Wherever a balcony slab, wall-to-floor junction, or other conductive path interrupts the insulation layer, heat escapes faster than the plane elements’ U-values account for — that shortfall is thermal bridging. At junctions it is quantified as the psi-value (linear thermal transmittance, W/mK): the extra heat loss per metre of junction beyond what the adjoining elements already lose, formally ψ = L2D − Σ(Ui × li), where L2D is the thermal coupling coefficient from a two-dimensional analysis per EN ISO 10211 and EN ISO 14683.
Psi-values sort into severity bands: below 0.01 W/mK is negligible — the Passive House target — 0.01–0.10 acceptable, 0.10–0.30 significant, and above 0.30 severe. The energy cost of a junction follows directly from climate: Q = ψ × length × HDD × 24 / 1000 turns a psi-value and the heating degree-days into annual kWh, so a detail that looks small per metre compounds over every metre of every floor line.
Bridges also make surfaces cold, which is the mould question. The temperature factor fRsi = (Tsi,min − Te)/(Ti − Te) measures how warm the coldest interior surface stays relative to the inside–outside difference; per EN ISO 13788, fRsi at or above 0.75 indicates no mould risk while below 0.72 condensation and mould are likely. A junction with ψ = 0.05 W/mK and fRsi = 0.80 therefore passes twice — acceptable heat loss, and a surface warm enough to stay dry.