Water resistive barriers are materials on the exterior of a building which are intended to resist liquid (bulk) water that has leaked, penetrated or seeped past the exterior cladding from absorbing into the exterior sheathing or concrete wall (depending on the application) and further into the wall assembly. Water resistive barrier materials can be mechanically fastened building wraps, fluid applied membranes, cellular plastic, self adhered, building paper or any other material that has been designed to resist liquid water. Water resistive barriers are combined with flashing and other supporting materials to ensure that there is a shingled effect to direct liquid water away from the exterior sheathing.
Testing the water resistance of a material is normally done using the boat method (ASTM D779), the "water ponding" method (CCMC 07102 section 6.4.5) or the hydrostatic head method (AATCC 127)