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Vapor Barrier

A moisture-impermeable membrane installed between a damp subfloor (usually concrete slab) and the finish flooring.

A vapor barrier blocks moisture vapor from migrating up through a slab or subfloor and into the finish flooring. Without one, slab moisture causes hardwood to cup, LVP to delaminate, and adhesive to fail.

Common vapor barriers include 6-mil polyethylene sheeting, paint-on epoxy sealers, and sheet products with attached vapor membranes. The right choice depends on the finish flooring's manufacturer specification and the slab's moisture vapor emission rate (MVER) measured in pounds per 1,000 square feet per 24 hours.

For any slab install, run a calcium chloride test or RH probe and document the result. If you're seeing MVER above 3 lbs or RH above 75%, install a vapor barrier or moisture mitigation system before laying the finish flooring.

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