Thermal bridges or cold bridges: Both terms refer to the same physical phenomenon, but they describe it from different perspectives. At points in the building envelope where heat transfer is locally increased, heat flows out of the building in winter, while in summer, heat penetrates from the outside. Whether these weak spots are called thermal bridges or cold bridges is not a question of physics, but of perspective. For architects, building physicists, and anyone who designs, builds, or renovates buildings, understanding these phenomena is fundamental: thermal bridges cause energy loss, mold damage, and structural damage—all of which can be prevented with careful design.

• What thermal bridges and cold bridges are, and why both terms mean the same thing
• Which physical laws govern the behavior of thermal bridges
• What types of thermal bridges exist and how they differ from one another
• How thermal bridges are calculated, evaluated, and taken into account in the design process
• Which standards and design parameters are relevant in practice
• Why thermal bridges cause mold growth and moisture damage
• How thermal bridges can be structurally avoided or minimized
• The role thermal bridges play in renovation, new construction, and energy efficiency

Thermal Bridges or Cold Bridges: Definition and Clarification of Terms

In building physics, the term “thermal bridge” refers to a point in a building’s thermal envelope where the heat flow is locally increased compared to the undisturbed area of the building component. This increased heat flow occurs because, at this point, either the geometry of the building component, the material properties, or a combination of both factors causes heat to flow more easily from the warm side to the cold side than in the standard cross-section. In winter, the warm side is the interior, and the cold side is the outside air; in summer, the relationship is reversed.

The term “thermal bridge” is not a technical physics term in its own right, but rather a colloquial expression that describes the same phenomenon from the occupant’s perspective. Anyone who touches a cold spot on an interior wall in winter or notices a noticeably cool corner in a room experiences the thermal bridge as a source of cold. From a physical standpoint, heat flows out of the room, not cold in, because cold is not an independent medium that moves on its own, but merely the absence of heat. Nevertheless, the term “cold bridge” has become firmly established in German usage and is frequently used by tradespeople, builders, and in popular guidebooks. In standards, technical publications, and scientific literature, only the term “thermal bridge” is used.

To understand both terms, it is important to recognize that the phenomenon is direction-independent. A thermal bridge is not a one-way street: it accelerates heat exchange in both directions. In the summer, the same spot that conducts heat outward in the winter can allow heat from outside to enter the building and contribute to rooms overheating. This bidirectional nature is often underestimated in practice but is relevant for the year-round evaluation of building envelopes.

Physical Principles: Heat Conduction, Heat Flux, and Temperature Coefficient

The physical basis of thermal bridges is heat conduction, described by Fourier’s law: The heat flux through a building component is proportional to the temperature difference between its two sides and to the thermal conductivity of the material, and inversely proportional to the thickness of the component. Thermal conductivity, referred to in building physics as the lambda value (λ) and expressed in watts per meter per kelvin (W/(m·K)), is the key material parameter. Concrete has a lambda value of about 2.0 W/(m·K); mineral wool ranges between 0.030 and 0.045 W/(m·K) depending on density; and steel reaches values around 50 W/(m·K). A steel component that penetrates the insulation layer therefore conducts heat about a thousand times more efficiently than a layer of mineral wool of the same thickness.

The heat transfer coefficient, known as the U-value (formerly the k-value), describes the heat flow through a structural element in a steady state, relative to the area and the temperature difference, in the unit W/(m²·K). The lower the U-value, the better the thermal insulation. Thermal bridges are generally not accounted for by the U-value of the standard cross-section, but rather by additional parameters that describe the local additional heat loss. For linear thermal bridges—that is, those that extend along a line—this is the psi value (Ψ), expressed in W/(m·K). For point-shaped thermal bridges, such as individual fastening anchors, the chi value (χ) in W/K is used.

The temperature factor fRsi is of central importance for assessing the risk of condensation on building component surfaces. It is defined as the ratio of the difference between the interior surface temperature and the exterior temperature to the total temperature difference between the interior and exterior air. An fRsi value of 1.0 would mean that the interior surface has the same temperature as the indoor air; a value of 0 would mean that it assumes the outdoor temperature. DIN 4108-2 specifies minimum fRsi values for thermal insulation in building construction to ensure that, under defined boundary conditions, the internal surface temperature does not fall below the dew point of the indoor air. According to this standard, this minimum fRsi value is 0.70 for residential buildings under standard conditions.

Types of Thermal Bridges: Geometric, Material, and Structural

In building physics, thermal bridges are classified into three main types based on their cause. This classification helps in selecting the appropriate countermeasure and correctly calculating the effects.

Geometric thermal bridges arise from the shape of the building component itself, without any significant differences in material. The classic example is the exterior corner of a building: a larger exterior surface area is opposed by a smaller interior surface area. The heat flowing through the masonry is distributed over a larger area on the exterior than on the interior, causing the interior surface in the corner to be cooler than on the undisturbed wall surface. Interior corners behave in the opposite way: Here, the interior surface area is larger than the exterior surface area, resulting in a slightly warmer interior surface. Geometric thermal bridges can only be eliminated to a limited extent through structural measures, as they result from the building’s geometry; however, their effect can be significantly reduced by using sufficient insulation thickness.

Material-related thermal bridges occur where materials with vastly different thermal conductivities are arranged side by side or one behind the other. The most common example in modern construction is concrete components that penetrate the insulation layer: reinforced concrete balcony slabs that are monolithically connected to the floor slab, steel columns in a wood-frame structure, fastening anchors for curtain wall panels, or concrete lintels above windows without insulation backing. Each of these elements creates a heat conduction path that locally bypasses the insulating effect of the surrounding structure. Material-related thermal bridges are generally the most serious because the differences in thermal conductivity between insulation materials and concrete or steel are extremely large.

Structural thermal bridges arise from the way building components are connected to one another. Joints between exterior walls and ceilings, between window frames and reveals, between roof structures and exterior walls, or between basement walls and floor slabs are typical locations where the insulation layer is interrupted, offset, or incompletely installed. These thermal bridges can often be avoided if sufficient care is taken with the design of details during the planning phase. At the same time, they are particularly common in practice because the interfaces between different trades and building components traditionally receive less attention than the standard cross-sections.

Thermal Bridges and Mold Growth: The Connection to Dew Point and Surface Temperature

The health and structural significance of thermal bridges lies not only in energy loss but, above all, in the risk of condensation and the resulting mold growth. Mold requires a persistently elevated surface moisture level to grow; this occurs when the surface temperature of a building component drops below the dew point temperature of the adjacent indoor air. At a thermal bridge, the internal surface temperature is locally lower than at the standard cross-section. If it falls below the dew point of the indoor air, water condenses directly on the surface.

However, mold begins to grow even before visible condensation occurs. The critical threshold is a surface relative humidity of about 80 percent that persists over extended periods. This surface humidity of 80 percent corresponds to a surface temperature that is about 2 to 3 Kelvin above the dew point of the indoor air. Thermal bridges that lower the interior surface temperature to this level thus create a persistent risk of mold growth even without visible condensation. This explains why mold appears in interior corners, behind furniture against exterior walls, or in radiator recesses even when the indoor humidity seems moderate.

In practice, the connection between thermal bridges and mold growth is often misjudged. When mold appears at the corner of an exterior wall, the occupants’ ventilation habits are cited as the sole cause. However, a careful analysis must always take both factors into account: the moisture load of the indoor air and the surface temperature at the affected location. If the surface temperature remains in the critical range due to a thermal bridge, mold growth is possible even with normal ventilation practices. The structural cause must then be addressed; a change in behavior alone is not sufficient.

Thermal bridges are particularly critical when combined with high levels of moisture production in the room. Bathrooms, kitchens, and bedrooms with many occupants generate significant moisture loads. If thermal bridges are also present in these rooms, lowering the interior surface temperature, the risk is greatly increased. Experts therefore recommend paying special attention to ensuring that all connections are designed without thermal bridges when planning wet areas and living spaces.

Calculation and Evaluation of Thermal Bridges in Design Practice

In design practice, thermal bridges are quantified in two ways: by using thermal bridge catalogs with tabulated Psi values for standard details, or through numerical simulation using the finite element method (FEM). Thermal bridge catalogs, such as those published by the Institute for Heat and Oil Technology (IWO) or the Passive House Institute, contain pre-calculated Psi values for common construction details that can be directly incorporated into energy balancing. This is possible provided that the actual construction detail matches the catalog entry.

For more complex or non-standard details, a numerical calculation is required. In this process, the building component is modeled as a two-dimensional or three-dimensional model, boundary conditions (internal and external temperatures, heat transfer resistances) are applied, and the heat flow is calculated. From the results, both the Psi value and the minimum interior surface temperature—and thus the fRsi value—can be derived. These calculations are performed by specialized building physics firms and are required for compliance with the Building Energy Act (GEG) as well as for Passive House certifications.

In the context of energy balancing under the GEG, thermal bridges can be accounted for either on a flat-rate basis by applying a surcharge to the average U-value of the building envelope or through detailed calculation of the individual Psi values. The flat-rate approach is simpler but results in a higher surcharge and thus a poorer energy balance. Those who document carefully planned details with minimal thermal bridges can reduce this surcharge and thereby achieve a better energy class. This creates a direct economic incentive for high-quality detailed design.

Thermographic inspections—that is, infrared images of the building envelope taken when there is a temperature difference between the interior and exterior—are a proven method for visualizing thermal bridges in existing buildings. On a thermographic image, thermal bridges appear as warmer areas on the exterior facade (because more heat escapes there) or as cooler areas on the interior wall. Thermographic images do not provide quantitative psi values, but they are very valuable for locating and documenting weak points in existing buildings.

Structural Prevention of Thermal Bridges: Principles and Detailed Solutions

The most important principle for preventing thermal bridges is the continuity of the insulation layer. The thermal insulation of the building envelope should form a seamless, closed layer that is neither penetrated by highly conductive materials nor interrupted at connections. This principle sounds simple, but in practice it is difficult to implement in many places because structural requirements, structural necessities, and traditional construction practices often work against it.

Balcony slabs are one of the best-known examples of structural thermal bridges, which can be effectively managed using modern methods. In the past, balconies were constructed as monolithic extensions of the floor slab, creating a massive thermal bridge. Today, so-called thermal separation elements—also known as Isokorb or thermal anchors—are available. These elements provide a structurally sound connection between the balcony slab and the floor slab while interrupting the heat transfer path with a layer of high-performance insulation. Depending on the product and design, the U-value of such connections is significantly lower than that of a monolithic connection.

For windows and doors, a common thermal bridge is found in the reveal. If the window frame is installed flush with the exterior wall or even within the exterior shell, an uninsulated area is created in the reveal, which locally lowers the interior surface temperature. The better solution from a building physics perspective is to install the window within the insulation layer—that is, as far outward in the structure as possible—so that the frame is enclosed by the insulation. This installation position significantly reduces the Psi value of the window connection while simultaneously improving the surface temperature of the reveal.

In the case of interior insulation used in the energy-efficient retrofitting of existing buildings—when exterior insulation is not possible for design or technical reasons—thermal bridges at the connections to the ceiling, interior walls, and windows are particularly critical. The interior insulation inevitably ends at these junctions, and the building components behind them remain thermally unimproved. Experts recommend extending the interior insulation as far as possible into these connection areas and performing a mathematical assessment of the thermal bridges to ensure that the fRsi value does not fall below the minimum value at any point.

Thermal Bridges in the Context of Energy Efficiency, Renovation, and Sustainability

The energy significance of thermal bridges increases with a building’s overall insulation standard. In a poorly insulated older building with U-values of 1.5 W/(m²·K) or higher on the exterior walls, thermal bridge losses are relatively small compared to total heat losses. In a well-insulated new building with U-values below 0.2 W/(m²·K), however, thermal bridges can account for a significant portion of total heat transmission losses—in some cases, more than 30 percent. This means that the better the insulation of the standard cross-sections, the more important it is to carefully design the details.

In the Passive House standard, which aims for a maximum heating demand of 15 kWh/(m²·a), a construction that is largely free of thermal bridges is a fundamental requirement. To this end, the Passive House Institute in Darmstadt has defined the criterion of “thermal bridge-free construction,” which stipulates a maximum linear thermal bridge effect of 0.01 W/(m·K) for all details. This criterion forces designers to carefully consider and calculate every connection, and has led to a multitude of innovative detailing solutions in practice.

In the energy-efficient retrofit of existing buildings, thermal bridges are an often-underestimated factor. If an exterior wall is retrofitted with insulation without carefully designing the connections to ceilings, windows, and the foundation, the remaining thermal bridges can consume a significant portion of the theoretical energy savings. At the same time, a retrofit that eliminates existing thermal bridges can raise interior surface temperatures to the point where an existing mold problem is permanently resolved. Eliminating thermal bridges is thus not only an energy-efficiency measure but also a structural and health-related one.

Sustainable construction in the broadest sense includes the durability of the structure. Thermal bridges that lead to condensation and mold growth damage the building fabric over the long term. Wooden components that become permanently damp lose their load-bearing capacity; steel reinforcements in concrete corrode; and insulation materials lose their effectiveness due to moisture penetration. Preventing thermal bridges therefore also contributes to the building’s service life and resource efficiency, as it avoids costly renovations resulting from moisture damage.

Understanding Thermal Bridges: The Foundation for High-Quality Construction

Thermal bridges—or cold bridges—are not a peripheral issue in building physics, but rather a central quality characteristic of every building envelope. Anyone who knows the difference between geometric, material-related, and structural thermal bridges; who understands how the Psi value and the temperature factor fRsi quantify their effects; and who has grasped the connection between thermal bridges, the dew point, and the risk of mold, has the tools needed for high-quality planning and execution.

Practical experience shows that thermal bridges almost always occur in the same places: at balcony connections, window reveals, building corners, base areas, and joints between different building components. These locations are well known, solutions are available, and standards specify clear minimum requirements. What is often lacking is consistent implementation in detailed design and careful execution on the construction site. Thermographic inspections after completion can help identify construction defects early on, before they lead to damage.

The growing importance of energy efficiency and climate protection in the construction industry makes addressing thermal bridges unavoidable. The more ambitious the energy standards, the more significant every weak point in the insulation layer becomes. Architects, civil engineers, and tradespeople who integrate this knowledge into their daily work make a measurable contribution to buildings that are sustainably energy-efficient, structurally sound, and comfortable for their occupants. Understanding thermal bridges ultimately means viewing buildings as physical systems in which every detail counts.