Thermal transmittance

Thermal transmittance (denoted by the letter U) is the quantity that expresses the steady-state heat flow that passes, per unit time, through 1 m² of surface for a temperature difference of 1 K between the two environments it separates. In practical terms, it measures the heat flux transmitted through a wall, roof, floor, or window when there is a temperature difference between indoors and outdoors: the lower the U-value, the better the insulating performance of the element.

Thermal transmittance is the reference parameter for the energy design of buildings and for verifying compliance with the limits set by current regulations. It is the value used by designers to size insulation, compare alternative construction solutions, and demonstrate compliance with minimum legal requirements. For this reason, unlike other thermophysical properties of materials (e.g., thermal conductivity λ), which describe the material, thermal transmittance describes the construction element as a whole, including surface resistances and all the layers that compose it.

Formula and units of measurement

Thermal transmittance is calculated as the reciprocal of the total thermal resistance of the element (RT), which includes the resistances of all layers and the internal and external surface resistances: 

U = 1 / RT
where:
Rₜ = Rsi + Σ(dᵢ / λᵢ) + Rse
Rsi = internal surface resistance   |   Rse = external surface resistance   |   dᵢ/λᵢ = resistance of each layer

The unit of measurement of thermal transmittance in the International System is W/(m²·K).

Surface resistances (Rsi e Rse) account for heat exchange by convection and radiation between the surface of the component and the air of the adjacent environments. Their values are defined by UNI EN ISO 6946 depending on the direction of heat flow: for horizontal heat flow (vertical wall), Rsi = 0.13 m²K/W and Rse = 0.04 m²K/W. Their inclusion in the calculation of U is mandatory and can significantly affect the U-value, especially for well-insulated components where surface resistances represent a significant portion of the total resistance.

Transmittance, resistance, and conductivity: the complete system

Thermal transmittance U is the “system” quantity that summarizes in a single value the thermal performance of a multilayer construction element. Its calculation starts from the properties of individual materials and is built by summing resistances: 

The relationship is direct: reducing U means increasing Rₜ i.e., increasing the thickness of insulating layers, using materials with lower λ, or both. Each additional layer - even an air layer, if confined - contributes to increasing the total resistance and thus reducing U. It should also be noted that U describes behavior under steady-state conditions, whereas real behavior, especially in summer, is dynamic and described by the periodic thermal transmittance (Yie).

Example of calculation of thermal transmittance U of a multilayer wall

Disclaimer: the following example is for educational purposes. The values used are simplified and do not replace a calculation compliant with technical standards for design or certification purposes. 

Consider an external wall composed of three layers: internal plaster (d = 1.5 cm, λ = 0.90 W/(m·K)), rock wool insulation (d = 12 cm, λ = 0.036 W/(m·K)), and external facing brick (d = 12 cm, λ = 0.60 W/(m·K)). 

The calculation of total thermal resistance proceeds by summation: 

R_plaster = 0,015/0,90 = 0,017 m²K/W

R_insulation = 0,12/0,036 = 3,333 m²K/W

R_brick = 0,12/0,60 = 0,200 m²K/W

3,720 m²K/W

Rt = Rsi + ∑R_strati + Rse = 0,13 + … + 0,04

U = 1 / 3,720 = 0,27 W/(m²·K)

The result - 0.27 W/(m²·K) - complies with the limits set by the Ministerial Decree of 28 October 2025 for the most severe climate zones. It should be noted that the 12 cm rock wool layer alone accounts for 89.6% of the total resistance: this confirms how decisive the λ of the insulating material is for the final result, and how surface and structural layer resistances contribute only marginally when insulation is adequate.

Transmittance of opaque elements 

For opaque components - walls, roofs, and floors - thermal transmittance is calculated according to UNI EN ISO 6946, which defines the calculation method for thermally homogeneous layered elements and provides reference surface resistance values.

For structures characterized by geometric or material inhomogeneities, the presence of thermal bridges, or complex construction configurations, numerical calculation methods (e.g., finite elements) according to UNI EN ISO 10211 are required.

The equivalent thermal transmittance of a real wall is influenced by the presence of thermal bridges, i.e., localized areas - such as structural nodes, columns, beams, and wall–slab junctions - where insulation continuity is interrupted and heat flow is locally higher.

UNI EN ISO 10211 and UNI EN ISO 14683 provide methods for determining the contribution of linear (ψ) and point (χ) thermal bridges, necessary for the correct evaluation of overall thermal performance.

Transmittance of windows and transparent elements 

For windows, doors, and curtain walls, thermal transmittance is determined using a different approach compared to opaque components, since the element consists of heterogeneous parts - glass, frame, and spacer - with different thermal performances. 

The reference standards are UNI EN ISO 10077-1 (simplified method) and UNI EN ISO 10077-2 (numerical method), which define the following quantities: 

• Ug (glass transmittance): determined according to UNI EN 673 by calculation or measured in the laboratory according to UNI EN 674. For double glazing with argon-filled cavity and low-emissivity coating, typical values range between 0.5 and 1.1 W/(m²·K).
  
• Uf (frame transmittance): determined by numerical calculation according to UNI EN ISO 10077-2 or by experimental testing. Multi-chamber PVC and wooden frames typically range between 1.0 and 1.4 W/(m²·K), while aluminum profiles without thermal break can exceed 5 W/(m²·K).
  
• Uw (window transmittance): overall value of the window, calculated as the area-weighted average of glass and frame, including the contribution of the linear spacer. It is the value declared in the Declaration of Performance (DoP) under the CPR Regulation and the one to be compared with regulatory limits. 

Performance note: for transparent components, thermal transmittance is not the only relevant parameter. The solar factor g (or SHGC) - which represents the fraction of solar energy transmitted indoors - and the light transmittance factor are complementary parameters in the overall assessment of the energy performance of transparent envelopes.

• UNI EN ISO 6946: reference standard for calculating thermal transmittance of building components consisting of thermally homogeneous layers. Defines standard surface resistances and the method for calculating total thermal resistance.
  
• UNI EN ISO 10077-1 and 10077-2: standards for calculating thermal properties of windows and doors. Part 1 defines the simplified method for overall transmittance (Uw); Part 2 defines the numerical method for determining frame (Uf) and spacer transmittance.
  
• UNI EN ISO 10211: numerical method for calculating heat flows and surface temperatures in the presence of two- and three-dimensional thermal bridges. Allows determination of linear (ψ) and point (χ) coefficients.
  
• UNI EN ISO 14683: provides tabulated values of linear thermal transmittance coefficients (ψ) for common thermal bridges. Represents a simplified alternative to numerical calculation according to UNI EN ISO 10211.
  
• UNI EN ISO 13789: defines heat transfer coefficients for transmission and ventilation in buildings. Allows aggregation of component transmittances to determine the global heat loss coefficient Hᵀ.
  
• UNI EN 673 / UNI EN 674: standards for determining thermal transmittance of flat glass: UNI EN 673 by calculation, UNI EN 674 by experimental guarded hot plate measurement.
  
• Ministerial Decree 28 October 2025 – Minimum Requirements Decree: establishes limit values for thermal transmittance of building envelope components, differentiated by climate zone and element type. Compliance is mandatory within the technical report under Law 10.
• Construction Products Regulation (EU) CPR: governs CE marking of construction products and introduces the Declaration of Performance (DoP), which includes technical performance such as thermal transmittance and, where applicable, environmental performance (LCA).

Thermal transmittance in industrial applications 

The concept of thermal transmittance is not limited to the building sector. The same physical principle - heat flow proportional to temperature difference and separator characteristics - applies in several industrial contexts: 

• Insulation of systems and pipelines: in industrial systems, heat losses from insulated pipes, tanks, and reactors are calculated using models based on linear heat loss coefficients (W/(m·K)), derived from summation of thermal resistances adapted to cylindrical geometry.
  
• Cold rooms and climatic chambers: sandwich panels used in these systems have very low transmittance values (typically 0.20–0.35 W/(m²·K)) to limit heat exchange and reduce refrigeration energy demand. Determination of transmittance is part of product qualification procedures.
  
• Heat exchangers: in heat exchangers, the overall heat transfer coefficient U is the main sizing parameter. It accounts for convective resistances of fluids, conductivity of the separating wall, and resistances due to fouling. The formulation is formally analogous to that used in building physics, although referring to more complex phenomena.
  
• Isothermal containers and thermal packaging: in temperature-controlled transport (pharmaceutical and food sectors), thermal transmittance is a key parameter determining container performance and thermal autonomy. Evaluation is performed through climatic chamber testing according to sector-specific protocols.

Certimac support

Certimac supports manufacturers, designers, and laboratories in the thermal characterization of building and industrial components. Within its thermophysical analysis services, the laboratory performs experimental measurement of thermal conductivity (λ) and thermal resistance (R) of constituent materials according to EN 12667, providing the input data for calculating transmittance U of components according to EN ISO 6946 and EN ISO 10077.

As a Notified Body under the Construction Products Regulation (CPR) and an ISO 17025 accredited laboratory by ACCREDIA, Certimac guarantees test results recognized at national and European level, usable for the Declaration of Performance and Conformity (DoPC), CE marking, and technical documentation required by energy regulations.

To learn more about thermal analysis services and evaluate the most suitable support for your case, visit the dedicated page, fill in the form at the bottom of the page, or contact our experts at materiali@certimac.it.