Passive Building Principles
Passive Building Principles
This domain tests how a passive building behaves as a thermodynamic system: how heat, air, and moisture move; how solar and internal gains offset losses; and how comfort and load limits are set before equipment is selected. Expect conceptual questions plus simple calculations.
Core rules and concepts
- Heat transfer has three paths: conduction through assemblies, convection through air leakage and ventilation, and radiation through windows and warm surfaces. A passive building controls all three, not just insulation.
- Use whole-assembly R/U-values, including framing, cladding attachments, and slab or balcony penetrations. Nominal cavity R-value is not a design value.
- Distinguish annual demand (kWh/m²·yr) from peak load (W/m² or kW). Demand drives source energy; peak load sizes the heating and cooling system.
- Internal and solar gains are useful only when they reduce useful heating; in summer or high-gain rooms they become overheating and cooling loads.
- Comfort is radiant and airtightness-driven: surface temperatures, drafts, and humidity matter as much as air temperature.
Worked example
A 120 m² house has an envelope UA of 55 W/K and 2,500 heating degree-days. Ignoring gains, transmission loss = 55 × 2,500 × 24 / 1,000 = 3,300 kWh/yr, or 27.5 kWh/m²·yr. If solar and internal gains recover 1,200 kWh, demand falls to 2,100 kWh/yr = 17.5 kWh/m²·yr. That is above a 15 kWh/m²·yr target, so improve UA or gains before changing the heater.
Traps
- Mixing R (m²K/W) and U (W/m²K); U = 1/R.
- Treating a thermal bridge as a small percentage instead of calculating it.
- Assuming all internal gains are beneficial in summer.
- Confusing airtightness (ACH50) with ventilation rate.
- Sizing equipment from annual demand rather than peak load.