Thermal bridge

Thermal bridge - A thermal bridge (also known as a cold bridge) is an area in the building envelope where heat escapes to the outside more quickly than in the surrounding building components. Causes include structural weaknesses (balcony connections, window reveals, ring beams), material-related differences (reinforced concrete slabs in insulated walls), or geometric factors (external corners). Thermal bridges increase heating energy consumption and, if ventilation is insufficient, lead to condensation and mold.

Types and Causes

There are three types: Structural thermal bridges - arise from changes in building materials, e.g., a reinforced concrete slab passing through an insulated exterior wall (balcony slab). Geometric thermal bridges - occur at points where the heat-emitting exterior surface is larger than the heat-absorbing interior surface, typically at building corners. Material thermal bridges - arise from differences in thermal conductivity between adjacent materials, e.g., mortar joints in masonry. Modern thermal bridge calculations according to DIN 4108 and DIN EN ISO 10211 quantify heat loss and are part of the energy performance certificate.

Thermal bridges are particularly problematic in practice due to their invisible nature: While an uninsulated facade is obvious, thermal bridges are often hidden-at window reveal joints, in roller shutter boxes, or in the transition from the basement to the ground floor. They are only noticed when mold or condensation appears. For buyers of existing properties, the following applies: Mold spots on exterior wall corners, behind furniture, or around windows and doors are a clear indication of thermal bridges and should be professionally assessed before purchase.

Effects and Remediation

Thermal bridges lead to heat loss of up to 10-20% of heating energy consumption and to low surface temperatures on the interior side of the room. If the surface temperature falls below the dew point (approx. 12 °C at a room temperature of 20 °C and 60% humidity), moisture condenses-mold growth begins at surface moisture levels above 80%. Renovation measures: Interior insulation with capillary-active materials (calcium silicate boards), exterior insulation with an external thermal insulation composite system (ETICS), thermal separation of balcony connections (retrofitting is only possible with significant effort), window connections with perimeter insulation.

Thermal separation of balcony connections-so-called Isokorb systems-is one of the most effective but also most expensive renovation measures. Retrofit installation requires extensive opening of the balcony and facade; costs range from 3,000-8,000 euros per balcony, depending on the balcony system. A significantly more affordable alternative for new construction: ensure thermally decoupled balcony structures are incorporated during the planning phase-this prevents thermal bridges from the start and costs only a few hundred euros in additional expenses for new construction.

Thermal Bridges and Energy Performance Certificates

Thermal bridges are accounted for in energy performance certificates either using a flat-rate method or a detailed method. With the flat-rate method (Appendix 2 to DIN 4108), a surcharge of 0.10 W/(m²·K) is added to the heat transfer coefficient. With a detailed thermal bridge calculation (DIN EN ISO 10211), the actual heat losses of the individual thermal bridges are calculated-this can result in significantly better or worse energy performance values. For high-quality new construction and renovations to KfW Efficiency House standards, the detailed thermal bridge calculation is standard.

Practical Tip for Homeowners in Nuremberg

We recommend that homeowners in the Nuremberg metropolitan area have a thermographic survey conducted if they experience mold problems or high heating costs. During the heating season (November-February), a thermal imaging camera makes thermal bridges visible-the cost is approximately 200-500 euros for a single-family home. Post-war buildings in Nuremberg from the 1950s-1970s (often in Langwasser, Schweinau, and Maxfeld) in particular have numerous untreated thermal bridges. Exterior wall insulation with an ETICS significantly reduces thermal bridges and is subsidized by the BAFA with up to a 20% grant.

Buyers inspecting an existing property in Nuremberg should explicitly address the issue of thermal bridges in buildings from the 1950s to the 1980s. During discussions with the seller, it is often possible to determine whether there have been mold problems or if heating costs are unusually high. As real estate agents, we help our clients properly assess such structural issues during the buying process-and, if in doubt, recommend an inspection by a building expert before signing the purchase agreement.

Frequently Asked Questions

Is mold always caused by thermal bridges?

Not always, but very often. Mold develops when the relative humidity on the wall surface is consistently above 80%-thermal bridges are the most common cause of this. Other causes include improper ventilation (insufficient ventilation when humidity levels are high), structural moisture (moisture from new construction, water ingress), and defective waterproofing. A thermographic inspection can determine whether a thermal bridge is the cause.

Can I insulate thermal bridges from the inside?

Yes, interior insulation is possible and is often the only option for facades on historic buildings or where exterior insulation is not feasible. Capillary-active materials such as calcium silicate boards (approx. 80-120 euros/m² including installation), which absorb and release moisture, are suitable. Caution: Improperly installed interior insulation can exacerbate moisture damage-planning should be handled by a building expert or energy consultant.

How much energy do I lose through thermal bridges?

In an uninsulated older building, thermal bridges can account for 10-20% of total heat loss. In an insulated building, the relative proportion of heat loss due to thermal bridges actually increases because the rest of the building envelope is better insulated, making the thermal bridges stand out as weak points. A professional thermal bridge calculation in accordance with DIN 4108 quantifies the additional heat loss and helps prioritize renovation measures.

Are thermal bridges mentioned in the purchase contract?

Thermal bridges are generally not explicitly mentioned as part of the contract-they fall under the general agreement regarding the property’s condition. If the seller is aware of mold damage attributable to thermal bridges and conceals this information, they may be obligated to remedy the issue or pay damages due to fraudulent misrepresentation. As a buyer, you should therefore actively inquire about thermal bridges and mold problems and have the answer confirmed.

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