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In the corner of the living room, a black spot suddenly appears on the white wall paint. Mould! How could this happen?
After consulting an expert, the situation is clear. There is a pergola above the living room. During the construction of the building, the concrete ceiling of the pergola was not thermally separated from the living areas. As a result, the entire ceiling of the living room has become a linear thermal bridge.

The corner of the room is particularly critical because the smaller, warm interior surface also meets the larger, cold exterior surface there. Consequently, the corner of the room has become the coldest point in the room where condensation occurs when warm, moist air meets this cold surface. If some time passes, the moisture on the surface, together with deposited dust and the paint, forms the ideal breeding ground for mould spores.

A thermographic image of the building in winter makes the thermal bridges visible: Radiator niches in particular, but also roller blind boxes, balcony connections, canopies and (uninsulated) roofs are then clearly visible in colour as red - i.e. warm - spots. In our picture below, the roof is insulated and the windows seem to have already been replaced.
Thermal bridges are especially problematic during the heating period because there is increased heat loss compared to the neighbouring building component surfaces. If the heating is switched off, the thinner wall in the area of the niche cools down more, moisture from the room air condenses on the cold wall and in the worst case mould forms.

Material-related thermal bridges occur where different materials meet. In a homogeneous component, the isotherms (= lines of equal temperature) run parallel to the component surface. In a component made of different materials, the isotherms run as curved lines in the "disturbed" areas. At the abutting areas they come to the surface, which leads to a room-side temperature reduction in the component areas with the lower thermal resistance. An example of a material-related thermal bridge is a rafter roof.

To comply with the Building Energy Act (GEG) and to provide evidence for the federal funding for efficient buildings (BEG), it is necessary to prepare an energy balance of the building. For this purpose, the losses through the components of the thermal envelope and the primary energy demand are considered. When calculating losses through the external components, a distinction is made between residential and non-residential buildings. The losses through the thermal envelope of residential buildings are represented by the transmission heat losses. The U-values of the building components, but also the thermal bridges are included in the calculation - according to the formula: 

HT=∑Ui*Ai*Fxi+UWB*Ai

The thermal bridge surcharge takes into account the additional heat losses due to thermal bridges, resulting in a larger total transmission heat loss through the building envelope. The thermal bridge allowance can be assumed as a lump sum, but can also be calculated explicitly.

Care must always be taken to avoid thermal bridges. For this purpose, the legislator has created a practical set of rules in German DIN 4108 Supplement 2, in which numerous thermal bridges are illustrated. This means that thermal bridges can already be minimised very well in new buildings.

• Cantilevered building components such as balconies or canopies should always be thermally decoupled from the heated enclosure surface. This can be achieved, for example, with so-called iso-baskets, which are connected to the façade by steel girders. The load-bearing thermal insulation element helps to minimise thermal bridges on projecting building components.
• If the statics permit, the heat losses between the floor slab and the façade can be reduced in new buildings by using chimney bricks. Kimm bricks are sand-lime bricks with high compressive strength and low thermal conductivity.
• When fixing insulation materials, care must be taken to avoid creating new thermal bridges. For example, stainless steel anchors should be avoided, as they have a much higher thermal conductivity or higher transmission heat losses than insulating materials and consequently create a thermal bridge. Plastic dowels or glued joints as well as a two-layer installation of the insulation boards are important means to avoid thermal bridges in the design.
• Furthermore, compact designs are to be preferred, whereby the so-called A/V ratio (ratio of the heat-transferring envelope area A to the building volume V or to the usable floor area) is to be taken into account.

Radiator niches and roller shutter boxes are particularly common thermal bridges in old buildings. In the past, radiators were always installed in niches under the window, as windows were the thinnest part of the façade and the radiators were supposed to compensate for this. Old radiators only gave off their heat through radiation and could therefore be placed in a niche. However, the niche represents a thermal bridge. If the windows are replaced in the course of a renovation, this increases the mould problem at thermal bridges, as the moisture can no longer escape from the room if it is not properly ventilated and settles on the cold wall. Therefore, when windows are replaced, the radiator recess should be insulated in any case, if the building does not receive a thermal skin anyway.