Expert in Optical & Thermal Measurement Solutions
Due to the availability of ASTM C518-21, we’ve updated our SAC-SINGLAS accreditation schedule to ASTM C518-21. New test reports will be issued based on ASTM C518-21 accordingly.
Please also refer to the updated sample test report.
We are a Singapore-based third-party test laboratory, providing lab test services of material optical & thermal properties. We use Xiatech TC3000E transient hotwire thermal conductivity meter, with the following key information.
The instrument is equipped with two probes, a thin film probe, and a needle probe.
The thin film probe is mainly for flat plate solid samples:
The thin film probe is mainly for bulk gel or soft soil samples:
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 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) |
| Glass | 1 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 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:
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.
OTM provides two online thermal transmittance (U-value) calculators:
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.
We are a Singapore-based third-party test laboratory, providing lab test services of material optical & thermal properties. We use Thermtest HFM-100 heat flow meter, with the following key information.
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:
Among the 3 thicknesses:
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.
The heat flow meter used by OTM supports two thickness control modes:
In practice, there are two scenarios:
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.
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:
OTM is SAC-SINGLAS (ISO 17025) accredited for insulation material thermal conductivity testing. We perform both internal and external quality assurance exercises regularly for consistent and reliable measurement accuracy.
Shown below are the photos of agar gel thermal conductivity testing using a thermal needle, according to ASTM D5334. The size of the thermal needle is 1.6 mm in diameter and 12 cm in length.
The thermal conductivity of agar gel (with 5-gram agar powder per liter of water) is tested, as part of the quality control measures when we test soil or similar gel-like materials.
We’ve helped a few customers in determining the thermal conductivity of thin materials, such as paint and coating, according to ASTM D5930.
In general, the thermal resistance of thin materials is negligible. In case it is necessary to determine the thermal conductivity of paint and coating. A pair of special samples, with thick paint or coating, need to be prepared, as illustrated below.
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.
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.
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:
Please refer to our thermal conductivity page for more details.
We are pleased to release our 2020 edition of laboratory profile, which is a 56-page comprehensive document, with rich information on our capabilities and many technical insights.
What are in the laboratory profile?
You may download a soft copy of our lab profile.
