The method described in the post is only applicable to one single layer of coating. For a surface coated with two or more layers of coatings, the method can only measure the top layer, but cannot determine the apparent thermal conductivity of multiple layers.
To determine the apparent thermal conductivity, the calculation method needs to be used. In this post, we use a two-layer coating system as an example:
Measure the thermal conductivity of each layer individually (k1 and k2). Note: the samples need to be prepared individually too, as the method can measure the top layer only.
Calculate the overall thermal resistance: R = d1/k1 + d2/k2, where d1 and d2 are the thickness of each layer.
Calculate the apparent thermal conductivity: k = (d1 + d2)/R
For the calculation of the overall thermal resistance, our online ETTV U-value calculator can be used. The R-value result reported is the overall thermal resistance of all layers in the system.
Thermal conductivity, thermal resistance, and thermal transmittance are the 3 commonly used properties related to material or system thermal insulation performance. The 3 properties and their differences are explained below.
Thermal conductivity (K-value)
Thermal conductivity represents the ability of a material to conduct heat.
Thermal conductivity is also called K-value and its unit is W/(m⋅K). The smaller the thermal conductivity, the better the thermal insulation performance.
Listed in the table below are the typical thermal conductivity ranges of selected building material types.
Building material type
Typical thermal conductivity range
Insulation material (e.g. polyurethane/polystyrene foam, mineral wool)
0.02 – 0.04 W/(m⋅K)
Wood, plywood, and gypsum board
0.1 – 0.5 W/(m⋅K)
Coating (e.g. paint), plastics, and rubber
0.1 – 0.5 W/(m⋅K)
Concrete (light weight or heavy weight), brick, and tile
0.5 – 2.5 W/(m⋅K)
Metal (e.g. stainless steel or aluminum)
15 – 200 W/(m⋅K)
Thermal conductivity is typically for homogeneous materials. For inhomogeneous materials (e.g. concretes or composite panels), their average thermal conductivity is referred to as the apparent thermal conductivity.
Thermal resistance (R-value)
Thermal resistance represents the ability of a material layer to resist heat transmission. Thermal resistance is calculated as:
Thermal resistance is also called R-value and its unit is (m2K)/W. The greater the thermal resistance, the better the thermal insulation performance.
Thermal resistance is always in terms of one or multiple material layers with fixed thicknesses:
Material layer: thermal resistance is applicable to layer-by-layer structures only.
Example 1: for a wall system with 3 layers: concrete + rock wool insulation + plaster, there is a thermal resistance for each material layer (or multiple layers combined together).
Example 2: for studs, frames and fasteners, and other materials without layered structure, there is no thermal resistance for such materials
Fixed thickness: thermal resistance is dependent on thickness.
Example: the insulation performance of a thin insulation material could be worse than a thick conductive material (e.g. 1 mm thick polystyrene foam vs. 100 mm thick glass)
Thermal transmittance represents the ability of a wall, roof or fenestration system to transmit heat. Thermal transmittance is calculated as:
Thermal transmittance is also called U-value and its unit is W/(m2K). The smaller the thermal transmittance, the better the thermal insulation performance.
Thermal transmittance is always in terms of a complete wall/roof/fenestration system and air-to-air thermal transmission.
Complete wall/roof/fenestration system: thermal transmittance is applicable to a complete wall/roof/fenestration system only.
There is no thermal transmittance of an individual material layer unless the wall/roof/fenestration system is formed by 1 material layer only
Example 1: for a wall system with 3 layers: concrete + rock wool insulation + plaster, there is a thermal transmittance of the complete system only, but no thermal transmittance of each material layer.
Example 2: for a double glazing unit (DGU) system with 3 layers: glass + air gap + glass, there is a thermal transmittance of the complete DGU system only, but no thermal transmittance of each layer.
Air-to-air thermal transmission: thermal transmittance includes thermal transmission through both indoor and outdoor air layers.
The indoor and outdoor air layers adjacent to a wall/roof/fenestration system introduce some additional thermal resistance too (called surface film resistance). The additional thermal resistances by air layers are always included in thermal transmittance calculations.
Thermal transmittance is therefore always dependent on the environmental conditions (such as wall/roof/fenestration orientation, indoor/outdoor airflow speed, and indoor/outdoor surface emissivity).
Example: a wall system and a roof system made of the same materials may be with different thermal transmittances, due to the different airflow speeds along a vertical surface (for a wall) and a horizontal surface (for a roof).
OTM provides two online thermal transmittance (U-value) calculators:
Thermal conductivity (K-value) vs. thermal resistance (R-value)
Thermal conductivity is independent of thickness; thermal resistance is dependent on thickness.
Thermal conductivity is for homogeneous materials (or averaged for inhomogeneous materials); thermal resistance is for material layers.
Thermal resistance (R-value) vs. thermal transmittance (U-value)
Thermal resistance can be in terms of an individual material layer in a wall/roof system; thermal transmittance is always in terms of a complete wall/roof/fenestration system and there is no thermal transmittance of an individual material layer (unless the wall/roof system is made of a single material layer only).
Thermal resistance excludes the effect of indoor and outdoor air layers; thermal transmittance includes the effects of indoor and outdoor air layers (air-to-air thermal transmission).
Thermal resistance is independent of environmental conditions; thermal transmittance is dependent on environmental conditions.
The discussions above follow typical engineering practices. In the academic context, the practices could be different.
If we use double glazing unit (DGU) glasses as an example, the engineering practice is to use its thermal transmittance (U-value), instead of the other two, for performance evaluations, though in the academic context it is still correct to calculate the apparent thermal conductivity and thermal resistance of a DGU glass.
In thermal conductivity testing according to ASTM C518, a test sample is clamped between two plates and compressed to certain thickness.
Some customers are concerned if thermal conductivity results are affected by the compression. This article aims to provide some explanations to this concern.
Conceptually, there are 3 thicknesses for a sample:
Uncompressed thickness: the thickness of a sample in the free state without compression;
Installation thickness: the thickness of a sample in the intended installation. In an installation, the sample may or may not be compressed;
Testing thickness: the thickness of a sample during thermal conductivity testing. During testing, a sample is always compressed for good thermal contact.
Among the 3 thicknesses:
The installation thickness could be the same as the uncompressed thickness (if the sample is not compressed in an installation), or smaller than the uncompressed thickness (if the sample is compressed in an installation).
The testing thickness is always smaller than the uncompressed thickness, as a sample is always compressed during testing.
The testing thickness should be as close to the installation thickness as possible, for fair product performance rating.
Testing thickness and thermal conductivity
When a sample is compressed to a smaller thickness, its density increases and the increased density affects the thermal conductivity measurement result.
A sensitivity study was performed by OTM in 2020. In the study, when the sample was compressed by 10%, the result variation was less than 1.7%. The thermal conductivity measurement result is not so sensitive to the testing thickness variation. If the compression is small (e.g. less than 5%), the result variation is negligible for general engineering applications.
Determination of testing thickness
The heat flow meter used by OTM supports two thickness control modes:
Automatic thickness: a sample is compressed by the instrument with a constant pressure of approximately 2.5 kPa and the sample thickness under compression is automatically measured as the testing thickness.
Manual thickness: a thickness is input manually and the sample is compressed to the manually input thickness as the testing thickness (provided that the sample can be compressed to this thickness with less than 2.5 kPa of pressure).
In practice, there are two scenarios:
Rigid or firm materials
Testing thickness of rigid or firm materials
For rigid materials (e.g. polystyrene foam or polyurethane foam) or firm materials (e.g. high-density rockwool), they cannot be compressed significantly in nomral installations (e.g. more than 5% of compression).
The testing thickness of a rigid or firm material is determined with the automatic thickness mode mentioned above.
Testing thickness of soft materials
For soft materials (e.g. low-density rockwool or glasswool), they can be compressed significantly in normal installations (e.g. more than 10% of compresssion).
The testing thickness of a soft material is determined with the manual thickness mode mentioned above.
The customer needs to declare the installation thickness of a soft material sample. The declared installation thickness will be used as the testing thicknes.
If an installation thickness is not declared, we will assume that the installation thickness is the same as the sample nominal thickness. If the installation thickness is the same as the uncompressed thickness, the sample may be compressed by up to 5% for good thermal contact.
OTM participated in a proficiency testing (PT) program recently, with satisfactory results. A PT program is for test quality evaluation of laboratories. Typically, samples with known results are distributed by the PT organizer to a group of participating laboratories. The results tested by the laboratories are compared by the PT organizer to evaluate the test quality of each laboratory.
Below is a summary of the PT program on thermal conductivity testing and the performance of OTM in this program:
Number of participating laboratories: 84
Test type: insulation material thermal conductivity test
Sample: polystyrene board, at 25 °C mean temperature
Average results from all laboratories: 0.0344 W/(m⋅K)
Result tested by OTM: 0.0344 W/(m⋅K), with Z-score = 0
Substrate (the blue color part): the substrate needs to be flat and rigid. The substrate type does not affect the measurement result. Typical substrate types include glass, metal plate, and wood plate.
The preferred substrate size is 50 mm x 50 mm; The minimum size is 30 mm x 30 mm and the maximum size is 100 mm x 100 mm.
Thick paint or coating (the yellow color part): the paint or coating needs to be applied onto one side of the substrate, with large enough thickness
The preferred thickness is 3 mm or larger; the minimum thickness is 1 mm.
Sample quantity: two samples in a pair are needed.
Shown below is the arrangement during testing.
The measurement probe is a thin film (less than 0.1 mm in thickness, the red color part) with a tiny wire inside (refer to our thermal conductivity test page for details). The probe is sandwiched between the two test samples, next to the paint or coating material.
Because the probe is in contact with the paint or coating material and the measurement duration is very short, it is equivalent to insert a tiny wire into a bulk material block made of the paint or coating. Only the thermal conductivity of the paint or coating is measured and the result is not affected by the substrate material.
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: