When installing brown cellulose insulation in an attic, how many inches of insulation are needed to have a R38 if the R factor is 3.1 per inch?

When installing brown cellulose insulation in an attic, how many inches of insulation are needed to have a R38 if the R factor is 3.1 per inch?

• A. 12
• B. 15
• C. 10
• D. 11

Answer: (A)

The key idea is to convert the desired R-value into an thickness using the insulation’s R-value per inch. You divide the target R by the R per inch: 38 ÷ 3.1 ≈ 12.26 inches. That means you need a bit over 12 inches of insulation to reach R38. Among the given options, 12 inches is the closest whole-number amount to that requirement, since 12 inches would give about R37.2, just under R38 but very close. The other choices are farther away from the needed thickness. In practice, to truly meet or exceed R38 you’d usually install enough to reach 13 inches or more, but with the options provided, 12 inches is the best match.

The key idea is to convert the desired R-value into an thickness using the insulation’s R-value per inch. You divide the target R by the R per inch: 38 ÷ 3.1 ≈ 12.26 inches. That means you need a bit over 12 inches of insulation to reach R38.

Among the given options, 12 inches is the closest whole-number amount to that requirement, since 12 inches would give about R37.2, just under R38 but very close. The other choices are farther away from the needed thickness. In practice, to truly meet or exceed R38 you’d usually install enough to reach 13 inches or more, but with the options provided, 12 inches is the best match.