Overview
ISO 21901:2021 - Thermal insulation - Test method for thermal diffusivity - Periodic heat method specifies a laboratory method to measure the thermal diffusivity of thermal insulation materials shaped as flat plates. The method uses a controlled, periodic (sinusoidal) temperature variation on one surface of a specimen and measures the temperature response at two locations to derive diffusivity from the measured phase lag and amplitude behaviour. The document was prepared by ISO/TC 163/SC 1 and is intended for standardized, repeatable measurement of diffusivity for a wide range of insulation materials.
Key topics and requirements
- Measurement principle: One-dimensional heat conduction with a trigonometric (periodic) temperature input on the heating surface. Thermal diffusivity is obtained from the phase lag between temperature signals measured at two points.
- Specimen shape and preparation: Test specimens must be flat plates with dimensions matched to the heater (±2 mm) and meet homogeneity requirements (see ISO 8302 referenced). Specimens are dried to constant mass (typically 105 °C ± 2 °C unless lower temperature is required to avoid alteration).
- Geometry limits and sensor considerations: Side heat losses and thermocouple diameter impose constraints on specimen thickness and lateral size to reduce edge effects; for example, a 3 mm thermocouple tube suggests a minimum thickness of about 20 mm in the guidance.
- Instrumentation: Temperature measurement instruments and measurement locations are specified; one surface is kept at constant (or controlled) low temperature while the opposite surface receives periodic heating.
- Calculations: The standard gives analytical solutions for phase lag and amplitude ratio based on angular frequency and a damping coefficient, enabling calculation of thermal diffusivity and, where material density and specific heat are known, thermal conductivity.
- Limitations: Materials that undergo phase change, outgassing, large expansion/shrinkage, or cracking during testing are excluded.
Applications and practical value
- Useful for manufacturers, test labs and R&D teams assessing thermal performance of insulation products (fibrous insulations, foams, boards, masonry, concrete) where diffusivity influences transient thermal behavior.
- Helps in product development, quality control, and specification compliance when transient heat transfer properties are required (e.g., building envelope analysis, high‑temperature insulation, thermal storage components).
- Enables calculation of thermal conductivity from measured diffusivity combined with density and specific heat - valuable for simulation inputs and performance comparisons.
Who should use this standard
- Materials testing laboratories and certification bodies
- Insulation manufacturers and product development engineers
- Building scientists and thermal analysts requiring standardized transient thermal property data
Related standards
- ISO 8302:1991 - Determination of steady‑state thermal resistance (guarded hot plate) - referenced for specimen homogeneity and related test practices.
Keywords: ISO 21901:2021, thermal diffusivity, periodic heat method, thermal insulation test, phase lag, thermal conductivity, test specimen, transient heat transfer.