Tile/Stone Talk: Moisture Migration

By Gregory Mowat, Forensic Tile Consultants

Moisture migration
Moisture migration

There are four fundamental elements to understanding moisture migration:

  1. Wet moves toward dry
  2. Hot moves toward cold
  3. High Pressure moves toward low pressure
  4. Gravity

For reference, look to the American National Standard Specifications for the Installation of Ceramic Tile and Handbook for Ceramic, Glass, and Stone Tile Installation for correctly choosing adopted installation methods.

Tile and Grout are not Waterproof!

Wet areas are tile surfaces that are either soaked, saturated, or subjected to moisture or liquids (usually water), such as tub enclosures, showers, laundry rooms, saunas, steam rooms, swimming pools, commercial kitchens, and exterior areas. As such, adequate design and installation should address proper moisture management to reduce moisture migration contributing to deterioration of tile and stone tile assemblies.

Moisture migration through/within concrete slabs, can cause/contribute to loss of bond of tile and stone assemblies and deterioration of tile and stone tiles. A typical symptom is visual efflorescence, usually first observed on grout or adjacent cracks in grout.

For homes that use natural air ventilation, moisture migration through concrete slabs is not common. Heating and air conditioning create interior pressures inside the home that allow vapor transmission, moisture wicking, percolating, migrating, and transmitting up through the concrete slab. A waterproof membrane meeting ANSI A118.10 installed between the concrete slab and tile or stone tiles reduces and/or prevents the interior pressure from affecting the moisture within the concrete slab from migrating.

Moisture migration from underneath concrete slab
Moisture migration from underneath concrete slab

Considering that moisture migration through concrete slabs is a topic that has been researched for decades, here are several points, problems and solutions that need to be understood, including the fundamental concepts included in reducing moisture migration through the concrete slab:

  1. Concrete slabs on-grade should be constructed over a vapor retarder;
  2. Concrete slabs on-grade should be constructed with a proper water cement ratio to create a discontinuous capillary pore structure in the concrete slab and reduce vapor transmission, moisture wicking, percolating, migrating, and transmitting up through concrete slabs;

Curing Concrete Slabs

It’s important to note that covering the poured concrete slab is recommended in preparation of the tile installation. When curing compounds are used, it must be removed by scarification (or as recommended by the manufacturer with a written warranty).

Further, curing compounds serve as bond breakers for tile and stone tile installations and allow vapor drive condensation to occur in the void spaces, unless the assembly is isolated from the concrete slab with the assembly providing proper structural integrity, as in a wire-reinforced mortar setting bed.

Also, using calcium chloride speeds the slab’s setting, while allowing for moisture migration (sometimes found with model homes).

Damp proofing the edges of the concrete slab adjacent to landscaping will help prevent the capillary flow of moisture and/or vapor transmission into the slab through the edges.

Finishing

  1. Burnished finish of concrete slabs is recommended and should be scarified prior to the installation of a waterproof or crack isolation membrane.
  2. Broom trowel finish with no curing compounds is required for good bond adhesion where no waterproof membrane or crack isolation membrane is part of the assembly.
  3. Waterproof membranes for thin-set ceramic tile and dimension stone tile installations function as barriers to positive liquid water migration.

The Natural Stone Institute recommends always installing a waterproof membrane where stone or stone tiles are to be installed as the finished surface. This membrane must meet ANSI A118.10 requirements for direct bond of tile or stone tile. Remember, not all ANSI A118.10 waterproof membranes are equal.

Moisture migration from voids underneath stone tiles.
Moisture migration from voids underneath stone tiles.

Where tile or stone tile is directly bonded to a waterproof membrane, the Acceptance Criteria for the International Code Council requires the waterproof membrane meet ANSI A118.10 requirements. A roofing membrane may be used if isolating from the roofing membrane with a wire-reinforced mortar setting bed.

Further, when a tile assembly is installed over an older concrete slab or a concrete slab with mastic residue, scarification is recommended to remove surface laitance and mastic residue.

Also of importance are the following:

  1. When a floor is being remodeled, scarification of the concrete slab is recommended where paint, drywall mud or cabinet overspray is on the concrete slab surface;
  2. Where a structural crack in the concrete slab exists greater than 1/16-inch and is with vertical displacement from the two sides of the crack, the concrete slab should be rejected until the concrete slab is replaced or repaired;
  3. Where concrete slab cracks occur less than 1/16-inch and no offset occur between the two sides of the crack, a crack isolation membrane meeting ANSI A118.12 is recommended or a membrane meeting both ANSI A118.10 and ANSI A118.12 is recommended;
  4. When tile and stone tiles are installed with direct bond to concrete slab, and water loss occurs from above the tile and stone tiles, the water loss can cause loss of bond to the concrete slab;
  5. Concrete slabs that fail to meet proper flatness tolerance should be corrected prior to proceeding with the installation;
  6. Do not use thin-set mortars to level or flatten the concrete slab;
  7. Do not install water soluble mastics with direct bond on concrete slabs on grade;
  8. Discussions should occur with the architect, general contractor, builder, and/or owner’s representative when planning for tile or stone tile assemblies over new concrete, or when observations are made of a deficient concrete slab prior to the installation.

Understanding the fundamentals and planning for reduced moisture migration is beneficial for successful long-term tile and stone tile installations. The tile industry does not require moisture measurement of concrete slabs. As such, waterproof membrane and crack isolation membrane manufacturers offer a limited warranty and liability when excess moisture contributes to the failure of membrane assemblies.

Common methods for concrete moisture measurements performed by others include: ASTM F1869 Anhydrous Calcium Chloride test; ASTM F2170 In-situ Relative Humidity Test; ASTM 4263 Concrete Moisture Test; and non-destructive electronic meters.

Exterior Concrete Slabs

Exterior concrete slabs should be sloped to drains or outfall prior to the installation. The tops of exterior walls should also be waterproofed to reduce moisture absorption into the wall assembly.

Moisture migration from voids underneath stone tiles with visual efflorescence.
Moisture migration from voids underneath stone tiles with visual efflorescence.

Proper planning includes movement joints (expansion joints) as required in the Annual Tile Council of America Handbook for Ceramic, Glass and Stone Tile Installation and the American National Standard Specifications for the Installation of Ceramic Tile ANSI A108, A118, A136. It’s crucial to note and recognize the following conditions as well:

  1. Uncompleted metamorphism is moisture in the veins/inclusions of stone and causing either expansion of the minerals or decomposing of the minerals causing blistering/spalling of the surface of the stone tile.
  2. Agglomerate- and resin-based tiles, concrete tiles, limestone (especially green and blue), marble, sandstone, serpentine and green marble, may all be subject to curling or warping with moisture migration or when used in wet areas.
  3. Soluble iron may bleed from slates in wet areas.
  4. Yellowing of light-colored marbles may occur with iron oxide in wet areas.

Proper Mortar Adhesion Reduces Moisture Migration

Wet moves toward dry, hot moves toward cold, high pressure moves toward low pressure, and gravity. Vapor drive condensation occurs when tiles have void spaces behind them. With the recommended 95% coverage, as stated in ANSI A108.5, vapor drive condensation is inhibited and thereby reduces moisture migration into the underlying assembly.

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