A builder wants to achieve an effective RSI of 4.5 m²·K/W in a 38x140 mm (2x6) exterior wall in British Columbia. If batt insulation with an RSI of 3.2 m²·K/W per 100 mm is chosen, what approximate thickness of continuous exterior rigid insulation (RSI 0.75 m²·K/W per 25 mm) would be required to compensate for a 15% framing factor?

Question: A builder wants to achieve an effective RSI of 4.5 m²·K/W in a 38x140 mm (2x6) exterior wall in British Columbia. If batt insulation with an RSI of 3.2 m²·K/W per 100 mm is chosen, what approximate thickness of continuous exterior rigid insulation (RSI 0.75 m²·K/W per 25 mm) would be required to compensate for a 15% framing factor?

Answer options: ✅ 50 mm

• 75 mm
• 100 mm
• 125 mm

Correct answer: 50 mm

Explanation: This requires a step-by-step calculation to account for internal stud-cavity insulation and the added exterior insulation, while considering the thermal bridging effect. The overall effective RSI calculation is complex, but approximately 50mm of rigid insulation helps bridge the gap to 4.5 by reducing thermal bridging.