A.Larry Elkin, a forensic mechanical engineer with Applied Building Sciences near Charleston, S.C., and author of several articles about floor moisture problems in humid climates, responds: Under the right conditions, any type of flooring that has a relatively impermeable surface — including laminate flooring, sheet goods, ceramic tile, and even wood flooring with a thick polyurethane finish — can act as a cold-side vapor retarder and lead to the situation you describe.

The problem seems to be more common in colder houses, particularly when the air conditioning is ducted through toe-kick registers rather than through wall or ceiling ducts. We've found fan-shaped patterns of rot damage in front of these registers, right where very cool supply air blows across the floor surface; this is caused by condensation,...

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