Crack isolation methods and techniques have been practiced for thousands of years to prevent excessive tile cracking. In ancient times, crack isolation involved locating a layer of sand beneath the setting bed so the tile installation could float over any movement below. To deal with expansion, regularly spaced grout joints were filled with tar which was covered with fine sand. The sand bed method is still being used today, although primarily on exterior applications where thick stone or cast concrete tiles are installed without grout.
For the majority of interior and exterior crack isolation work, a sheet or liquid-applied membrane system is used. They are called systems because each brand has components that work together to provide the desired level of protection required for a specific installation. For example, a liquid-applied system may include a base liquid, gel, or paste, and a reinforcing fabric. A sheet system is generally composed of the sheet, an adhesive for laminating the sheet to the setting bed, and a sealant for closing seams. In addition to basic sheet or liquid-applied systems, there are self-adhering reinforced sheets that are used with a companion primer to maximize adhesion. The latest type of crack isolation is built into, and becomes a property of thinset mortar and requires no reinforcing fabric. Crack isolation thinset mortars cost considerably more than regular thinset mortars, but the extra cost is usually offset by labor and material costs for traditional sheet or liquid-applied systems. It is very important to keep in mind that not all crack isolation systems provide the same level of performance, and each brand offers varying degrees or warrantee protection – or none at all.
As with any other method of installing tiles, a network of movement joints has to be built into any installed crack isolation system. The primary function of any crack isolation system is to overcome substrate movement. Ironically, tiles installed over a crack isolation membrane must be free to move. Any tile installed over a crack isolation membrane without movement joints are subject to both cracking and shearing. With any crack isolation systems, as stated above, there are no 100% guarantees against cracking, but when no movement joints are incorporated into an installation, any warrantee offered by the manufacturer will not be honored. All ceramic and stone tile installations – with or without a membrane system – require a network of movement joints.
Finally, some membrane systems can provide waterproofing protection as well as crack isolation, and at least one (Nobleseal SIS), offers crack isolation, waterproofing, and sound reduction protection.