A contractor installs a vapor barrier system in a crawl space on Kauai. The area is 1,200 sq ft and requires 6-mil polyethylene with 6-inch overlaps. How much material is needed accounting for 10% waste?
Correct Answer
C) 1,450 sq ft
Base area 1,200 sq ft + 10% for overlaps (120) + 10% waste (132) = 1,452 sq ft, rounded to 1,450 sq ft.
Why This Is the Correct Answer
The correct calculation applies two separate 10% additions. First, 10% for overlaps: 1,200 × 1.10 = 1,320 sq ft. Then 10% waste on that total: 1,320 × 1.10 = 1,452 sq ft, which rounds to 1,450 sq ft. Both the overlap allowance and waste factor must be applied sequentially.
Why the Other Options Are Wrong
Option A: 1,250 sq ft
1,250 sq ft applies only a minimal addition (~4%) and does not account for either overlaps or waste adequately. This would severely under-order material.
Option B: 1,375 sq ft
1,375 sq ft results from applying roughly one 10% factor but not both. It captures either the overlap OR the waste but not both compounded adjustments.
Option D: 1,500 sq ft
1,500 sq ft overstates by applying a 25% total addition rather than two sequential 10% factors (10% + 10% compounded = 21%, not 25%).
Memory Technique
Two-step multiplication: 'Overlap first, waste second.' Write it out: 1,200 → ×1.10 → 1,320 → ×1.10 → 1,452 → round → 1,450.
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