer insulation used in aerospace and cryogenic systems can achieve extremely low apparent or equivalent heat-transfer rates. However, these systems should not be directly compared with ordinary solid insulation materials using a single intrinsic thermal-conductivity value, because their performance depends strongly on vacuum level, layer configuration, boundary temperatures, and system geometry.
III. Why Can Aerogels Have Lower Thermal Conductivity Than Still Air?
One of the keys lies in nanoscale pore structure.
At room temperature and atmospheric pressure, gas molecules travel a characteristic average distance between collisions known as the mean free path. For air, this distance is on the order of tens of nanometers.
When pore dimensions become comparable to or smaller than the mean free path of the gas molecules, collisions between gas molecules and pore walls become increasingly important.
As a result, gas molecules become less effective at transporting thermal energy through the pore space.
This phenomenon is commonly associated with the Knudsen effect.
In simplified terms:
When pores become sufficiently small, gas-phase heat conduction can be reduced below that of unrestricted still air.
Aerogels also typically possess very high porosity, often exceeding 90%.
Because the solid skeleton occupies only a small fraction of the total volume, solid-phase heat conduction can also be reduced.
At the same time, the tortuous structure of the solid framework increases the complexity of heat-transfer pathways.
Radiative heat transfer can be further controlled through material composition, pore structure, and, in some formulations, infrared opacifiers.
The exceptionally low thermal conductivity of aerogel therefore does not arise from one mechanism alone.
It results from the combined suppression of:
solid conduction, gas conduction, convection, and radiative heat transfer.
IV. Three Important Qualifications When Discussing the “Lowest” Thermal Conductivity
The phrase “lowest thermal conductivity” sounds straightforward, but meaningful comparisons require clearly defined boundary conditions.
Qualification 1: Temperature Matters
Thermal conductivity depends on temperature.
A material that performs exceptionally well near room temperature may behave differently at cryogenic or elevated temperatures.
At higher temperatures, radiative heat transfer can become increasingly important, while at low temperatures the relative contributions of different heat-transfer mechanisms may change.
Therefore, there is no meaningful universal ranking without specifying the temperature range.
Qualification 2: Intrinsic Material Properties and System-Level Performance Are Different
Aerogel is a porous solid material.
A VIP is an engineered system containing a core material inside a gas-tight barrier under reduced pressure.
MLI is another system that depends on multiple reflective layers and high vacuum.
Their reported performance values therefore represent different physical configurations.
A VIP may achieve an effective thermal conductivity far below that of its core material, but its performance depends on maintaining the vacuum. If the barrier is damaged and the vacuum deteriorates, thermal performance can decline substantially.
For this reason, material-level λ-values and system-level effective thermal conductivity should not be treated as directly interchangeable.
Qualification 3: The Lowest Thermal Conductivity Is Not Always the Best Engineering Solution
A material or gas with an extremely low thermal conductivity may still be impractical because of:
Cost
Mechanical strength
Thickness
Flexibility
Manufacturability
Durability
Moisture sensitivity
Vacuum requirements
Sealing requirements
Weight
Safety
Application constraints
For example, xenon has very low thermal conductivity, but its high cost limits widespread use.
Similarly, vacuum insulation panels provide outstanding thermal performance but require a sealed vacuum enclosure and are vulnerable to puncture or barrier damage.
Therefore, engineers usually ask a broader question:
Which material provides the best balance of thermal conductivity, thickness, weight, durability, processability, cost, and application compatibility?
V. An Engineering Perspective on Ultra-Low-Conductivity Materials
If the discussion is limited to insulation materials that can function as solid or flexible material layers near room temperature, several categories stand out.
Silica Aerogel
Silica aerogel is among the best-known solid materials with ultra-low thermal conductivity.
Typical high-performance values may fall in the range of approximately:
0.012–0.020 W/(m·K)
However, conventional monolithic silica aerogel is inherently brittle.
For practical engineering applications, aerogel is therefore often incorporated into blankets, fibers, or composite structures to improve handling and mechanical durability.
Flexible Nanoporous Insulation
Flexible nanoporous materials represent another approach to combining low thermal conductivity with mechanical flexibility and processability.
For applications such as apparel and footwear, properties including:
Flexibility
Sewability
Cutability
Low thickness
Moisture management
Durability
may be just as important as achieving the absolute lowest λ-value.
For example, Y-Warm, a flexible nanoporous thermal insulation material, has a thermal conductivity of approximately 0.020 W/(m·K) under specified test conditions.
Its engineering significance therefore lies not simply in pursuing the lowest possible thermal conductivity, but in combining low thermal conductivity with flexibility, low thickness, and compatibility with wearable-product manufacturing.
Polyurethane Foam
High-performance rigid polyurethane insulation typically has thermal conductivity values around:
0.020–0.030 W/(m·K)
depending on formulation, blowing agent, density, temperature, and aging conditions.
It remains one of the most widely used high-performance conventional insulation materials, particularly in buildings, refrigeration, and industrial applications.
Conclusion
There is no single answer to the question:
“Which material has the lowest thermal conductivity?”
The answer depends on whether we are comparing:
Pure materials
Composite materials
Gases
Porous solids
Vacuum-assisted insulation systems
Cryogenic systems
Room-temperature engineering products
At room temperature, silica aerogel and other high-performance nanoporous materials are among the solid insulation materials with the lowest known thermal conductivity, while vacuum insulation systems can achieve even lower effective values by suppressing gas-phase heat transfer.
From an engineering perspective, however, the most important question is usually not:
Which material has the absolute lowest thermal conductivity?
but rather:
Which material provides the required thermal performance while also meeting the application's requirements for thickness, weight, flexibility, durability, manufacturability, safety, and cost?
That distinction is essential when moving from laboratory material properties to real-world thermal insulation design.