A builder is calculating the required ventilation for an attic space that measures 40 feet by 30 feet. Using the 1:150 ratio required by Michigan Building Code, what is the minimum net free ventilation area needed?
Correct Answer
C) 8.0 square feet
Attic area = 40 × 30 = 1,200 sq ft. Required ventilation = 1,200 ÷ 150 = 8.0 square feet net free area.
Why This Is the Correct Answer
Step 1: Attic floor area = 40 ft × 30 ft = 1,200 sq ft. Step 2: Required net free ventilation area = 1,200 ÷ 150 = 8.0 sq ft. The 1:150 ratio means 1 square foot of ventilation for every 150 square feet of attic floor area.
Why the Other Options Are Wrong
Option A: 5.6 square feet
5.6 square feet results from dividing by an incorrect number (e.g., 1,200 ÷ 214 ≈ 5.6). This does not correspond to any standard ventilation ratio and is mathematically incorrect for the 1:150 ratio.
Option B: 12.8 square feet
12.8 square feet would result from using a ratio of approximately 1:94, which is stricter than required but not a standard code ratio. The calculation does not align with the 1:150 requirement.
Option D: 10.4 square feet
10.4 square feet would result from dividing 1,200 by approximately 115, which is not the 1:150 code ratio. This answer may arise from arithmetic errors in applying the ratio.
Memory Technique
150 square feet of attic needs 1 square foot of air. Think of it as '150 → 1.' Divide total attic area by 150 to get the required ventilation area.
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