Passivhaus Design: Structural Implications

Passivhaus targets a maximum 15 kWh/m²/yr heating demand — roughly 80% lower than typical South African new build. Achieving it pushes structural design in specific directions, with cost and complexity consequences.

The five Passivhaus principles

• Continuous insulation
• Airtightness (0.6 ACH @ 50Pa)
• Thermal-bridge-free construction
• Triple-glazed high-performance windows
• MVHR (mechanical ventilation with heat recovery)

Structural consequences

Wall thickness

To reach Passivhaus U-values (typically <0.15 W/m²K), walls become 350–500mm thick. Timber frame or SIPs commonly used; brick-and-block with cavity widely 250mm+ achievable but heavy.

Thermal bridging

Structural elements crossing the insulation envelope conduct heat:

• Balcony cantilevers — use thermal break connectors (Schöck Isokorb)
• Steel beams penetrating walls — wrap and isolate
• Foundations — perimeter insulation, raft on insulation, or thermally broken slab edge
• Window reveals — extend insulation into reveal

Foundations

Common approach: insulated raft (300mm+ XPS or EPS under raft), edge insulation, no cold bridges at slab perimeter.

Airtightness

Service penetrations seal individually; framing junctions sealed with airtight tape; airtight membrane wraps complete envelope. Structural drawings must accommodate sealing strategy.

Cost premium

• Passivhaus over SANS / NBR baseline: +8–15% capital cost
• Operating cost savings: 70–90% reduction in heating bills
• Simple payback: 7–15 years depending on energy prices

Certification

• Designer-led approach with PHPP (Passivhaus Planning Package) modelling
• Site test (blower door test) for airtightness
• Final certification by approved certifier

EnerPHit (retrofit)

Retrofit version of Passivhaus with relaxed targets (25 kWh/m²/yr). Important standard for South African existing building retrofit at scale.

MCFAR works with Passivhaus designers on certified South African projects.

Frequently Asked Questions