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Thermal bridges

Last modified by Claudia Bless on 2026/08/26 10:21

These coefficients (psi-values) are used to calculate losses by thermal bridges. The coefficients are given per unit of element size (in most cases per meter). The sizes of elements are automatically calculated from zone geometry, but may be also given by the user. Local changes in geometrical or bridge coefficients are done under Loss factor for thermal bridges in the zone form.

The numerical psi-value of each thermal bridge type depends on the selected measure of envelope area (Wall/slab dimensions in Thermal bridges form). The following external area measures are defined in ISO 13789:

Internal: The external wall/roof area is calculated as seen from the inside of each zone (wetted by indoor air). There will then be gaps in the external wall/roof at each internal wall or slab. A positive psi-value must be given for each thermal bridge in order to take account of the heat transfer through these gaps. This setting does however not affect the surface area for solar radiation absorption and thermal exchange with the outside, which is always the external area facing the outside air.
Overall internal: As internal, but includes also the cross section area of the inner walls and slabs that are connected to an external surface. Half the cross section area is attributed to each adjacent zone.
External: The external wall/roof/floor area of each zone is calculated as seen from the outside (including corners). The cross section area of the external floor slab is not included in the area of the external walls. The area of the internal surface facing the inside air is however always used for solar and thermal interaction with the zone air and surfaces.
External incl. floor slab: As External, but includes also the cross section area of the external floor slab in the area of external walls.

In addition to these, an additional measure is defined in IDA ICE for the purpose of conservation of total volume:

Preserve wall volume: As External, but only half of the cross section area of the external walls (and roof) is included.

The envelope area definitions are applied to all envelope areas, except for floors connected to the ground, where the cross section area of the inner walls always is included in the floor area. Half of the cross section area of an inner wall is attributed to the floor area of each of the adjacent zones. No thermal bridge is computed for a joint between an inner wall and a floor connected to the ground. This is according to ISO 13370, which governs heat losses to the ground.

The drawings of these five Envelope area definitions at the top of the help page show sections of a room with external floor, wall and roof. The red line shows the Envelope area according to the definition.

The envelope area definitions affect the size of the external wall construction that is used, but they do not affect the size of the inside or outside surface areas that interact with the surroundings regarding solar radiation, convection and thermal radiation.