What is a thermal bridge and why it matters

A thermal bridge is a localised zone of the building envelope where thermal resistance is reduced compared to the rest of the wall, floor or roof. It arises from: (1) geometric discontinuity (external corners, edges, wall-floor junctions); (2) material discontinuity (RC columns or beams embedded in brick wall); (3) insulation crossing (steel beams, balcony brackets). Heat flow at a thermal bridge is concentrated: flux density (W/m²) can be 2-5× that of the plain wall, causing: higher energy loss, local drop in internal surface temperature (with condensation and mould risk), and in extreme cases plaster cracking from thermal stress. In an unrefurbished building thermal bridges can generate 15-30% of total losses, with significant impact on energy class and heating cost. Standards EN ISO 14683 (simplified methods with abaci) and EN ISO 10211 (2D/3D finite element analysis) provide quantification methods.

The fundamental formula: Q_pt = ψ · L · ΔT

Instantaneous loss through a linear thermal bridge is:

Q_pt = ψ · L · ΔT  [W]

where ψ (psi) is linear thermal transmittance (W/(m·K)), depending on node geometry and stratigraphy; L is bridge length (m); ΔT indoor-outdoor difference (K). Example: "column embedded in wall" bridge with ψ = 0.30 W/(m·K), 3 m column height, ΔT = 20 K → Q_pt = 18 W. On a whole building with many nodes (columns, floors, corners, sills) the sum can easily reach 300-800 W. For yearly analysis multiply by equivalent full-power heating hours: temperate climate 2000-2500 h/year; cold climate up to 3200 h.

Typical ψ values for main nodes

ψ values are tabulated in EN ISO 14683 (Table A.2) or in national handbooks. Indicative values in W/(m·K):

• Column-wall node: 0.15 (good insulation) to 0.80 (exposed column, no insulation).
• Floor-wall node: 0.25-0.60 uncorrected, 0.05-0.10 with "thermal break" (Schöck Isokorb, Halfen).
• External wall corner: 0.10 (with insulation) to 0.60 (without).
• Window sill: 0.15-0.45 depending on frame integration.
• Cantilever balcony (RC): 0.80-1.20 without thermal break, reducible to 0.15-0.25 with Isokorb connectors.
• Roof-wall node: 0.10-0.25 with well-designed insulation continuity.
• Ground floor-wall: 0.30-0.60 without perimeter strip, 0.10-0.20 with.

"Thermal break" is the main correction strategy: interrupting conductive material continuity (RC, steel) with a high-thermal-resistance load-bearing insulating element (rigid PUR, Foamglas, Isokorb with stainless steel bars in polystyrene box).

How to compute ψ: from tables to 2D FEM

Three normative methods of increasing precision and complexity exist:

1. Tabular method (EN ISO 14683 Table A.2 or national annexes): pre-tabulated ψ values for standard configurations. Precision ±30-50%. For pre-sizing.
2. Abacus method (commercial databases, ANIT, CENED software): abaci recalculated for typical local stratigraphies. Precision ±20-30%. Market standard for energy certification.
3. 2D finite element analysis (EN ISO 10211): 2D FE thermal analysis in steady state. Software: Therm (LBNL, free), Trisco, HTflux, Comsol, Physibel. Precision ±5-10% with good meshing. Mandatory for NZEB buildings, PassivHaus certification.

2D FEM procedure: draw node with mm precision, assign thermal conductivity λ of each material (concrete 2.0; brick 0.4-0.6; EPS 0.036; mineral wool 0.036; wood 0.13), set boundary conditions (indoor 20 °C with R_si = 0.13; outdoor 0 °C with R_se = 0.04), solve steady thermal field, calculate ψ by subtraction from total conductance.

Mould and condensation: the f_Rsi factor

Beyond energy loss, thermal bridges cause local drop in internal surface temperature that may lead to condensation and mould. Standard verification (EN ISO 13788) uses temperature factor f_Rsi:

f_Rsi = (T_si − T_e) / (T_i − T_e)

Must be f_Rsi ≥ 0.72 (for temperate climates with 65% RH and T_i = 20 °C). If f_Rsi < 0.72, condensation occurs in normal winter conditions → mould within 10-30 days. Typical problematic nodes: uncorrected external corners (f_Rsi = 0.55-0.65); RC balconies without thermal break (0.50-0.60); column-wall junctions (0.60-0.70). This calculator doesn't directly compute f_Rsi (requires 2D FEM), but for ψ > 0.60 W/(m·K) the assessment includes a mould risk warning.

Thermal bridge correction strategies

• External insulation (ETICS): continuous insulation on all perimeter walls, covering columns, corners, sills. Reduces standard-node ψ by 60-80%. Typical thickness 8-16 cm EPS/XPS/mineral wool with λ = 0.032-0.040.
• Balcony thermal break (Schöck Isokorb, Halfen HIT): structural connectors replacing continuous slab with stainless steel bars in polystyrene box (80-120 mm thick), reducing ψ from 0.80-1.20 to 0.15-0.25 W/(m·K). Cost 150-300 €/m of balcony, but amortised by loss reduction.
• Window sill and jamb correction: adhesive membranes, compressive tape (Kompriband), thermal-break insulating materials (Purenit, Compacfoam) as window support. Reduces ψ 40-60%.
• Ground floor perimeter insulation: 1-2 m XPS strip along external perimeter (vertical under foundation), or continuous insulation under crawlspace. Reduces ψ 50-70%.
• Internal insulation: solution for constrained heritage buildings. Reduces plain wall losses but increases mould risk at untreated bridges. Requires hygrothermal analysis.
• Sprayed PUR foam: for filling irregular cavities around beams/columns crossing roof insulation.

Thermal bridge and building energy class

Thermal bridge quantification is mandatory in Energy Performance Certification (APE) (Italian D.M. 26/06/2015) and in building energy demand calculation per UNI/TS 11300-1. Standard procedures:

• Simplified method: flat 5% transmission demand increase if bridges are corrected; 10-20% for uncorrected existing; 30-50% for pre-1980 buildings without insulation.
• Detailed method (mandatory for NZEB from 2019): per-node calculation with 2D FEM and sum. Required for PassivHaus.

Energy class effect: proper thermal bridge management can raise class from E to B, with significant market value impact.

How to use the calculator

Enter the linear transmittance ψ in W/(m·K) of the node you want to analyse (read from EN ISO 14683 tables, ANIT abaci, manufacturer manuals, or engineer's 2D FEM calculation). Typical values: 0.05-0.10 well-designed nodes with insulation; 0.20-0.40 standard nodes; 0.60-1.20 critical uncorrected nodes. Enter length L in metres (3 m column: L = 3; 10 m floor: L = 10; 2.7 m tall external corner: L = 2.7). Enter ΔT in K (20 K temperate climate; 25 K cold). Enter yearly heating hours (2000-2500 temperate; 2500-3200 cold), generator efficiency (0.90 traditional boiler; 0.95 condensing; 3-4 heat pump SCOP), energy price in €/kWh (gas 0.10; electric 0.28) and CO₂ factor (natural gas 0.20; diesel 0.27; electric EU mix 0.28). Calculator returns linear conductance H_pt, instantaneous heat loss Q_pt, yearly useful and primary energy loss, yearly cost, CO₂ emissions and quality assessment.