Modelling a Balcony Thermal Bridge

The cantilevered concrete slab is the classic worst-case thermal bridge. Here is how to model it and judge it by the numbers.

Why Balconies Matter

A cantilevered balcony slab is structural concrete punching straight through the insulation layer — a continuous fin of λ ≈ 1.8–2.5 W/(m·K) material connecting the heated interior to the outdoor air. In literature and thermal bridge catalogues, an uninsulated continuous slab typically shows ψ-values in the region of 0.5–0.9 W/(m·K), while a slab with a structural thermal break element is typically in the region of 0.1–0.3 W/(m·K) — one of the largest single improvements available in a building envelope. The exact value depends on slab thickness, wall build-up and the break product, which is why you model it.

Beyond heat loss, the cold slab depresses the interior surface temperature at the floor–wall junction — so this detail needs both the ψ-value and the fRsi check.

Step 1 — Draw the Cross-Section

Model the vertical section through the junction:

• Wall: the full build-up (e.g. masonry or concrete with exterior insulation), extending at least 1 m above and below the slab (EN ISO 10211 cut-off rule).
• Floor slab: the interior slab with its floor build-up, extending at least 1 m into the room.
• Balcony slab: the cantilever, continuous with the floor slab. EN ISO 10211 allows truncating projecting parts 1 m from the wall face.
• Variant B (recommended comparison): duplicate the project and insert a thermal break — a region of insulating material (λ on the order of 0.1 W/(m·K) for typical load-bearing break elements, per the manufacturer's datasheet) where the slab crosses the insulation plane.

The rectangle tool, snap-to-grid and boolean operations in the geometry editor make this a few minutes' work; or import the architect's DXF/DWG section directly.

Step 2 — Materials

• Concrete for the slab and balcony (EN ISO 10456 design value from the built-in library).
• Insulation (EPS, XPS or mineral wool) for the wall insulation layer.
• Screed, plaster, masonry as present in the real build-up — thin but conductive layers still matter near the junction.
• A custom material for the thermal break element, using the manufacturer's declared equivalent conductivity.

Step 3 — Boundary Conditions

• Interior surfaces (room side of wall and floor): 20 °C, R_si = 0.13 m²·K/W for the heat-flow/ψ calculation — switch to R_si = 0.25 m²·K/W for the EN ISO 13788 condensation check.
• Exterior surfaces (outside wall face, balcony top, bottom and edge): 0 °C (or your design temperature), R_se = 0.04 m²·K/W.
• Cut-off planes (top/bottom of wall, end of floor slab, truncated balcony end): adiabatic.

Step 4 — Solve and Evaluate

1. Mesh and solve; inspect the temperature map and isotherms — with no thermal break you will see the isotherms pulled sharply toward the interior along the slab.
2. In Results → PSI, define the flanking components (wall above/below with its 1D U-value, floor as appropriate to your dimension convention) and read Ψ.
3. Read fRsi and the critical point — for balconies it usually sits at the wall–floor junction on the room side.
4. Repeat for the thermal-break variant and compare: Ψ, fRsi, and the isotherm pattern. Document both in a PDF from the Sheets tab.

Modelling Tips

• Steel reinforcement crossing the break (in real break products) is already accounted for in the manufacturer's equivalent λ — do not model individual bars in 2D.
• A 2D model assumes the detail is uniform along the facade. Point fixings, brackets and balustrade posts are 3D effects outside the 2D scope — note this in the report.
• Check mesh independence near the slab/insulation corner, where gradients are steepest — refine until the heat flow stabilizes.