Q: I have seen manufacturers recommend putting a drainage mat in front of rigid insulation in a cavity to prevent mortar bridges. Is this necessary? It seems that if there are mortar bridges and water against the insulation, there is still generally a water resistive barrier on the surface of the backup, which would drain it to the flashing. It seems a full-height drainage mat would add unnecessary costs to the project.
A: I generally use a full-height drainage mat when the cavity in front of the insulation is 1 inch or less. In these cases, there is a much greater possibility that mortar bridges will occur and block the fairly narrow drainage space. Although I agree that a full-height drainage mat may not be necessary if there is a good water resistive barrier on the face of the back up, I generally prefer a redundant approach.
If the air space becomes much less than ¾ of an inch, it is very difficult to build the wall because there is no knuckle space for the mason to hold the brick. As long as the drainage mat is fairly compressible, it is possible to lay up the wall with a ¾-inch air space filled with a 3/8-inch-thick drainage mat.