What is the net difference in thickness when substituting EPS for polyiso to maintain an R-38 roof insulation? Round to the nearest tenth. ______ inches

• A. 2.7
• B. 3.2
• C. 4.0
• D. 5.0

Answer: (B)

The main idea is to compare materials by their R-value per inch and then find how much thickness is needed to reach the same total R-38. Since R = (R per inch) × (thickness), determine the thickness each material would need to reach R-38, then subtract.

Using typical values used in this context: polyiso ≈ R-6.0 per inch and EPS ≈ R-4.0 per inch. The thickness for polyiso to reach R-38 is 38 ÷ 6.0 = 6.333 inches. The thickness for EPS to reach R-38 is 38 ÷ 4.0 = 9.5 inches. The net difference in thickness is 9.5 − 6.333 ≈ 3.167 inches, which rounds to 3.2 inches.

So substituting EPS for polyiso to maintain the same R-38 would require about 3.2 more inches of thickness. Real-world values can vary with installation and product specifics, but this is the calculation the question is testing.