According to the Spanish Technical Building Code (T.I.B. – C.T.E.), a thermal bridge is:
An area of the building envelope in which is evidenced a variation of the uniformity of the construction, either by a change of its thickness; by a change of the materials; by an intersection of materials of different conductivity; etc, which entail a reduction of the heat resistance compared to the rest of the envelope. Thermal bridges are parts of buildings where the possibility of surface condensation and moisture is increased.
Thermal bridges in the building envelope cause a change in its thermal behavior (lower thermal resistance and non-homogeneous); the interior conditions (sometimes formation of condensation and mold, due to the lower temperature of the inner surface); or even in the comfort, since the reduction of the inner surface temperature affects the indoor radiant temperature, and this can cause discomfort situations, even when the interior air temperature is acceptable.
As for the TIB – CTE, the most commont thermal bridges are:
- Thermal bridges embedded in the envelope:
- Supporting estructure in facades
- Around windows, doors and rooflights
- Shutter boxes
- Other embeded thermal bridges (repeated within an element)
- Thermal bridges where several planes intersects:
- Floors which penetrate the thermal envelope
- Wall/roof junctions
- Roofs with parapet
- Roofs without parapet
- Wall/floors junctions:
- Wall with floor slab
- Wall with retaining walls
- Corners of walls
- Junctions between overhangs and walls
- Junctions between interior partitions with external walls/roofs/floors
They can also be divided into linear or punctual: Linear thermal bridges are those whose cross-section is uniform along one direction, while the punctual thermal bridges are those occurring in a localized point of the envelope, and not along a line.
In these particular zones is necessary to calculate the surface temperature factor (Frsi), and the linear thermal transmittance Ψ, all as described in the UNE-EN-ISO 10211.
The temperature distribution and heat flux thought a building element can be calculated if the boundary conditions and the construction details are known. The conductivity values of the materials and construction products can be calculated according to the standard ISO 10456, or obtained from reliable tabulated values (e.g. those from the TIB-CTE)
The bidimensional model conditions, conjunction lenght, calculation rules… in order to obtain the lineal thermal trnsmittance are defined in the above mentiones ISO standard.