A concrete slab foundation in Hawaii requires a vapor barrier. Due to high humidity and frequent rainfall, what type of vapor barrier is most appropriate?
Correct Answer
D) 6-mil polyethylene with sealed joints
Hawaii's high humidity requires 6-mil polyethylene vapor barriers with properly sealed joints to prevent moisture penetration from below the slab.
Why This Is the Correct Answer
In Hawaii's high-humidity, high-rainfall environment, 6-mil polyethylene sheeting with sealed (lapped and taped) joints is the most appropriate vapor barrier under a concrete slab. The 6-mil thickness provides durability against punctures during concrete placement, and sealed joints prevent moisture from migrating through seams—a critical concern given the persistent ground moisture in Hawaii.
Why the Other Options Are Wrong
Option A: Asphalt-impregnated felt paper
Option A ('Asphalt-impregnated felt paper') is an older, lighter material that performs adequately in drier climates but does not hold up well under the sustained moisture conditions in Hawaii. It is more permeable and more prone to degradation over time in tropical conditions.
Option B: Spray-applied membrane
Option B ('Spray-applied membrane') is used primarily for above-grade waterproofing on foundation walls and below-grade vertical surfaces. While effective in those applications, it is not the standard choice for under-slab vapor barriers on residential slabs in Hawaii.
Option C: 4-mil plastic sheeting
Option C ('4-mil plastic sheeting') is lighter than 6-mil and more susceptible to tears during concrete placement. The thinner material is more easily punctured by aggregate or reinforcing steel, compromising the vapor barrier's integrity. Hawaii's conditions demand the heavier 6-mil specification.
Memory Technique
Hawaii = 6-mil sealed. Six islands, six mils: just as Hawaii has multiple islands surrounded by water, your vapor barrier needs the thicker 6-mil protection with sealed joints to keep all that surrounding moisture out.
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