A thermal bridge is a localized area of the building envelope characterized by thermal inhomogeneity, where thermal resistance is lower than in the surrounding areas.
At a thermal bridge, heat flow becomes locally concentrated, preferentially passing through zones with higher conductivity. This results in increased heat losses compared to the theoretical uniform behavior of the element, a reduction in internal surface temperature, and, under unfavorable conditions, a risk of surface condensation and mold growth.
Thermal bridges are generally present in all buildings and occur at geometric variations, discontinuities in insulation, or intersections between different construction elements. They cannot be completely eliminated but can be quantified, controlled, and reduced through appropriate design and construction solutions. Their assessment is subject to verification in accordance with current regulations.
Classification of thermal bridges
Thermal bridges are classified into three main categories based on their origin:
In practice, the most critical thermal bridges are often structural ones. A reinforced concrete element (λ ≈ 1.8–2.5 W/(m·K)) inserted in an insulated wall introduces a significant thermal discontinuity that can hardly be compensated by simply increasing the insulation thickness in adjacent areas.
In buildings constructed before the 1990s, untreated structural thermal bridges are often one of the main causes of high heat losses and surface condensation issues.
How a thermal bridge is quantified: ψ and χ
The effect of thermal bridges on the building energy balance is expressed through additional coefficients that integrate the thermal transmittance of plane surfaces:
- ψ (linear thermal transmittance coefficient) [W/(m·K)]
Represents the heat flow per unit length associated with a linear thermal bridge (e.g., wall–slab junctions, wall–window junctions, corners).
- χ (point thermal transmittance coefficient) [W/K]
Represents the heat flow associated with point elements (e.g., anchors, brackets, fasteners).
The ψ coefficient is determined as the difference between the total heat flow obtained by numerical calculation and the contribution of plane surfaces:
ψ = L2D −∑ Ui ⋅ bi
where:
- L2D = linear thermal transmittance obtained from two-dimensional simulation
- Ui = thermal transmittances of plane surfaces
- bi = corresponding reference lengths
The numerical calculation of ψ is performed according to UNI EN ISO 10211 using finite element modeling. Alternatively, for standard configurations, UNI EN ISO 14683 provides tabulated values and thermal bridge atlases.
Tabulated values are generally conservative compared to numerical calculation; the latter is preferable during design optimization phases.
As an indication, some typical ψ values for common linear thermal bridges in buildings with external insulation systems are:
Note: the values reported are purely indicative and strongly depend on geometry, stratigraphy, insulation continuity, and construction details. They must not be used for design verification or energy certification without calculation in accordance with UNI EN ISO 10211 or UNI EN ISO 14683.
Effects of thermal bridges: heat losses, comfort, and condensation
Thermal bridges produce three main, interrelated effects:
- Increase in heat losses: the additional heat flow associated with thermal bridges increases the overall heat transfer coefficient by transmission Hᵀ of the building and, consequently, the heating energy demand. In poorly insulated buildings, thermal bridges may account for a significant share of total envelope losses (approximately 20–40%). In high-performance buildings, where opaque surface transmittance is very low, the relative impact of thermal bridges may become predominant.
- Reduction of internal surface temperature: at thermal bridges, the internal surface temperature is lower than in surrounding areas. This reduction depends on joint geometry, material thermal properties, and boundary conditions. Lower surface temperatures may cause thermal discomfort due to radiation effects and increase the risk of condensation.
- Risk of surface and interstitial condensation: when the internal surface temperature falls below the dew point temperature of indoor air, surface condensation occurs. This phenomenon promotes mold growth, leading to hygienic and aesthetic issues. Verification of condensation risk, including at thermal bridges, is required by current regulations within hygrothermal assessments.
Regulatory framework and hygrothermal verification: requirements and methods
The management of thermal bridges is governed by a structured regulatory framework that includes both energy performance calculation and hygrothermal verification of the building envelope.
- EN ISO 10211 - Numerical calculation of thermal bridges
Reference standard for numerical calculation of heat flows and surface temperatures in the presence of two- and three-dimensional thermal bridges. It defines boundary conditions, accuracy requirements for numerical models, and procedures for determining ψ and χ coefficients. It represents the most accurate method for thermal bridge assessment and is used in software compliant with the EN ISO 52000 series.
- EN ISO 14683 - Tabulated linear thermal transmittance coefficients
Provides tabulated ψ values for the most common thermal bridge types. It includes:
- method A: predefined (conservative) values
- method B: atlas-based values
- method C: reference to numerical calculation according to UNI EN ISO 10211
In design practice, method B is frequently used for standard verifications.
Reference standard for the assessment of the risk of surface and interstitial condensation in building components, including thermal bridge areas. The surface temperature factor fRsi is a key parameter for ensuring indoor comfort and building hygiene conditions. It measures the temperature difference between the internal surface and the external environment and is calculated as:
fRsi = (Tsi − Te) / (Ti − Te)
Tsi = internal surface temperature | Ti = internal temperature | Te = external temperature.
For standard indoor conditions (20 °C, 50% RH), EN ISO 13788 sets a limit value of fRsi ≥ 0.70 to prevent mold growth. Verification is performed by extracting the minimum internal surface temperature at the critical point of the thermal bridge from numerical simulation (EN ISO 10211) and calculating the corresponding factor. If fRsi < 0.70, the thermal bridge is at risk of mold formation and must be corrected.
- Ministerial Decree 28 October 2025 (Minimum Requirements Decree) - Verification obligations
Effective from 3 June 2026, the decree substantially updates requirements related to thermal bridges compared to the previous D.M. 26 June 2015. The main obligations introduced or confirmed are:
- Verification of internal surface temperature: for all significant thermal bridges, it must be verified that the internal surface temperature does not fall below the dew point under design conditions, nor below the mold risk threshold according to EN ISO 13788. Verification must be documented in the technical report attached to the design.
- Calculation of Hᵀ including thermal bridge contribution: the global heat transfer coefficient by transmission Hᵀ must include contributions from linear (Σ ψᵢ · lᵢ) and point (Σ χⱼ) thermal bridges. Compliance with Hᵀ limits therefore requires quantitative evaluation of thermal bridges.
- Mandatory treatment in new buildings: for new constructions, the decree requires design solutions that limit thermal bridges, with explicit reference to insulation continuity and control of ψ values at main junctions.
- Major renovations (first level): in major renovations involving more than 50% of the envelope, thermal bridges must be addressed alongside insulation of opaque surfaces, documenting the achieved reduction compared to the existing condition.
- UNI EN ISO 52016 – Energy demand calculation including thermal bridges
Standard within the EN ISO 52000 series for hourly or monthly calculation of building thermal energy demand. It defines how to integrate ψ and χ coefficients into the dynamic energy balance, ensuring consistency between thermal bridge assessment and overall energy performance calculation required for the Energy Performance Certificate (EPC).
- UNI/TS 11300-1 – National implementation
Italian technical specification implementing and integrating European standards for calculating building thermal energy demand within the national regulatory framework. It defines climatic boundary conditions, reference values for surface resistances, and application criteria for thermal bridge coefficients in EPC calculations (DOCET, EDILCLIMA, etc.).
Construction solutions for thermal bridge mitigation
The reduction of thermal bridges is primarily based on ensuring continuity of the insulation layer at locations where thermal discontinuities occur. Solutions vary depending on construction type:
- Continuous external insulation (ETICS): external insulation systems represent one of the most effective solutions for reducing structural thermal bridges in façades. By ensuring insulation continuity over columns, beams, and slabs, ψ values at main junctions are significantly reduced. Effectiveness depends on proper detailing, especially at wall–roof, wall–slab junctions, and around openings.
- Thermal break elements in slabs: at wall–slab junctions, the use of thermal break elements reduces ψ while maintaining structural continuity. These elements are made of low-conductivity materials or composite systems (e.g., GFRP or thermally broken steel elements) and are widely used in new constructions.
- Internal insulation: when external insulation is not feasible (e.g., due to architectural constraints), internal insulation reduces heat losses of plane surfaces but is generally less effective in correcting structural thermal bridges. In such cases, hygrothermal verification according to UNI EN ISO 13788 becomes particularly critical.
- Correct window installation detailing: the junction between window and wall is one of the most common linear thermal bridges. The position of the window within the wall assembly (internal, intermediate, or aligned with the insulation layer) significantly affects the ψ value. In most cases, installation aligned with the insulation layer, ensuring continuity of insulating materials, is the most effective solution.
Certimac support
Proper management of thermal bridges starts from accurate knowledge of the thermophysical properties of the materials used in the structure. Certimac performs experimental characterization of thermal conductivity (λ), thermal resistance (R), and transmittance of construction materials - insulation materials, masonry units, concretes, composites - providing the input data required for numerical modeling of thermal bridges according to EN ISO 10211 and for hygrothermal verification according to EN ISO 13788.
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. Test Reports issued by the laboratory can be directly used as input in thermal bridge calculation software and in technical documentation required by current regulations.
For information on thermal analysis services, please visit the dedicated Thermal Analysis page, fill in the form at the bottom of the page, or write to materiali@certimac.it.
