Thermal conductivity of thin materials (paint, coating, metal sheet)

We are sometimes requested to test the thermal conductivity of thin materials, such as paint, coating and metal sheet.

Negligible thermal resistance of thin materials

For a wall (or a roof) system, the influence of such thin materials to the overall wall system U-value is negligible.

Shown below is an example calculated with our online ETTV U-value calculator. it is obvious that the thermal resistance of a 0.2 mm thick paint layer with 0.2 W/(m⋅K) thermal conductivity is only 0.001 (m²K)/W, which is negligible comparing to the thermal resistances of other layers (e.g. concrete, plaster, or insulation wool).

For thin metal sheets, e.g. 0.7 mm thick aluminium plates, the thermal resistance is further smaller, as the thermal conductivity of metal is much larger.

The reason is that thermal resistance is dependent on both thermal conductivity and thickness, with the following relationship:

Thermal resistance = Thickness / Thermal conductivity

In practice, due to the small thickness of thin materials (typically less than 1 mm), it is not practical to reduce the thermal conductivity of thin materials to achieve better insulation.

In wall/roof U-value calculations, the thin materials can be simply ignored. It is not meaningful to get the thermal conductivity of thin materials.

Thin material thermal conductivity testing

It may be still necessary to determine the thermal conductivity of thin materials. For example, the thin material is not used in a wall/roof system, but in a system with low thermal resistance.

For such scenarios, we can test the thermal conductivity of thin materials according to ASTM D5930, with the following practices:

• For low thermal conductivity paint and coating, the paint/coating can be applied onto flat substrates for testing.
  • There is no specific requirement on the substrate type, as long as the substrate surface is flat. We recommend flat metal plates or glass plates.
  • The preferred substrate size is 50 mm x 50 mm (minimum size: 30 mm x 30 mm; maximum size: 100 mm x 100 mm). There is no requirement on the substrate thickness, as long as it is strong enough.
  • The thickness of paint or coating should be thick enough (3 mm or thicker preferred, 1 mm minimum)
  • 2 pieces of painted/coated samples are requried
• We cannot test materials with large thermal conductivity [i.e. > 10 W/(m⋅K)], for example, aluminium or stainless steel.

Please refer to our thermal conductivity page for more details.