Horizontally installed Water-Resistive Barrier (WRB). Is it a good idea?
Water-Resistive Barrier (WRB)
For exterior wall construction, the International Building Code (IBC) has required a water-resistive barrier (WRB) behind the exterior wall cladding ever since the first edition of the IBC was published in the year 2000. A WRB is defined in Chapter 2 of the IBC as, “a material behind an exterior wall covering that is intended to resist liquid water that has penetrated behind the exterior covering from further intruding into the exterior wall assembly.” Examples of common WRB materials include felt paper or Grade D paper, synthetic fibers woven into sheets (think Tyvek®), or even liquid-applied WRB that can be sprayed onto the exterior wall sheathing. These products are intended to protect exterior walls, which are vertical construction, with the WRB applied vertically to the face of the walls.
Follow the Manufacturer’s Instructions
Per Chapter 14 of the IBC, “Exterior Walls,” we see in section 1403 that the requirement for WRB is to “provide a continuous water-resistive barrier behind the exterior wall veneer.” The WRB is required to be installed along with flashing to direct any moisture that gets behind the cladding, to the flashing for discharge out of the wall assembly. The IBC doesn’t specifically discuss the horizontal application of WRB but does require compliance with the manufacturer’s instructions, and generally, all of the WRB manufacturers prohibit the flat or horizontal application of their products. Make sure to read and follow these instructions for proper material performance.
The reason you don’t want to install these products flat is that they’re not intended to resist long-term moisture exposure and pressure. In vertical construction, the WRB intercepts moisture that gets behind the cladding, but the water then travels down the face of the WRB where it should discharge at the bottom termination of the wall. When these products are installed flat, water that infiltrates the cladding will sit on the WRB and not drain out, and long-term exposure to this type of moisture will overwhelm the WRB product and damage the construction behind it. It’s very important to remember that standard WRB products are simply not intended to be installed flat.
Once again, you must follow the manufacturer’s instructions when installing this type of product, but just as a general rule, it is always a bad idea to install WRB in a horizontal application. For horizontal applications, a waterproof membrane, metal flashing, or other means to protect the horizontal construction should be used.