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Q.What's the best way to vapor-seal a basement slab before tiling over it?

A.Dave Gobis, executive director of the Ceramic Tile Education Foundation, responds: Surface-sealing a slab before applying tile is not required or recommended by the tile industry. Tile can readily tolerate basement slab moisture, as long as you choose the appropriate tile and tile setting material. In fact, applying sealer to concrete is likely to cause problems for tile by closing the pores that are required for cement-based compounds to bond to the concrete slab.

In any case, you definitely cannot use mastic, premixed thinset, or any latex-modified product in this application. If the slab is perfectly sealed, the material will not adhere because the pores will be closed; on the other hand, if any moisture does rise out of the slab, the adhesive will probably deteriorate and lose the bond that way.

For ceramic tile in a basement with a history of moisture issues, your best choice would be a vitreous tile (with a water absorption of .5% to 3%) or a semivitreous tile (with a water absorption of 3% to 5%). That would allow the use of regular dryset mortar instead of the latex- or polymer-modified material recommended by manufacturers of porcelain tile, which is impervious (less than .5% absorption). Standard dryset, unlike many latex and polymer formulations, will cure very well in a damp environment. Make sure the surface is clean, free of sealers, and free of any standing water before application.

One caution: In most instances of high moisture, alkalinity is also present, which may cause efflorescence (a white powdery deposit that typically appears first at tile edges). If the slab is alkaline, talk to your local concrete and masonry supply house about treating the slab with a cleaning product like Sure Klean Concrete Cleaner from Prosoco (www.prosoco.com). But if the alkalinity is continually migrating through the slab, it may be caused by excessive subsoil moisture. In such instances, chemical treatment will not have a lasting effect and you may have to take more expensive measures, such as drainage improvement or, in rare cases, slab reconstruction.