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Interior walls on floor slabs are often calculated two-dimensionally without the influence of the ground. The result will be strongly on the safe side because the “insulating effect” of the ground is not taken into account (see Figure 1). As a matter of fact the thermal bridge of an interior wall on a floor slab depends on the distance from the floor slab edge. The Ψ-value improves with increasing distance of the interior wall to the floor slab edge.
In the case of a specific building the position of the interior wall is known. The Ψ-value can then be calculated based on the same method as in the section on "Exterior wall on floor slab". For this, first a conductance without taking the interior wall into account is determined and then the conductance with the interior wall taken into account at the corresponding point. However, this requires a three-dimensional calculation which will be too time-consuming in most cases.
As a two-dimensional approximation, which is also used in the context of certification for a construction system, it is recommended that the ground is taken into account and the distance of the interior wall from the floor slab edge assumed to be 1.5 metres. This procedure is demonstrated below using an example.
Modelling of the exterior wall can be omitted, but adiabatic cutting of the front surface of the floor slab will then be necessary. The Ψ-value results from the difference in the conductances of the undisturbed floor slab and the floor slab disturbed by the interior wall.
Information! Instead of adiabatic cutting of the front surface it usually makes more sense to depict the exterior wall as well because the connection of the exterior wall to the floor slab will be calculated in any case and the interior wall can be quickly placed into the corresponding model. The Ψ-value being sought will thus be the difference in the conductances with and without the interior wall. Another advantage is that the surface temperatures determined in this way will be assessed as more meaningful. The nearer the interior wall is to the floor slab edge, and therefore to the exterior wall, the more on the safe side the surface temperatures will be, because these increase in line with the increasing distance to the edge.