Prefabricated Passive House Apartment in Edmonton

Case Study: Prefabricated Passive House Apartment in Edmonton

Scenario A 12-unit prefabricated timber-frame apartment building targeting PHI certification in Edmonton, AB (Climate Zone 7, HDD₆₅ = 6,200). The design used factory-built wall panels with integrated structural sheathing, insulation, and air barrier. Key constraints included minimizing on-site labor, avoiding thermal bridging at panel joints, and achieving U ≤ 0.10 W/m²·K per PHI standard — requiring precise R-value validation before panel fabrication.

Given Data

• Exterior OSB sheathing: R = 0.12 m²·K/W
• 140 mm dense-packed cellulose (λ = 0.038 W/m·K): R = 140/1000 ÷ 0.038 ≈ 3.68 m²·K/W
• Interior gypsum + service cavity: R = 0.10 m²·K/W
• Air film resistance (both sides): Rₐᵢᵣ = 0.17 m²·K/W (ASHRAE default for still air)

So r_values = [0.12, 3.68, 0.10] and r_air = 0.17.

Calculation

• Rₜₒₜₐₗ = (0.12 + 3.68 + 0.10) + 2 × 0.17 = 3.90 + 0.34 = 4.24 m²·K/W
• U-value = 1 / 4.24 ≈ 0.236 W/m²·K

But this exceeds the Passive House target (U ≤ 0.10). Further analysis revealed that the standard Rₐᵢᵣ = 0.17 assumes still indoor/outdoor air — inappropriate for Edmonton’s high winds (winter wind speed > 5 m/s). Per ISO 6946 Annex B, exterior air film resistance drops to Rₑ = 0.04 m²·K/W under windy conditions. Revised calculation:

• Rₜₒₜₐₗ = 3.90 + 0.04 + 0.17 = 4.11 m²·K/W → U = 0.243 W/m²·K (worse).

The team switched to vacuum insulation panels (VIPs) in the service cavity: added R = 1.20 m²·K/W. New Rₜₒₜₐₗ = (0.12 + 3.68 + 1.20 + 0.10) + 0.04 + 0.17 = 5.31 → U = 0.188 W/m²·K — still short. Final solution: increased cellulose depth to 210 mm (R = 5.53), retained VIPs only at window reveals. Final Rₜₒₜₐₗ = (0.12 + 5.53 + 1.20 + 0.10) + 0.04 + 0.17 = 7.16 → U = 0.139 W/m²·K, then reduced framing factor to 12% and added thermal break at sole plates — verified via THERM: U = 0.097 W/m²·K.

Lesson Air film resistance is not universal: always adjust Rₐᵢᵣ based on local wind exposure and surface orientation — using default 0.17 in cold, windy climates risks significant U-value overprediction and certification failure.