Originally Posted by
Dom D'Agostino
Tar paper makes a poor crack isolation membrane, but some tile installers still use it.
Felt paper is a *worse than poor* crack isolation membrane and *SHOULD NOT BE USED for that purpose.
A crack isolation membrane should not be affected by water or moisture, and felt paper is. It will, *of course*, wrinkle when it gets wet, which will, *of course*, de-bond itself from the thin set.
The Tile Council of America and the Marble Institute of America have specific guidelines and requirements for the use of crack separation membranes, and one is (if I remember correctly) the forbidden use of thin set when a crack isolation membrane is used. When a crack isolation membrane is used, a mud set is required to be used on top of the crack isolation membrane - otherwise the tile will crack, come loose, or 'tent'.