Thermal conductivity of 90° rotated mineral wool material

Related services Thermal conductivity

We noticed that some customers provided us with samples with 90° rotated mineral wool insulation material. Shown below is an example. The mineral wool material was rotated by 90°. The test result is significantly higher than the typical thermal conductivity of mineral wool materials. The reasons are explained in this post.

As shown in the sketch below. the mineral wool fibers are aligned in a layered structure.

  • In the normal installation, the heat flow direction is perpendicular to the layered structure. The thermal resistance is greater and the thermal conductivity is lower.
  • In the 90° rotated installation, the heat flow direction is parallel to the layered structure. The thermal resistance is smaller and the thermal conductivity is greater.

A comparison is presented in this page. In this study, the thermal conductivity is 39% higher when the mineral wool material is 90° rotated.

Apparent thermal conductivity of multiple layers of coatings

Related services Thermal conductivity

We have a post on the measurement of thin paint and coating thermal conductivity with the ASTM D5930 method. Many surfaces are coated with multiple layers of coatings. How to determine the apparent thermal conductivity of such multilayer coating systems?

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:

  1. 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.
  2. Calculate the overall thermal resistance: R = d1/k1 + d2/k2, where d1 and d2 are the thickness of each layer.
  3. 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.

Our instrument: hot wire thermal conductivity meter

Related services Thermal conductivity

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.

Transient hotwire thermal conductivity meter
  • Measurement principle: transient hot wire
  • Measurement range: 0.001 – 10 W/(m K)
  • Temperature range: room temperature only

The instrument is equipped with two probes, a thin film probe, and a needle probe.

Thin film probe

The thin film probe is mainly for flat plate solid samples:

  • 2 samples in a pair are needed, with the thin film probe sandwiched between the 2 samples
  • Minimum sample size: larger than 25 mm × 25 mm in length and width, and thicker than 0.3 mm in thickness
Needle probe

The thin film probe is mainly for bulk gel or soft soil samples:

  • Needle size: 1.6 mm in diameter and 12 cm in length
  • The needle is to be inserted in the sample for measurement

Related lab services

Thermal conductivity (K-value), thermal resistance (R-value), and thermal transmittance (U-value)

Related services Thermal conductivity

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 typeTypical thermal conductivity range
Insulation material (e.g. polyurethane/polystyrene foam, mineral wool)0.02 – 0.04 W/(m⋅K)
Wood, plywood, and gypsum board0.1 – 0.5 W/(m⋅K)
Coating (e.g. paint), plastics, and rubber0.1 – 0.5 W/(m⋅K)
Glass1 W/(m⋅K)
Concrete (light weight or heavy weight), brick, and tile0.5 – 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.

Thermal transmittance (U-value)

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.

Notes

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.

Our instrument: heat flow meter

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.

Thermtest HFM-100 heat flow meter

Key specifications:

  • Preferred sample size: 300 mm × 300 mm
  • Maximum sample thickness: 100 mm
  • Plate temperature range: -20 – 70 °C
  • With high thermal conductivity kit

Related lab services

Sample thickness in ASTM C518 thermal conductivity testing

Related services Thermal conductivity

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.

3 thicknesses

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
  • Soft 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.

Proficiency testing on insulation material thermal conductivity test

Related services Thermal conductivity

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/(mK)
  • Result tested by OTM: 0.0344 W/(mK), with Z-score = 0

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.

Agar gel thermal conductivity testing

Related services Thermal conductivity

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.

Needle probe in agar gel
Standalone needle probe