Thermal bridges – energy weak spots in the building envelope

Climate change and increasing demand for energy mean that a sustainable strategy to increase energy efficiency is required. For several years now, numerous measures have therefore been in place to reduce energy consumption. This has had a positive impact: insulation standards are improving and energy requirements are being perceptibly reduced.

As a result of these developments, weak spots in the building envelope which are not remedied are becoming more significant. The same is true of thermal bridges: their status as troublemakers for total heat loss in a building is crystallised in building ratings in accordance with the German Energy Saving Ordinance (EnEV).

What are thermal bridges?

Thermal bridges are points in the thermal building envelope where a difference in thermal energy transfer is perceptible. They cause more thermal energy to flow from inside to outside. As a result the surface temperature in the room decreases more quickly than in adjacent areas.

Various types of thermal bridges

With thermal bridges, a distinction is made between the various reasons why they occur:

  • Thermal bridges resulting from materials:If various materials with varying thermal conductivity and insulating qualities are juxtaposed, thermal bridges resulting from materials are created (e.g. reinforced steel struts or cement in the walls or internal walls which break the insulation layer).
  •  Geometric thermal bridges:If the heat-absorbing internal surfaces and the heat-emitting external surfaces are different in size, this leads to geometric thermal bridges (e.g. building edges or building corners).
  • Thermal bridges resulting from construction:Connections between building components within composite thermal systems cause construction-related thermal bridges (e.g. balconies, roller shutters, gutters).

Effects of thermal bridges

There are many reasons to reduce thermal bridges. This is because the consequences of thermal bridges have a negative effect on the existing building and its users.

Consequences of thermal bridges:

  • Increased heating requirement and energy consumption:The increased heat outflow leads to low internal surface temperatures. The cold surfaces only radiate small amounts of heat – this has a negative effect on thermal comfort. This is balanced by increased energy requirements. Energy consumption increases!
  • Inadequate living hygiene:The interaction of warm, moist air on cold surfaces leads to the formation of condensation. Together with dust and wallpaper paste or paint, this is an ideal feeding ground for mould. This has the potential to cause health issues!
  • Risk of structural damage:Constant condensation can cause the building to become damp. If the building structure is penetrated, this can cause long-term damage to the structural engineering of the building. Permanently damp building components cause increased thermal conductivity. This then reinforces the thermal bridge. 

How to deal with thermal bridges

Both in new builds and in renovation projects, technical planning should be used to handle thermal bridges. This is the only way to minimise negative consequences. Planners and tradespeople have an equal responsibility to guarantee that the necessary measures are taken to minimise thermal bridges.

German Energy Saving Ordinance (EnEV): construction engineering regulations

The German Energy Saving Ordinance (EnEV) includes regulations for accounting for thermal bridges and energy evaluations concerning them. The regulations include keeping the effect of construction-related thermal bridges on the annual primary energy requirement as low as possible.

Evaluation and ratings in relation to thermal bridges

The German Energy Saving Ordinance includes various processes to determine this:

  1. The evaluation can take place on a flat-rate basis using thermal bridge corrective values.
  2. A simplified calculation procedure in relation to thermal bridges takes place by assigning standard, flat-rate sample solutions. 
  3. Detailed calculations allow the thermal conductivity coefficient to be calculated.

The plan to minimise thermal bridges can be individually adapted to the construction situation by means of a detailed calculation.

Reduction of thermal bridges

It is not possible to construct a building with no thermal bridges at all. In particular, geometric thermal bridges at building corners cannot be completely prevented. However, the number of thermal bridges can be reduced using architectural tricks. Terraced houses are the perfect example of how to deal with geometric thermal bridges: the number of building corners can be drastically reduced through this method of development.

Qualified evaluations, detailed planning and technical expertise in implementation can minimise the negative effects of thermal bridges. The aim is to avoid the surface temperature of the internal walls falling. The use of insulating products for the building envelope also contributes to reducing energy loss – it is therefore possible to construct an energy-efficient building despite thermal bridges!

Expert opinion: How it's done is decisive!

“Not every thermal bridge is a concern!” according to Jacek Goehlmann, Head of Technical Services at tremco illbruck. “Each change in geometry and change in materials in the building envelope represents a thermal bridge. This means that every window joint within the wall construction is a thermal bridge. That certainly doesn't mean we don’t build windows! The question is therefore not if but rather how.”