ISO 7345:2018

Overview

ISO 7345:2018 - "Thermal performance of buildings and building components - Physical quantities and definitions" is an ISO standard that defines the physical quantities, symbols and units used to describe thermal performance and energy transfer in buildings and building elements. Focused on the built environment, it standardizes terms such as heat flow, heat flux, thermal conductivity, thermal resistance, transmittance (U‑value), linear transmittance (Ψ‑value), heat capacity and related thermophysical properties. The third edition (2018) updates terminology and symbols to align with modern practice and explicitly notes where definitions differ from ISO 80000‑5.

Key Topics

• Basic thermal quantities: heat (Q), heat flow rate (Φ), areal heat flux (q) and linear heat flux (ql).
• Material properties: thermal conductivity (λ), thermal resistivity (r), specific heat (c), thermal diffusivity (a) and thermal effusivity (b).
• Element and surface metrics: thermal resistance (R), areal thermal conductance (L), surface heat transfer coefficient (h).
• Building-scale metrics: thermal transmittance (U, U‑value), linear thermal transmittance (Ψ, Psi‑value), volumetric and areal heat transfer coefficients (Fv, Fs/H′).
• Energy performance terms: ventilation rate (n) and conventions for subscripts (i, e, s, cd, cv, r, etc.).
• Annex A: a more rigorous mathematical treatment of the concept of thermal conductivity for homogeneous materials.
• Standardized symbols and SI units for all quantities to ensure consistent communication in calculations, test reports and modelling.

Applications

ISO 7345:2018 is used wherever consistent definitions and units for thermal quantities are required:

• Preparing and interpreting U‑value and Ψ‑value calculations for building components and thermal bridges.
• Setting input data for building energy simulations and compliance modelling.
• Producing product datasheets and test reports for insulation materials and building elements.
• Guiding laboratory and field measurements of heat flow and surface coefficients.
• Supporting building designers, energy assessors, architects, thermal engineers, manufacturers, test laboratories and code officials in ensuring consistent terminology and comparable results.

Related Standards

• ISO 80000‑5 (quantities and units - note: definitions in ISO 7345 may differ where specific to buildings)
• ISO 7945, ISO 9251, ISO 9346, ISO 9229, ISO 9288 - complementary vocabularies and vocabularies for thermal insulation and heat transfer in building contexts.

Keywords: ISO 7345:2018, thermal performance, thermal conductivity, U‑value, Ψ‑value, thermal resistance, heat transfer coefficient, building envelope, thermal bridges, energy performance of buildings.