By Richard P. Goldberg AIA, CSI; Professional Consultants International, LLC
and
Robert Scavone Jr., Total Flooring Contractors, Inc.
Contrary to what you may have heard, tile and stone floors are not immune to problems associated with moisture vapor emission (MVE) through concrete slabs. In response to the increasing number of moisture vapor-related problems that have occurred in the tile industry, the National Tile Contractors Association’s Technical Committee recently approved a new technical document (G-13; General Statement on Moisture Emissions) for the NTCA Reference Manual.
In the past, ceramic tile assemblies were relatively simple; tile was absorptive and somewhat permeable, and often adhered directly to a concrete slab or a mortar bed with moisture-resistant cement–based mortars. In addition, tile installed over un-bonded mortar beds was typically isolated from moisture vapor by loose- laid asphalt building paper or polyethylene plastic sheeting.
The popularity of the thin-set method of installation and the proliferation of impermeable materials, such as resin agglomerate tiles, large format porcelain tiles, epoxy adhesives, as well as moisture-sensitive specialty products such as bonded waterproofing/crack isolation membranes and underlayments, have made tile assemblies more susceptible to deterioration from excessive MVE. The combination of these contemporary materials and today’s extremely aggressive construction schedules can result in problems unless you understand some simple concepts and preventative measures.<
Moisture vapor emission occurs when there is an area of high vapor pressure, such as damp concrete or cool, wet soil, and areas of low vapor pressure, such as a warm, dry building interior. The pressure differential, while relatively small, causes the moisture vapor to migrate towards the dry area. Moisture vapor can collect and condense beneath impermeable materials, and cause damage to the tile assembly.
Excessive MVE can originate from a variety of sources. Residual moisture in curing concrete (especially lightweight concrete), and even moisture from tile installation mortars are common sources of moisture. In the absence of a suitable vapor retarder beneath a concrete slab on-grade, capillary action from high water tables and damp soil are additional sources of moisture. While a properly designed and constructed vapor retarder will likely provide adequate protection, beware of plain polyethylene plastic sheeting consisting of recycled plastics, as this material can be easily punctured, and can also decompose over time.
The following tests are now commonly used to measure moisture vapor in concrete, listed in order of most accurate to least accurate:
When materials such as impermeable porcelain tile or membranes are placed over a concrete slab, they create a barrier to vapor movement. If moisture vapor emission through the slab is excessive, moisture vapor can collect and condense under the tile or membrane. Residual moisture from mortars can also get encapsulated between tile and membranes and cause problems, especially to moisture-sensitive natural and engineered stone tile.
The collection of moisture and resulting increase in the alkalinity of the concrete, and not the actual exertion of pressure, is the primary cause of problems in tile assemblies. Moisture can activate naturally occurring alkaline salts in concrete, creating a variety of chemical reactions with natural and engineered stone tiles, adhesives, grouts and membranes.
The collection of moisture on the underside of certain varieties of natural stone or resin agglomerate tiles can cause differential expansion, resulting in warping of the tile, which in turn can lead to partial or complete delamination. Excessive MVE can also cause surface spalling of some natural stones with organic content, such as limestone. In addition, mold growth is a concern if materials such as latex mastic adhesives with organic content are used for tile or membrane installation.
Testing for MVE and analyzing test results is a complex endeavour that falls outside the tile contractor’s scope of expertise, and therefore the specification and analysis of such testing should be the responsibility of the design professional. Similarly, tests should be conducted by an independent test agency or consultant under the direction of a general contractor. Nonetheless, tile contractors should be prepared to take a proactive approach to prevent moisture vapor problems by understanding a few simple rules:
Visit the following link for more information on moisture vapor emission testing: proconweb.com/index.htm
Richard P. Goldberg, AIA, CSI is an architect and president of Professional Consultants International, Consulting Architects & Engineers. Goldberg specializes as a technical consultant for the ceramic tile, stone, masonry and concrete industries, and can be e-mailed at [email protected].
Robert Scavone Jr. is vice president of Total Flooring Contractors, Inc. The company specializes in large tile and stone commercial projects and has completed work in 24 states and Puerto Rico. Scavone Jr. is also a member of the NTCA Board of Directors and holds a seat on the association’s Technical Committee. He can be reached by e-mail at [email protected].
This article was printed in the March 2008 issue of TileLetter magazine, the official publication of the National Tile Contractors Association. For more information, call 601-939-2071 or visit www.tileletter.com or www.tile-assn.com.