Table of Contents
Principle and relevance of the heat flow meter method
The Heat Flow Meter Method (HFM) is based on the fundamental principle of heat transfer through a material. A sample is placed between two plates with defined temperatures – one with a warm surface and one with a cold surface (1). The resulting temperature difference causes heat to flow through the sample, which is precisely measured using heat flow sensors. The actual measurement takes place under stationary conditions, whereby a constant heat flow is detected.
The method guarantees high measurement accuracy with typical deviations of less than 2% and is suitable for quality monitoring in the production process of insulation boards. The specific thermal conductivity (λ, measured in W/(m-K)) is calculated using the Fourier equation:
\lambda = \frac{Q \cdot d}{A \cdot \Delta T}
\)
Where Q is the heat flow, d is the sample thickness, A is the sample area and ΔT is the temperature difference.
Measuring procedure in detail:
- The sample is placed in the measuring device and contacted on both sides by the hot and cold plates
- The panels generate a defined temperature profile
- The temperature difference and the resulting heat flow are recorded by heat flow sensors
- The specific thermal conductivity is calculated
- The sample thickness is recorded automatically by high-precision sensors
Application in the production process of various insulation materials
Mineral wool
Mineral wool is characterized by its low thermal conductivity and pronounced anisotropy. The fiber orientation has a significant influence on the thermal conductivity, which is why the measurement method must be validated for different installation positions (2). In the Heat Flow Meter, the thermal conductivity is determined perpendicular to the board direction. In production control, the method provides a fast, non-destructive and repeatable measurement.
Special challenges with mineral wool:
- Fiber orientation and binders require pre-treatments such as curing to obtain reliable values
- Sample geometry and boundary conditions must be precisely adhered to
- Anisotropy effects must be taken into account in the direction of the heat flow
PU foam
PU foam is a high-performance insulating material whose thermal conductivity is significantly influenced by various parameters such as density, cell gas composition, moisture and ageing. The thermal conductivity increases slightly with age, as gases diffuse within the cell structure (3).
Critical factors for PU foam:
- Ageing effects due to gas exchange in the cell structure
- Density and moisture content significantly influence measurement results
- Measurement under standard conditions (e.g. 10°C average temperature) guarantees comparability
- Long-term behavior is recorded using standardized methods in accordance with EN 13165
Significance for the construction industry and energy efficiency
The Heat Flow Meter method offers decisive advantages for the modern construction industry in the quality assurance of insulation materials. An in-process application enables improved production control. This contributes significantly to the quality assurance of insulation materials and the minimization of energy losses in buildings.
Advantages for the industry:
The HFM method works in accordance with recognized standards such as EN 12667, EN 13165, ASTM C518 and ISO 8301. The system represents a cost-efficient alternative to more complex measuring methods and is characterized by high precision with low-maintenance operation and short downtimes. The certification conformity supports market access and serves as reliable proof of quality.
Method-specific challenges and their control
The precision of the Heat Flow Meter Methode depends largely on correct sample preparation and execution. Uniform sample surfaces and correct thickness measurement are crucial, as measurement errors in the sample thickness have a direct effect on the measured thermal conductivity (see above: Fourier equation)
Critical control points:
- Sample preparation with plane parallelism and homogeneous density
- Minimize edge effects by carefully adhering to the sample size
- Steady-state equilibrium must be reached before reading the measurement results
- Reduce contact resistance between sample and measuring plates
- Ensure temperature field homogeneity over the entire sample area
Modern HFM systems use Peltier heating and cooling technology, which ensures low downtimes and minimal maintenance. This reduces downtime.
Scientific validation and comparability
The reliability and precision of the Heat Flow Meter method has been proven by numerous scientific studies. Comparative measurements in the form of round robin tests show deviations of less than 2 % between different laboratories and measuring devices, which underlines the high reproducibility and comparability of the method. Tests on anisotropic and moist samples prove the flexibility of the method, and comparative studies with other measuring methods such as the Guarded Hot Plate show a good correlation. In addition, long-term studies document the stability and repeatability of the results. The scientific basis of the HFM method is continuously being expanded through research work that deals with specific material challenges and optimization of the measurement technology. The method has proven to be a reliable tool, particularly when investigating the effects of moisture and ageing.
Conclusion
The Heat Flow Meter method is an indispensable method for modern quality control of insulation materials. The combination of short measurement time of typically 15 minutes, high accuracy with less than 2% deviation and non-destructive testing makes it the ideal tool for production monitoring of mineral wool and PU foam insulation boards, enabling standard-compliant measurements for certification and proof of quality and providing scientifically validated and reproducible results. At the same time, it offers a cost-effective alternative to more complex testing methods and is versatile for different insulation types and thicknesses. At a time when energy efficiency and sustainable building materials are becoming increasingly important, the Heat Flow Meter method is an essential tool for ensuring material quality. It enables manufacturers to produce high-quality insulation materials and check their thermal conductivity, thereby making an important contribution to reducing energy consumption in buildings.
List of sources
(1) TEC-Science: Heat-Flow-Meter Method for the determination of thermal conductivity https://www.tec-science.com/de/thermodynamik-waermelehre/waerme/heat-flow-meter-verfahren-zur-bestimmung-der-warmeleitfahigkeit-hfm/
(2) PMC: Determination of Thermal Properties of Mineral Wool Required for Fire Behavior Modeling and Structural Applications
https://pmc.ncbi.nlm.nih.gov/articles/PMC10488771/
(3) High Performance Insulation: Thermal insulation materials made of rigid polyurethane foam https://highperformanceinsulation.eu/wp-content/uploads/2016/08/Thermal_insulation_materials_made_of_rigid_polyurethane_foam.pdf
