A wall assembly in Vancouver (mild climate) uses 38x89 mm (2x4) studs and has a total clear-wall RSI of 3.0 m²·K/W. If a client wants to upgrade the insulation from R-12 batt (125 mm thickness, RSI 2.1 m²·K/W) to R-20 batt (140 mm thickness, RSI 3.5 m²·K/W) and add an exterior layer of 25 mm rigid insulation (RSI 0.75 m²·K/W), what would be the approximate new clear-wall RSI value, assuming negligible change in film resistances and sheathing?

Question: A wall assembly in Vancouver (mild climate) uses 38x89 mm (2x4) studs and has a total clear-wall RSI of 3.0 m²·K/W. If a client wants to upgrade the insulation from R-12 batt (125 mm thickness, RSI 2.1 m²·K/W) to R-20 batt (140 mm thickness, RSI 3.5 m²·K/W) and add an exterior layer of 25 mm rigid insulation (RSI 0.75 m²·K/W), what would be the approximate new clear-wall RSI value, assuming negligible change in film resistances and sheathing?

Answer options:

• RSI 3.8 m²·K/W ✅ RSI 4.1 m²·K/W
• RSI 4.6 m²·K/W
• RSI 5.0 m²·K/W

Correct answer: RSI 4.1 m²·K/W

Explanation: The original clear-wall RSI is 3.0. The insulation upgrade from R-12 (RSI 2.1) to R-20 (RSI 3.5) changes the cavity insulation component by 3.5 - 2.1 = 1.4. Adding 25 mm rigid insulation (RSI 0.75) directly adds to this. So, 3.0 (original) - 2.1 (old batt) + 3.5 (new batt) + 0.75 (rigid) = 4.15. The clear wall calculation primarily involves summing layer RSIs.