Cupped Hardwood Flooring

Uneven Expansion: Moisture in the subfloor, coming from a humid crawl space, causes hardwood strip flooring to cup upward as the bottom surface of the flooring expands while the upper surface does not.

Protect Against Vapor: First, eliminate the moisture load from below. Example shown uses vapor-impermeable rigid foam insulation attached to the underside of the joists, with air-sealed joints. Fiber insulation between joists is optional; if used, an air space below the subflooring should be included.

Sand and Refinish: Only after removing the moisture source and allowing the flooring to dry out and restabilize should the flooring be sanded flat again.

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