Why are We Still Having Problems with Moisture?

Damage to vinyl composition tile (VCT) in a retail store caused by efflorescence. A vapor retarder installed under several inches of coarse crushed stone below the slab was found to be extensively punctured and ineffective. Moisture rising through the slab evaporated at the gap between adjacent tiles, depositing salts that destroyed the vinyl binder in the tiles. The entire tile installation (>50,000 sq. ft.) had to be replaced.

Warehouse floor efflorescence from combination of subslab moisture movement and cleaning chemicals. Regulated food-handling environments like this must be cleanable and remain kept clean.

Warehouse floor constructed with no vapor retarder. Even though the floor appears shiny and smooth, each 15x20-ft. sawcut panel actually was dramatically curled as much as 3/4-in. from center to corner (inset). Omitting the vapor retarder does not eliminate slab curling.

A concrete floor slab, placed in April 2007, for a 100,000 sq. ft. warehouse. Lack of protection during concrete placement caused extensive tears and punctures (inset) in the polyethylene vapor retarder. The subslab membrane must be treated with the same care as a roof membrane in order to be effective for the life of the slab.

A 6-inch diameter core hole through a concrete slab showing vapor retarder (arrow) underneath a blotter/cushion layer. The plastic sheet was placed over granular fill and was then covered with several inches of additional granular fill that was moistened and compacted. The vapor retarder suffered extensive punctures rendering it ineffective against moisture intrusion.

Vapor retarder after removal from subbase. Hundreds of pinhole punctures are visible (see arrows).

This warehouse under construction has a plastic sheet vapor retarder only in the corner of the building where the architect or owner anticipates the initial tenant will build offices. If moisture-sensitive floor finishes are installed beyond this area, there will be no subslab vapor protection.

Modern electronic thumb-size relative humidity probes now are available that can be placed below the surface of the concrete slab to monitor drying without interfering with construction or use of the floor. The devices provide real-time relative humidity and temperature at the push of a button.

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