The ISO 13370 ground model

The model calculates the thermal resistance of the ground according to the ISO 13370 standard. This resistance is added to the heat resistance of the floor or underground wall construction. It is affected by the size and shape of the base area of the building body, its depth below the ground level and the ground conductivity.

An example is given in Figure 1 where layers 1-2 are the floor construction. The two outermost layers (here 3-4) are prescribed by the standard. They consist of a 0.5 m layer of the ground material (soil), which is the outermost material given under “Ground properties” in the General tab, and a 0.1 m virtual layer, whose conductivity gives the calculated heat resistance for the ground. This ground resistance is used for all zones connected to the ground, independent of their location in the building, but the heat resistances of the constructions may of course differ. An insulation layer in the ground is treated as part of the floor/wall construction.

Figure 1. Description of the layers of the floor construction and the ground for a zone when opening the floor at advanced level.

The virtual ground temperature (Tv) is a weighted average between the annual average air temperature and its monthly average, with a calculated phase shift. The weighting depends on the base area of ​​the building in relation to its perimeter. If this ratio is larger, the annual average is given greater weight. However, from IDA ICE 5.2, different ground temperatures are used for “edge” zones with external walls and “internal” zones without external walls, but giving the same area weighted mean value (Tv) as before.

An example can be seen in Figure 2, where the logged variables Tamb is the phase shifted monthly average outdoor air temperature, Te is the ground temperature for the edge zones, Ti is the ground temperature for the internal zones (equals here the annual average outdoor temperature), and Tv is the ground area weighted mean value of Te and Ti.

Figure 2. Logged ground temperatures (described above)

All building bodies located at z=0 or lower will be included in the calculation of the thermal resistance and the used ground temperatures. If building bodies are intended solely for shading, they should be raised to a level slightly higher than the 0-level.

A weakness of the model is that the thermal resistance in the ground is the same for all zones regardless of their location in the building. A zone in the center of a building therefore has the same thermal resistance of the ground as a zone with an external wall. Earlier also the virtual ground temperatures, to which the heat losses are counted, were the same for these two cases, but since IDA ICE 5.2 they differ between edge and inner zones, see above.

The thermal resistance of the ground is different for basement walls and basement floors. However, the same ground temperature is used for basement walls as for basement floors in edge zones.

The standard is made for the case where the building body base is filled with zones. The program still allows you to simulate cases where the building body is partially empty. In such case, you may get the following warning: “Significant part of external floor or basement walls is not covered by zones. To build an accurate ISO-13370 ground model, provide reasonable default construction for 'Slab towards ground'.” Zone multiples can be used so that the floor and basement wall area of ​​the building body gets filled up.

The program also gives a warning if the basic geometry is too complicated: “Calculation of heat transfer through the basement walls with ISO 13370 may not be correct because the shape of the basement walls is either unknown or too complicated.” This can happen, for example, if you manually connect the floor to the ground when there is no ground surface in the building body.

The ISO-13370 standard uses the periphery and the base area of the building bodies connected to the ground for the calculation of the heat losses through the ground. If more than one building body is used, they will be treated as merged if they are exactly adjacent to each other, but with any small distance they will be treated as separated and may then unintentionally give a wrong result. It is therefore important to lock the building bodies in the Floor plan after they are created, so that they are not moved by accident.        

If part of the of building is underground due to a sloping ground, it is most correct if the building's basement plan is at an average level (depth) in the ground. Walls or wall parts can then, when needed, be manually connected to either the facade or the ground. Having the basement plan at the zero level is also a possibility, which will give only slightly higher heat losses and thus a safety margin.