What is a 'thermal bridge'?

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ponte-termico-risolto-isotermeIn our articles, as we write about Passive Houses, thermal comfort and energy efficiency, we often assign great importance to the term 'thermal bridge'. We'll try to explain here what this is all about.

A 'thermal bridge' is a localized area of the building thermal envelope where the thermal resistance is not consistent (whether because of structural geometry or material composition). In such an area, the heat flow between inside and outside is different - usually higher - compared to the rest of the structure.

In simpler terms, thermal bridges occur in local structural elements within the envelope structure, especially if these elements cut through insulation layers. Geometrical thermal bridges are any edge of the building.

With regards to energy efficiency,

a thermal bridge usually causes a higher heat flow. These localized higher heat flows have to be accounted for in the overall energy balance of the building. This calculation has to be carried out with a specific finite element software, according to EN ISO 10211. Unfortunately, most professionals in Italy stick to older norms and design practices.

With regards to occupant health and comfort,

these localized weaker areas have a lower interior temperature compared to the rest of the components of the thermal envelope (walls, roof etc.). This situation causes thermal discomfort ("I feel cold"), and can foster the growth of mold or the presence of condensation. For this reason, it is necessary to develop construction documentation (including accurate construction detail) and evaluate these thermal bridges with finite element calculations according to EN ISO 13788. This detailed design process is extremely important in case of deep energy retrofit, to provide thermal comfort and avoid the risk of mold.

In the design of high quality and comfortable buildings, such as passive houses, it is of extreme importance to eliminate all thermal bridges caused by interruption/reduction of insulation layers. As far as geometrical thermal bridges, even if it is not possible to eliminate all edges of a building (unless you want to live inside a sphere), it is nonetheless important to choose a simple shape for the thermal envelope.

Not only does a compact building have less geometrical thermal bridges than a complex one, but it is also cheaper to build and more resistant to seismic forces.

In the case of a deep energy retrofit, it may not be possible to entirely eliminate all structural thermal bridges. In these cases, it is necessary to design the retrofit in order to grant health and comfort, providing an even interior temperature, and to minimize as much as possible heat losses.