A building requires R-30 insulation in the attic. If using fiberglass batt insulation with R-3.14 per inch, what thickness is needed?
Correct Answer
A) 9.6 inches
R-30 ÷ R-3.14 per inch = 9.55 inches, which rounds to approximately 9.6 inches. This calculation determines the required thickness of insulation to achieve the desired R-value.
Why This Is the Correct Answer
Option B is correct because it uses the proper formula for calculating insulation thickness: Total R-value needed ÷ R-value per inch = Required thickness. When we divide R-30 by R-3.14 per inch, we get 9.55 inches. This rounds to 9.6 inches, which matches option B. This calculation ensures the insulation will meet the required thermal resistance performance.
Why the Other Options Are Wrong
Option C: 10.2 inches
Option C (10.2 inches) is incorrect because it would provide an R-value of approximately 32 (10.2 × 3.14), which exceeds the required R-30 and represents unnecessary material cost.
Option D: 8.5 inches
Option A (8.5 inches) is incorrect because it would only provide an R-value of approximately 26.7 (8.5 × 3.14), which falls short of the required R-30 rating.
Memory Technique
Remember 'R-Total ÷ R-per-inch = Thickness' - think 'RT/RP=T' or 'Real Thick Requires Total Per-inch'
Reference Hint
Florida Building Code - Energy Conservation chapter, or insulation manufacturer specifications tables for R-values per inch
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