Overview
ISO 9251:1987 - "Thermal insulation - Heat transfer conditions and properties of materials - Vocabulary" is an ISO vocabulary standard developed by ISO/TC 163 (Thermal insulation). Published as the first edition in 1987, it provides standardized definitions for terms used in the field of thermal insulation to describe heat transfer conditions (e.g., steady state, non‑steady state, periodic, transient) and the properties of materials (e.g., porosity, local porosity, porous media types, density, isotropy/anisotropy).
This vocabulary standard promotes consistent use of terminology across testing, specification, research and product documentation for thermal insulation and related engineering disciplines.
Key topics
ISO 9251 defines and clarifies essential technical terms, including:
- Heat transfer conditions
- Steady state: parameters do not vary with time
- Non‑steady state: parameters vary with time
- Periodic state: non‑steady state with repeating parameter values
- Transient state: evolution from an initial state toward steady or periodic
- Heat transfer: energy transmission by conduction, convection, radiation or combinations
- Material properties
- Porosity and local porosity - total void volume definitions and point‑wise averaging
- Classification of porous media (fibrous, granular/loose‑fill, cellular, interconnected)
- Homogeneous / heterogeneous and isotropic / anisotropic media distinctions
- (Mass) density - bulk and material density considerations
The standard also cross‑references related vocabulary in the ISO thermal insulation series (see Related Standards).
Applications
ISO 9251 is practical for:
- Standardizing terminology in test methods, lab reports and certification for thermal insulation materials
- Supporting clear product specifications and procurement documents for manufacturers and suppliers
- Enabling consistent communication in building engineering, HVAC design, refrigeration, industrial insulation and material R&D
- Helping regulatory bodies, compliance auditors and researchers interpret measurement results and compare data across studies
By providing precise definitions, ISO 9251 improves clarity in technical documentation, reduces misinterpretation in testing and design, and assists in harmonizing international practice.
Who should use it
- Insulation manufacturers and product developers
- Test laboratories and certification bodies
- Architects, building engineers and HVAC designers
- Materials scientists and researchers
- Standards writers and regulatory authorities
Related Standards
Part of a series on thermal insulation vocabulary and quantities; related documents include:
- ISO 7345 (Physical quantities and definitions)
- ISO 9346 (Mass transfer - quantities and definitions)
- ISO 9229 (Thermal insulating materials and products - vocabulary)
- ISO 9288 (Heat transfer by radiation - quantities and definitions)
Keywords: ISO 9251, thermal insulation vocabulary, heat transfer conditions, porosity, steady state, transient state, porous medium, material properties.