✅ For Best Experience: This article covers advanced technical stuff. To get the most out of this guide, we recommend starting with our introduction to the topic: Boundary Types Guide.
General Modeling Rules
Before you begin, keep these key points in mind for any ground floor junction:
Include Skirting Boards: Always draw any skirting boards as part of the internal junction detail.
Model the Ground: You must include the ground below and adjacent to the floor in your model. The size of this ground area and its thermal properties should be determined according to official standards like BS EN ISO 13370.
How to Model Solid Ground Floor
Your drawing should include the wall, the solid floor slab, the insulation, and the block of ground underneath it as specified by the standards.
Boundary Conditions: The internal and external temperatures are applied to the wall and floor surfaces as usual. The boundary conditions for the ground area are calculated according to the procedure in BS EN ISO 13370.
How to Model Suspended Ground Floor
Suspended floors have a ventilated crawl space or void underneath them, which requires an extra step. Your drawing will include the wall, the suspended floor structure, and the ground below. The surfaces inside the underfloor void are then assigned the calculated intermediate temperature (Tu) as their boundary condition.
The Intermediate Temperature (Tu): The underfloor space is not at the full external temperature. You must first calculate its intermediate temperature (Tu). This is based on a heat balance that considers the ventilation of the space (e.g., air vent size, wind speed).
How to Model Door Threshold
A door threshold is different from a typical wall-to-floor junction because there is no wall construction below the door.
You should model the threshold detail itself. Where the threshold meets the floor, apply an adiabatic boundary (a zero heat flow line), just as you would for a window sill.
Calculation Note. Because there's no wall, the part of the thermal bridging formula that accounts for heat loss through the wall is removed from the calculation.