Innovation·Powers the better World
Let’s Start with the Short Answer
Determining whether a thermal conductivity value is “low enough” requires more than looking at a single number. Two questions should be considered.
First, does the material meet the basic requirements for thermal insulation?
Under certain engineering specifications for equipment and pipeline insulation, materials with thermal conductivity values below approximately 0.12 W/(m·K) under defined operating conditions may be classified as thermal insulation materials. The exact threshold, however, depends on the applicable standard, mean temperature, material type, and intended use.
Second, how low is the value relative to other insulation materials?
In practical engineering applications, thermal conductivity values can be broadly divided into several performance ranges.
Tier 1: 0.040–0.045 W/(m·K) — Mainstream Insulation Range
Many conventional insulation materials fall within this range.
These materials often rely heavily on trapped, relatively still air for their thermal performance. Fibrous structures or polymer foams create pores or cells that restrict air movement and reduce heat transfer.
Typical materials may include certain mineral wools, expanded polymers, and other conventional insulation products.
Tier 2: 0.030–0.035 W/(m·K) — High-Performance Conventional Insulation
Materials in this range generally use more advanced structural or formulation strategies.
For example:
Graphite-modified EPS incorporates infrared-absorbing or infrared-reflecting additives to help reduce radiative heat transfer.
PUR and PIR rigid foams may use closed-cell structures filled with blowing gases whose thermal conductivity is lower than that of air.
In this range, both material chemistry and microstructure become increasingly important.
Tier 3: Around 0.026 W/(m·K) — The Still-Air Reference
Dry, still air near room temperature has a thermal conductivity of approximately 0.026 W/(m·K).
This value is an important benchmark because many conventional fibrous and cellular insulation materials derive much of their performance from trapping air.
However, it should not be treated as an absolute physical limit for all insulation materials. Advanced materials can reduce gas-phase heat transfer below the value of unrestricted still air by controlling pore size, gas composition, pressure, or nanoscale structure.
Tier 4: Approximately 0.015–0.020 W/(m·K) — Nanoporous Insulation
High-performance nanoporous materials can achieve thermal conductivity values below that of still air.
Laboratory-scale silica aerogels may reach values of approximately 0.012–0.015 W/(m·K) under suitable conditions, while many practical aerogel composites and other nanoporous insulation materials fall in the range of approximately 0.015–0.020 W/(m·K).
Y-Warm, a flexible nanoporous thermal insulation material, has a thermal conductivity of approximately 0.020 W/(m·K) under specified test conditions.
Tier 5: Approximately 0.004–0.008 W/(m·K) — Vacuum Insulation Systems
Vacuum insulation panels, or VIPs, can achieve effective thermal conductivity values of approximately 0.004–0.008 W/(m·K).
The word “effective” is important here.
These values describe the performance of an engineered insulation system that combines an evacuated core with a gas-tight barrier. They do not represent the intrinsic thermal conductivity of a single homogeneous material.
III. What Conditions Must Be Considered?
A thermal conductivity value is meaningful only when the conditions under which it was measured are clearly understood.
Condition 1: Temperature
Thermal conductivity is temperature-dependent.
For many insulation materials, thermal conductivity increases as the mean temperature rises.
Rock wool, for example, may have a thermal conductivity of approximately 0.035–0.040 W/(m·K) near room temperature, while its value can be significantly higher at several hundred degrees Celsius.
Even around room temperature, differences in mean test temperature can affect the reported result.
For this reason, λ-values measured at different temperatures should not be compared without adjustment.
The most important question is not simply:
“What is the thermal conductivity?”
but rather:
“At what mean temperature was this value measured?”
Condition 2: Aging and Moisture Content
Some insulation materials change over time.
Rigid polyurethane and PIR foams, for example, may initially contain low-conductivity blowing gases inside their cells. As these gases gradually diffuse out and are replaced by air, thermal conductivity may increase.
This is why long-term or aged performance values are often more meaningful than freshly manufactured values.
Moisture can have an even greater impact.
Liquid water has a thermal conductivity of approximately 0.6 W/(m·K), far higher than that of still air.
If an insulation material absorbs water or becomes damp, its effective thermal conductivity may rise significantly.
Therefore:
dry-state and wet-state thermal conductivity values should not be treated as interchangeable.
Condition 3: Initial, Declared, and Design Values
The same insulation product may be associated with several different λ-values.
These may include:
Initial Value
The value measured shortly after production.
Declared Value
A manufacturer-declared performance value determined according to the relevant product standard and statistical or aging requirements.
Design Value
The value engineers use for actual design calculations after considering factors such as temperature, moisture, aging, and installation conditions.
For this reason, two data sheets may appear to report different thermal conductivity values even when the underlying materials are similar.
Condition 4: Application Requirements
A “low” thermal conductivity value has different significance depending on the application.
In building insulation, where substantial thickness may be available, a material with a λ-value of 0.035–0.040 W/(m·K) can provide excellent overall thermal resistance when used at sufficient thickness.
In apparel, footwear, gloves, aerospace, and compact equipment, however, available space may be measured in millimeters.
In these applications, the relevant engineering question becomes:
How much thermal resistance can the material provide within a very limited thickness and weight?
Y-Warm, for example, is designed as a thin, flexible insulation material for applications where space, flexibility, and wearability are important. Certain Y-Warm products are approximately 0.7 mm thick, allowing insulation to be incorporated into products without relying solely on bulky lofted structures.
For clothing systems, however, overall warmth should not be judged by material thermal conductivity alone. Garment thermal resistance, construction, fit, air layers, moisture conditions, and whole-garment measurements such as clo are also important.
Frequently Asked Questions
Q: Is a thermal conductivity of 0.04 W/(m·K) considered low?
A: Yes. A value of approximately 0.04 W/(m·K) is generally considered low compared with ordinary structural materials and falls within the normal range of conventional insulation products.
However, whether it is “low enough” depends on the application.
For building walls, where several centimeters of insulation can be used, 0.040 W/(m·K) may be entirely adequate.
For applications in which only a few millimeters of thickness are available, lower thermal conductivity can become much more important.
Q: What does the “still-air reference” of 0.026 W/(m·K) mean?
A: It refers approximately to the thermal conductivity of dry, still air near room temperature and atmospheric pressure.
It is an important benchmark because many traditional porous insulation materials work by trapping air and restricting its movement.
Advanced nanoporous materials can reduce gas-phase thermal conduction below the value of free still air by confining gas within extremely small pores.
Q: Two insulation products differ by only 0.003 W/(m·K). Is the lower value worth paying extra for?
A: It depends on the application.
If thickness can easily be increased, a modest difference in λ may be compensated for by using slightly more material.
If thickness is fixed, however, even a relatively small difference in thermal conductivity can produce a meaningful difference in thermal resistance.
The correct comparison should therefore consider:
Required R-value
Available thickness
Cost per unit area
Weight
Durability
Installation constraints
Long-term performance
rather than λ alone.
Q: Why do different manufacturers report different thermal conductivity values for similar materials?
A: Several factors can cause differences, including:
Mean test temperature
Test method
Density
Sample thickness
Moisture content
Initial versus aged performance
Declared versus design values
For a meaningful comparison, the test conditions must be comparable.
Q: What is the difference between a declared value and a design value?
A: A declared value represents a standardized product-performance value reported according to the relevant product standard.
A design value is the value used by engineers for real-world calculations after applying corrections for actual operating conditions such as temperature, moisture, aging, and installation.
The design value may therefore be higher than the declared value.
Q: If VIPs have such low thermal conductivity, why are they not used everywhere?
A: VIPs provide exceptional thermal performance, but they also have practical limitations.
These may include:
Sensitivity to puncture or barrier damage
Loss of performance if the vacuum deteriorates
Limited ability to cut or modify panels on site
Edge and joint thermal bridges
Higher cost
More complex installation requirements
Therefore, the material with the lowest effective thermal conductivity is not always the most practical choice.
Q: Are the standards for “low thermal conductivity” the same for buildings and industrial insulation?
A: No.
Building insulation is generally evaluated under conditions close to normal service temperatures, whereas industrial insulation may operate at much higher or lower temperatures.
A material with a thermal conductivity of 0.08 W/(m·K) at a mean temperature of 300°C may still be appropriate for a high-temperature industrial application even though that value would not be considered particularly low for room-temperature building insulation.
The application temperature must always be considered.
Q: Does the lowest thermal conductivity automatically mean the best and most energy-efficient insulation?
A: No.
A low λ-value primarily describes resistance to conductive heat transfer through the material.
Real system performance also depends on:
Material thickness
Radiative heat transfer
Convection
Thermal bridges
Joints and seams
Moisture
Installation quality
Aging
Mechanical durability
The lowest λ-value does not automatically produce the best real-world insulation system.
Q: Can the warmth of clothing be evaluated using thermal conductivity?
A: Thermal conductivity can be used to characterize the insulation material itself, but it does not fully describe the warmth of a finished garment.
For clothing, overall thermal resistance is often more relevant.
One commonly used unit is clo:
1 clo = 0.155 m²·K/W
A garment’s clo value reflects the thermal insulation provided by the clothing system as a whole, including material layers, trapped air, garment construction, and fit.
Conclusion
So, what thermal conductivity should be considered “low”?
There is no single universal threshold.
As a broad engineering reference:
0.040–0.045 W/(m·K) is typical of many conventional insulation materials.
0.030–0.035 W/(m·K) represents high-performance conventional insulation.
Around 0.026 W/(m·K) corresponds roughly to still air near room temperature.
0.015–0.020 W/(m·K) is characteristic of certain high-performance nanoporous insulation materials.
0.004–0.008 W/(m·K) can be achieved by vacuum insulation systems under appropriate conditions.
But no thermal conductivity value should be interpreted without considering temperature, test method, aging, moisture content, thickness, and application conditions.
In practical engineering, the best insulation material is not necessarily the one with the lowest λ-value. It is the one that delivers the required thermal resistance while meeting the real-world constraints of thickness, weight, durability, manufacturability, safety, comfort, and cost.