A residential wall assembly in Ontario consists of 38x140 mm (2x6) wood studs at 400 mm O.C., 12.7 mm gypsum board, 12.7 mm exterior oriented strand board (OSB), and 12.5 mm fibre-cement siding. If the stud cavities are fully filled with fiberglass batt insulation (RSI 2.85 m²·K/W per 100 mm), what is the approximate effective RSI of the clear-wall section assuming 15% framing factor for the studs?

Question: A residential wall assembly in Ontario consists of 38x140 mm (2x6) wood studs at 400 mm O.C., 12.7 mm gypsum board, 12.7 mm exterior oriented strand board (OSB), and 12.5 mm fibre-cement siding. If the stud cavities are fully filled with fiberglass batt insulation (RSI 2.85 m²·K/W per 100 mm), what is the approximate effective RSI of the clear-wall section assuming 15% framing factor for the studs?

Answer options: ✅ RSI 3.9 m²·K/W

• RSI 3.5 m²·K/W
• RSI 3.1 m²·K/W
• RSI 2.7 m²·K/W

Correct answer: RSI 3.9 m²·K/W

Explanation: Calculate the RSI for 140 mm fiberglass: (2.85/100) * 140 = 3.99 m²·K/W. This question asks for the clear-wall section, meaning the section away from thermal bridges like studs. The RSI of the insulation itself is the dominant factor in the clear wall when fully filled. Add surface and sheathing resistances, but the insulation is key.