SIST EN ISO 7345:2018

Overview

EN ISO 7345:2018 (ISO 7345:2018) is a vocabulary standard that defines the physical quantities, symbols and SI units used to describe the thermal performance of buildings and building components. Prepared by ISO/TC 163 and adopted by CEN as EN ISO 7345:2018, the standard focuses on quantities relevant to heat transfer and energy use in the built environment. It includes definitions, formulae and an informative Annex A explaining the concept of thermal conductivity. Note: some definitions are tailored to building thermal performance and can differ from ISO 80000-5.

Key Topics

The standard provides formal definitions, symbols and units for core thermal quantities used in building physics and energy assessment:

• Heat (quantity of heat Q) and heat flow rate (Φ = dQ/dt)
• Areal heat flux (areal density of heat flow rate q = dΦ/dA) and linear heat flux (ql = dΦ/dl)
• Thermal conductivity (λ) and thermal resistivity (r), with the vector relation between heat flux and temperature gradient
• Thermal resistance (R) and areal/linear thermal resistance; reciprocal thermal conductance (L)
• Thermal transmittance (U‑value) and linear thermal transmittance (Ψ‑value) for thermal bridges
• Surface coefficient of heat transfer (h), heat capacity (C), specific heat capacity (c) and thermal diffusivity (a)
• Clarification on the use of “areal” versus “linear” terminology and updated symbol conventions

Applications

EN ISO 7345:2018 is a foundational reference for anyone working on building energy performance, thermal calculations or product data:

• Building designers and architects calculating U‑values and assessing thermal bridges (Ψ‑values)
• Energy assessors and compliance officers preparing energy performance certificates and code submissions
• HVAC and building services engineers using consistent thermal property definitions in simulations
• Manufacturers and test laboratories producing thermal datasheets for insulation, cladding and building elements
• Researchers and standardization bodies ensuring consistent terminology across reports, standards and software

Using ISO 7345:2018 ensures consistent interpretation of thermal parameters in modelling, lab reports and regulatory documents - improving comparability and reducing errors in energy performance assessments.

Related standards

ISO 7345:2018 is intended to be used alongside other building thermal vocabularies and test standards, for example:

• ISO 80000-5 (quantities and units - note: some definitions differ)
• ISO 9251, ISO 9346, ISO 9229, ISO 9288 and ISO 7945 (related thermal insulation vocabularies and properties)

Keywords: ISO 7345:2018, EN ISO 7345:2018, thermal performance, U‑value, Ψ‑value, thermal conductivity, thermal resistance, heat flow rate, building envelope, energy performance.