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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.

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    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.

    Harry Whitver

    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.

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    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.

    Harry Whitver

    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.

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    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.

    Harry Whitver

    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.

Wood flooring, as everyone knows, is beautiful, sustainable, and durable. But wood is a natural material, and it will move in response to changes in moisture content.

In a conditioned building where relative humidity is stable, you can trust wood flooring to stay flat. But if there’s a moisture source from underneath the floor—such as a damp crawl space—the underside of the wood flooring can take on moisture and expand, while the upper surface, exposed to drier air, does not. The result is cupping of the individual wood flooring strips. In fact, cupping can happen simply because the plywood subfloor is at a high moisture content when you nail down the flooring—perhaps because of rain during framing.

To prevent the problem, make sure the house has good vapor barriers between the ground and the framing. Install a vapor barrier under the foundation slab, or between the framing and the vented crawl space. Check the subfloor moisture content before installing finish floors (it should be 12 percent or less). If floors have already cupped, eliminate the moisture source and allow the wood to stabilize before you sand the floors again—otherwise, when the moisture does dissipate, the newly sanded floors could shrink back down and become convex instead of flat.