Building envelope: thermal insulation, thermal bridges, airtightness and construction details
Insulation and the thermal-bridge-free rule
Opaque envelope U-values in a Central European Passive House typically land at 0.10–0.15 W/(m²K), driven by the 15 kWh/(m²a) target rather than by a fixed code. The stronger rule is “thermal-bridge-free”: every junction should reach Ψ ≤ 0.01 W/(mK), a linear loss small enough to ignore. If that is not achievable, enter the actual Ψ in PHPP; default allowances are deliberately pessimistic.
Airtightness and detail continuity
The airtight layer, usually internal plaster or a membrane, must be continuous and inspectable. Penetrations for cables, ducts and pipes are the usual failure points. The certificate requires n50 ≤ 0.6 h⁻¹ from a blower-door test, normally the average of pressurization and depressurization using the measured volume. Plan the layer on drawings before construction: window connections, floor slab junctions, roof–wall intersections, service penetrations.
Worked example: quantifying a junction
A balcony slab penetrates the wall. The 2D calculation gives L2D = 0.42 W/(mK). Reference flanking areas: wall U = 0.14 W/(m²K) over 1.5 m, slab U = 0.20 W/(m²K) over 0.5 m. Ψ = 0.42 − (0.14 × 1.5) − (0.20 × 0.5) = 0.42 − 0.21 − 0.10 = 0.11 W/(mK). That is eleven times the target; for a 10 m balcony line at ΔT = 30 K it adds 0.11 × 10 × 30 ≈ 33 W/K, enough to shift the heating load. The fix is a structural thermal break, re-detailed and recalculated to bring Ψ near zero.
Traps
Claiming “thermal-bridge-free” without 2D or 3D calculation. Forgetting that Ψ depends on the flanking U-value reference areas, so lengths and sign conventions must match the detail exactly. And assuming a good n50 result proves good detailing when ducts or the attic hatch were taped shut for the test.