Exposed perimeter, characteristic dimension B′, ground thermal conductivity, and slab-on-ground vs suspended floors.
Simple U-value methods (ISO 6946) assume heat flows in a straight line through the construction to the external environment. For ground floors, the heat path is more complex — it spreads laterally through the ground before reaching the external environment, and the ground itself acts as thermal mass that moderates heat loss. ISO 13370:2017 accounts for this using an equivalent thickness method that incorporates floor geometry, ground thermal conductivity, and foundation type. Using ISO 6946 for a ground floor would give a significantly incorrect result. This page covers the ground floor's own U-value; a ground-floor or basement wall/floor junction's ψ-value uses a related but distinct BR 497 convention — see the ψ-value Calculations & FEM FAQ.
The exposed perimeter P is the length of the floor perimeter that is adjacent to the external environment or to unheated space. For a detached house, this is the full perimeter of the ground floor. For a semi-detached house, it excludes the shared wall with the neighbour — only the three exposed sides count. For a mid-terrace house, only the two end walls count. Getting the exposed perimeter right is important — an incorrect value directly affects the calculated U-value. Psiclops uses your input value and includes it in the stored result for traceability.
B' is the characteristic floor dimension defined in ISO 13370:2017 as the floor area divided by half the exposed perimeter (B' = A / 0.5P). It represents the effective width of the floor for heat loss purposes. B' determines which of the two ISO 13370 equations applies — for shallow slabs (where the equivalent thickness dt is less than B') and for deep slabs (where dt is greater than or equal to B'). Psiclops calculates B' automatically and reports it alongside the result.
Ground thermal conductivity λg describes how well the soil beneath the building conducts heat. ISO 13370:2017 Table 2 provides default values by soil type: clay or silt 1.5 W/(m·K), sand or gravel 2.0 W/(m·K), homogeneous rock 3.5 W/(m·K), and unknown 2.0 W/(m·K). Psiclops defaults to 1.5 W/(m·K) — clay or silt — rather than the standard's own 2.0 W/(m·K) 'unknown' figure, because clay or silt is the most common UK ground condition and gives a realistic starting point for the large majority of projects, not because it is the more conservative choice. If your own ground conditions are genuinely unconfirmed rather than simply not yet looked up, the standard's own 'unknown' value of 2.0 W/(m·K) can be entered directly instead. A site-specific value from a ground investigation report can be used if available — this will give a more accurate result.
Slab-on-ground (the most common UK new build foundation) sits directly on the ground with no void beneath it. Heat loss is calculated using the ISO 13370 equivalent thickness method. A suspended floor has a ventilated void between the floor deck and the ground. The void introduces an additional heat transfer path — air in the void exchanges heat with both the floor above and the ground below, and the void itself is ventilated to the outside. ISO 13370 accounts for the void ventilation rate (the area of ventilation openings per unit of exposed perimeter) in a separate calculation. Suspended floors typically have higher heat loss than well-insulated slabs because the void is difficult to insulate effectively.
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