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With increasing demands on energy efficiency and sustainability, the precise characterization of the thermal properties of insulation materials is coming to the fore. The thermal conductivity (λ) is the key parameter for evaluating the insulation performance – both when new and over the entire life cycle of a building material. But how reliably can these values be measured and evaluated, especially for modern materials such as polyurethane foams, aerogels or fiber-based insulation materials? The Laser Flash Method (LFA) has established itself as a highly precise and dynamic solution in this field.
Principle and advantages of the Laser Flash Analyzer method
Focus on materials: polyurethane, aerogels, fibers
Polyurethane
Polyurethane (PU) foams show excellent insulation performance with typical λ-values below 0.026 W/(m-K). Their advantage lies in their fine pore structure, which suppresses gas phase conduction. However, scientific studies by Wagner (University of Stuttgart) show that the thermal conductivity increases slowly over the service life, as the propellant gas in the cells is gradually replaced by air. Laboratory measurements provide reliable temperature dependencies, particularly in the case of moisture absorption or ageing, which is essential for long-term assessment (Wagner, 2010).
Aerogels
Aerogels, especially silica and carbon aerogels, are setting new standards in insulation with values of less than 0.015 W/(m-K), but are also challenging in terms of measurement technology. Porosity, anisotropic structures and high scattering of particle sizes require methods with high spatial and temporal resolution.
In the case of aerogel-based materials, it has been shown that the combination of tests with dried samples and samples exposed to moisture enables a reliable statement to be made about the influence of ageing and moisture input on thermal conductivity. Studies by Lakatos et al. (2025) show that the thermal conductivity of aerogel can initially increase after short-term temperature exposure, but remains remarkably stable under real building conditions (Lakatos et al., 2025).
Fibers
Fiber-based insulation materials (e.g. glass, rock wool-based or natural fibers) benefit from the flexibility typical of LFA. The ability to measure both in-plane and out-of-plane thermal conductivities means that anisotropy (preferential heat flow along the fiber orientation) can also be quantified – crucial for realistic component evaluations.
LFA method comparison: When is which measurement method optimal?
The choice of a suitable measurement method for thermal properties depends heavily on the material, the desired accuracy and the boundary conditions. While stationary methods such as the Guarded Hot Plate (GHP) or Heat Flow Meter (HFM) according to DIN EN 12664 and DIN EN 12667 have their established role in standardized testing, the LFA method shows clear advantages in specific areas of application.
Stationary methods (GHP/HFM) are particularly suitable for:
- Large, homogeneous samples at room temperature
- Direct determination of thermal conductivity without additional material parameters
- Standard-compliant quality testing for certifications
- Materials with very low thermal conductivity (<0.1 W/(m-K))
Laser flash analysis, on the other hand, offers decisive advantages:
- Temperature-dependent measurements: LFA covers ranges from -100°C to over 1000°C, while GHP/HFM are mostly limited to 10-70°C
- Small sample volumes: LFA requires only a few cm² of material, ideal for expensive developing materials such as aerogels
- Fast measurement cycles: An LFA measurement takes minutes instead of hours with stationary methods
- Inhomogeneous or anisotropic materials: The ability to measure small samples allows local differences to be detected and directional differences to be tested
- Ageing studies: the high reproducibility enables precise tracking of material changes
The superiority of the LFA is particularly evident in the characterization of modern insulation materials: while a GHP measurement on an aerogel panel takes several hours and requires large sample areas, the LFA delivers highly precise data in just a few minutes, even from small material samples.
Applications in the insulation industry
The LFA method is used in many different ways in the insulation industry:
Quality control in production: In the industrial production of insulation materials, the LFA method enables a significantly higher test frequency than conventional methods thanks to short measurement times. The rapid feedback on thermal properties allows process fluctuations to be detected and counteracted at an early stage, for example in the event of variations in the blowing agent content of foams.
Material development for extreme conditions: In the development of high-temperature insulating materials for industrial applications, the advantage of the wide temperature range of LFA becomes apparent. Continuous temperature ramps can reveal critical phase transitions and structural changes that would not be visible with point measurements. This information is essential for the optimization of material formulations.
Reliability of thermal conductivity values over the life cycle
The realistic assessment of insulation performance over decades remains a key challenge. Moisture and ageing in particular can have a significant impact on λ in some cases. The LFA method is sensitive enough to detect even small effects caused by gas diffusion, embrittlement or long-term ageing and thus creates the basis for reliable ageing forecasts:
Moisture load
Water significantly increases the thermal conductivity, as the pore structure is now filled with a more conductive medium. LFA measurements on material samples under defined climatic conditions make it possible to quantify these effects and thus make a model-based forecast of the long-term insulating effect.
Structural changes
In the case of aerogels, shrinkage after drying, pore enlargement or different proportions of different pore sizes can lead to altered heat conduction properties. The combination of spatially resolved measurement and parallel structural analysis (e.g. SAXS, SEM) sets LFA apart from conventional methods.
Ageing effects
Polyurethane can lose its diffusion density over time, which manifests itself in increasing thermal conductivity values. LFA analyses of batches and load series provide robust data for quality assurance purposes.
Measurement precision and influencing factors
The accuracy of the laser flash measurement is determined by various factors:
- Specimen thickness and geometry: The exact determination of the specimen thickness is critical, as errors here have a quadratic effect on the result
- Surface treatment: Different absorption properties influence the temperature increase and thus the measuring accuracy
- Temperature stability: Fluctuations in the sample ambient temperature can lead to measurement uncertainties
- Material changes: Ageing effects influence both the actual material values and the reproducibility of the measurements
By controlling and documenting these factors, the laser flash method can also guarantee the highest accuracy and reliability for life cycle analyses of insulation materials.
Conclusion: LFA as the key to life cycle assessment of modern insulation materials
The Laser Flash Analyzer method provides fast, high-resolution and precise experimental data on the thermal conductivity of a wide range of insulation materials, making it the ideal tool not only for material development, but also for predicting service life in the construction industry. construction industry. In combination with structural analysis methods and cyclical ageing tests, LFA-supported measurement technology opens up new possibilities for quality assurance and optimization of energy-efficient building products in laboratory and research practice.
Scientific evidence shows that the long-term stability of the different material classes varies: While polyurethane shows a slight but predictable increase in thermal conductivity over decades, high-quality aerogels exhibit extreme long-term stability under normal conditions of use. The LFA method makes it possible to precisely quantify these ageing processes and thus create a reliable basis for sustainable construction planning.
References
- ASTM E1461: Standard Test Method for Thermal Diffusivity by the Flash Method. ASTM International.
- Wagner, K. (2010): Simulation and optimization of the thermal insulation capacity of closed-cell PUR rigid foams. Dissertation, University of Stuttgart. Online: https://elib.uni-stuttgart.de
- Heinemann, U. et al. (2020): Long-Term Performance of Super-Insulating Materials in Building Applications. IEA-EBC Annex 65, Subtask I State-of-the-Art Report.
- Lakatos, Á. et al. (2025): Identifying the alteration in the thermal properties of aerogel materials. ScienceDirect. Online: https://www.sciencedirect.com
