A new home has 2,200 square feet of conditioned floor area with 10-foot ceilings. Calculate the minimum continuous ventilation rate required under Michigan's adoption of the International Residential Code.
Correct Answer
D) 95 CFM
Using the formula: CFM = (Floor area × 0.03) + (Bedrooms + 1) × 7.5. Assuming 3 bedrooms: (2,200 × 0.03) + (4 × 7.5) = 66 + 30 = 96 CFM, closest to 95 CFM.
Why This Is the Correct Answer
Michigan uses IRC Section M1507.3 for whole-house mechanical ventilation. The formula is: Q (CFM) = 0.03 × Floor Area (sq ft) + 7.5 × (Number of Bedrooms + 1). Assuming 3 bedrooms: Q = (0.03 × 2,200) + 7.5 × (3 + 1) = 66 + 30 = 96 CFM. The closest answer choice is 95 CFM, which aligns with the IRC's requirement. (Note: ceiling height does not directly modify the IRC ventilation formula; it would apply in other ventilation code sections.)
Why the Other Options Are Wrong
Option A: 105 CFM
105 CFM would result from a higher bedroom count assumption (e.g., 4 bedrooms): (0.03 × 2,200) + 7.5 × (4 + 1) = 66 + 37.5 = 103.5 ≈ 105 CFM. If the question assumed 4 bedrooms, 105 CFM would be plausible—but 3 bedrooms is the standard assumption for 2,200 sq ft homes, yielding 95–96 CFM.
Option B: 115 CFM
115 CFM corresponds to an even higher bedroom assumption or a different formula application—perhaps (0.03 × 2,200) + 7.5 × (5 + 1) = 66 + 45 = 111, or another variant. No standard bedroom assumption for a 2,200 sq ft home reaches 115 CFM under the IRC formula.
Option C: 85 CFM
85 CFM would result from using only 2 bedrooms: (0.03 × 2,200) + 7.5 × (2 + 1) = 66 + 22.5 = 88.5 ≈ 85–90 CFM. While mathematically close, a 2-bedroom assumption is below typical for 2,200 sq ft, and 85 CFM is not the best answer.
Memory Technique
IRC Ventilation Formula: '3 cents per square foot plus 7.5 per person slot.' The 0.03 factor is like 3 cents per sq ft of house. The 7.5 × (bedrooms + 1) accounts for each occupant 'slot.' Add them up for total CFM needed.
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