Thermal Bridge and Airtightness Dilemma in a Panel-Block Retrofit
A 1985 precast concrete panel apartment block in Sofia (climate zone D) is being renovated. The design adds 120 mm EPS to the facade, replaces windows with triple-glazed units (Uw = 0.9 W/m²K), and insulates the roof. The consultant notes that the panel joints and balcony slabs create linear thermal bridges; the original design assumed Ψ = 0.10 W/mK at panel joints and Ψ = 0.30 W/mK at balcony slabs, but the contractor proposes simply covering balcony slabs with EPS and leaving the slab edge exposed. Indoor surface condensation has been reported at the base of external walls in winter. The client wants an EPC class jump from D to B and asks whether the proposed detailing will achieve it.
1. Assess the thermal-bridge treatment and its effect on condensation risk and the EPC outcome.
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Thermal bridges. Covering balcony slabs with EPS while leaving the slab edge exposed does not eliminate the linear bridge; it can worsen the Ψ value because the slab remains a conductive path through the insulation. The correct approach is a structural thermal break at the slab connection or continuous insulation wrapping the slab edge. Panel joints must be sealed and insulated with a compatible detail; simply butting EPS panels leaves a gap.
Condensation. The reported condensation at the wall base is consistent with a cold bridge and/or high indoor humidity. Adding external insulation generally raises the internal surface temperature - if the insulation is continuous. If the base detail is discontinuous, the surface temperature may remain below dew point. I would require a hygrothermal check (e.g., Glaser or WUFI) at the base and at balcony connections, and verify the temperature factor fRsi ≥ 0.7 (or the national requirement) to avoid mould.
EPC outcome. The EPC calculation uses the actual Ψ values from the detailing. If the contractor's detail yields Ψ = 0.30 or worse at balconies and unsealed joints, the transmission heat loss will be higher than the design assumed, and the class jump to B may not be achieved. The consultant should demand a thermal-bridge catalogue or calculated Ψ values before signing off the design, and re-run the EPC with those values.
2. What verification steps would you require before accepting the as-built EPC?
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1. Thermal-bridge values. Obtain calculated or catalogue Ψ values for panel joints, balcony slabs, window reveals, and the wall base. Confirm they match the values used in the EPC. Reject assumed default values if measured/calculated ones are available.
2. Airtightness. Require a blower-door test (n50). Panel blocks often have poor airtightness; a target of n50 ≤ 1.5–2.0 h⁻¹ (or the design value) should be specified, with remedial sealing if exceeded.
3. Insulation continuity. Site inspection with thermal imaging in winter to detect gaps, compressed EPS, and cold spots at the base and balconies.
4. Window installation. Check that windows are installed with proper sealing and that the Uw = 0.9 value is supported by test certificates and correct frame/glazing combination.
5. Condensation check. Confirm the fRsi criterion at the base and at any remaining bridge.
6. Documentation. As-built drawings, material certificates, and the updated EPC calculation using verified values. If any value differs from design, the EPC must be recalculated before issuance.