Balcony Thermal Bridge: Default Ψ Versus a Thermal Break
On a 12-unit passive-house project, 12 concrete balcony slabs each 2.5 m wide penetrate a 200 mm mineral-wool insulated timber-frame wall. The structural engineer has drawn the slab continuous through the insulation; the architect proposes to accept the PHPP default Ψ = 0.60 W/(mK). A proprietary thermal-break bracket is available with a calculated Ψ = 0.15 W/(mK), but it adds cost and coordination. The wall U-value is 0.14 W/(m²K), indoor design is 21°C, outdoor design is -12°C, and indoor RH is 50%. The project has 85 kKh heating degree hours. The first PHPP run gives heating demand 14.5 kWh/(m²a), close to the 15 limit. The client is worried about mould at the slab edge in bedrooms and asks whether the thermal break is worth it.
1. Calculate the annual heat loss through the balcony penetrations with and without the thermal break, and compare it with the remaining heating-demand margin.
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Total penetration length is 12 × 2.5 m = 30 m. Using Q = Ψ × L × HDD in kKh: without a break, 0.60 × 30 × 85 = 1,530 kWh/a; with the break, 0.15 × 30 × 85 = 382.5 kWh/a. The saving is about 1,147.5 kWh/a. For a 1,200 m² TFA building this is about 0.96 kWh/(m²a). The current heating demand is 14.5 kWh/(m²a), so the margin to the 15 kWh/(m²a) limit is only 0.5 kWh/(m²a). In other words, accepting the continuous slab consumes roughly twice the remaining margin and would push the model to about 15.5 kWh/(m²a) before any other as-built penalties. That is a certification failure unless offset elsewhere. The calculation is approximate because HDD and Ψ are project assumptions, but the direction is robust: a continuous concrete slab is not a minor detail on a project this close to the limit.
2. Assess the condensation and mould risk at the slab edge, using the temperature factor. Is the thermal break justified on durability grounds?
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The temperature factor is f_Rsi = (T_si − T_e)/(T_i − T_e). With no break, a realistic 2D calculation might give f_Rsi ≈ 0.71; then T_si = −12 + 0.71 × (21 − (−12)) = 11.4°C. At 21°C and 50% RH the indoor dew point is about 10°C, so the surface is above dew point but its local RH is roughly 90%, which is in the mould-risk range, especially in a bedroom with furniture close to the wall. With the thermal break, f_Rsi might be about 0.88, giving T_si = −12 + 0.88 × 33 = 17.0°C and a local RH near 60%, which is much safer. These are estimates and must be confirmed by a 2D thermal-bridge calculation using the actual bracket and slab geometry. The break is justified not only by the 1,147.5 kWh/a saving but by durability and comfort: a cold internal corner is a long-term moisture risk that is expensive to fix after occupancy.
3. What thermal-bridge evidence should go into the PHPP and construction package?
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The PHPP should use a project-specific Ψ from a 2D calculation or a certified component value, not the conservative default, and the correct 30 m length must be entered. The construction package should include the bracket specification, the 2D isotherm and Ψ report, and a detail showing continuity of the insulation and airtight layer at the slab. The site team needs an inspection point before the slab is cast or before the facade is closed, because a bracket installed with a gap or a missing insulation return changes the calculated value. The certifier should also see that other penetrations at the balcony - rainwater pipes, electrical conduits and door thresholds - are treated consistently. If the thermal break is value-engineered out later, the PHPP must revert to the higher Ψ and the heating demand must be rechecked against 15 kWh/(m²a).