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CLO is widely used to describe the thermal insulation provided by clothing. However, CLO alone does not fully describe the intrinsic thermal performance of an ultra-thin insulation material such as Y-Warm.
The reason is simple: CLO measures thermal resistance, and thermal resistance depends not only on how effectively a material limits heat transfer, but also on how much material thickness is present.
Understanding this distinction is important when comparing ultra-thin insulation with conventional lofted materials such as down and synthetic fiberfill.
1. What Does CLO Measure?
CLO is a unit used to express the thermal resistance of clothing and clothing systems.
1 clo corresponds to a thermal resistance of 0.155 m²·K/W.
The relationship is:
CLO = R / 0.155
where:
R = thermal resistance, m²·K/W
A higher CLO value therefore indicates greater thermal resistance for the clothing system being evaluated.
CLO is particularly useful for assessing how much insulation a complete garment or clothing ensemble provides under defined test conditions.
2. Why Does Thickness Affect Thermal Resistance?
For a homogeneous material layer under simplified, steady-state one-dimensional heat-conduction conditions, thermal resistance can be approximated as:
R ≈ d / λ
where:
d = material thickness, m
λ = thermal conductivity, W/(m·K)
This relationship illustrates a fundamental principle:
Thermal resistance depends on both material thickness and thermal conductivity.
If two homogeneous materials have the same thermal conductivity, the thicker layer will provide greater thermal resistance.
If two layers have the same thickness, the material with the lower thermal conductivity will provide greater thermal resistance.
Therefore, thickness and thermal conductivity must be considered together.
3. Why CLO Works Well for Traditional Lofted Insulation
Traditional insulation materials such as down and synthetic fiberfill create thermal resistance primarily by forming a thick, lofted structure containing large amounts of relatively still air.
Increasing loft generally increases the effective thickness of the insulating layer.
Greater thickness, in turn, generally produces greater thermal resistance, provided that the structure continues to restrict internal air movement.
This is why CLO is highly useful when evaluating complete garments and conventional loft-based clothing systems.
4. Why Y-Warm Is Different
Y-Warm takes a different structural approach to thermal insulation.
Instead of relying on substantial loft, Y-Warm uses an engineered closed-cell porous structure with nanometer-scale cell walls to achieve low thermal conductivity within an ultra-thin material layer.
Typical Y-Warm insulation layers are approximately 0.7–1 mm thick.
Because the material itself is extremely thin, its absolute thermal resistance is necessarily influenced by the small value of d in:
R ≈ d / λ
This means that an ultra-thin layer can have a relatively modest absolute thermal resistance even when the material itself has very low thermal conductivity.
That distinction is critical.
A lower CLO value for a very thin layer does not necessarily mean that the material has poor intrinsic thermal insulation properties. It may simply reflect the limited physical thickness of the layer being measured.
5. CLO and Thermal Conductivity Answer Different Questions
CLO and thermal conductivity are related to thermal insulation, but they describe different aspects of performance.
CLO asks:
How much thermal resistance does this clothing system provide?
Thermal conductivity asks:
How readily does heat conduct through the material under defined test conditions?
A lower thermal conductivity indicates that the material conducts heat less readily.
For designers working under strict thickness or weight constraints, thermal conductivity is therefore an important material-level parameter because it helps indicate how efficiently the available thickness is being used to resist conductive heat transfer.
However, thermal conductivity should not replace CLO when evaluating a complete garment. The two metrics serve different purposes.
6. A Simple Example
Consider two hypothetical homogeneous insulation materials:
Material A
Thickness: 10 mm
Thermal conductivity: 0.040 W/(m·K)
Material B
Thickness: 1 mm
Thermal conductivity: 0.020 W/(m·K)
Using the simplified relationship:
R = d / λ
Material A:
R = 0.010 / 0.040 = 0.25 m²·K/W
Material B:
R = 0.001 / 0.020 = 0.05 m²·K/W
Material A provides greater total thermal resistance because it is ten times thicker.
However, Material B has only half the thermal conductivity.
This illustrates why total thermal resistance and intrinsic material thermal conductivity should not be treated as the same property.
The thicker material may provide greater absolute insulation, while the lower-conductivity material may provide more thermal resistance per unit thickness.
7. How Should Ultra-Thin Insulation Be Evaluated?
For an ultra-thin insulation material such as Y-Warm, no single metric provides a complete picture.
A more meaningful evaluation should consider:
Thermal conductivity (λ) — how readily heat conducts through the material
Thickness (d) — the physical thickness of the insulation layer
Thermal resistance (R) — the resistance provided by the actual layer
Thermal resistance per unit thickness — useful for thickness-constrained comparisons
CLO — useful for evaluating the insulation performance of clothing systems
Compression behavior — particularly important where insulation is subjected to pressure
Moisture performance — because moisture can significantly influence thermal comfort and insulation behavior
Complete garment performance — including construction, fit, air layers, shell fabrics, seams, and environmental conditions
The appropriate metric therefore depends on the question being asked.
CLO vs. Thermal Conductivity vs. Thermal Resistance
Metric | What It Describes | Most Useful For |
CLO | Thermal resistance of clothing or a clothing system | Garments and clothing ensembles |
Thermal conductivity (λ) | How readily a material conducts heat | Comparing intrinsic conductive behavior of materials |
Thermal resistance (R) | Resistance provided by a layer or system | Comparing actual insulation layers |
R per unit thickness | Thermal resistance relative to thickness | Thickness-constrained insulation design |
Conclusion
CLO is an important measure of clothing insulation, but it should not be used alone to judge the intrinsic thermal performance of an ultra-thin insulation material.
Traditional down and fiberfill achieve insulation largely through macroscopic loft and thickness.
Y-Warm uses a different structural approach: an engineered closed-cell porous structure with nanometer-scale cell walls designed to achieve low thermal conductivity within an ultra-thin material layer.
For this reason, evaluating Y-Warm requires distinguishing between material properties and system-level insulation performance.
The distinction can be summarized simply:
CLO describes how much thermal resistance a clothing system provides.
Thermal conductivity describes how readily heat conducts through the material.
Thickness connects the two.
For ultra-thin insulation, all three need to be considered together.