Whole-Building Energy Modeling (WUFI Passive)
Whole-Building Energy Modeling (WUFI Passive)
This domain tests whether you can build a defensible WUFI Passive model: geometry, assemblies, thermal bridges, ventilation, gains, HVAC, and outputs that match Phius criteria. The exam emphasizes inputs and QA, not software button-clicking.
Core rules and concepts
- Model the thermal envelope as conditioned volume; adjacent garages, attics, and crawlspaces are separate zones or boundary conditions.
- Enter each assembly with real layer properties and thicknesses. A wall U-value in the model must match the enclosure drawing, including continuous insulation and framing.
- Thermal bridges are entered explicitly, using psi values or equivalent U adjustments, not left as a default percentage.
- Ventilation inputs include flow rate, recovery efficiency, frost protection, and distribution losses. Internal gains and schedules must be documented, not left at software defaults.
- Check outputs against the standard: heating and cooling demand, peak load, source energy, and airtightness. A passing model that contradicts drawings is a QA failure.
Worked example
A 150 m² model shows heating demand 17 kWh/m²·yr against a 15 target. The window area is 30 m² at U = 1.1 W/m²K. Upgrading to U = 0.8 changes UA by 30 × (1.1 − 0.8) = 9 W/K. With 2,500 K·day, annual savings ≈ 9 × 2,500 × 24 / 1,000 = 540 kWh/yr; divided by 150 m² = 3.6 kWh/m²·yr. Predicted demand drops to about 13.4 kWh/m²·yr, so the measure is sufficient before considering gains and bridges.
Traps
- Using nominal R-values instead of whole-assembly U-values.
- Forgetting thermal bridges at balconies, slabs, and window installs.
- Default internal gains or schedules that flatter the result.
- Mixing conditioned floor area with gross floor area.
- Reporting demand without checking peak load.