What Is a Water-Resistive Barrier?

Q. We have noticed green panels on the exterior of some homes under construction. What are they, and what do they do?

Most likely, you have seen one type of WRB, or water-resistive barrier. Before putting on my building-geek hat, let me explain what a WRB is and why a home needs one.

Most residential claddings should be expected to admit some rain during their service life. The continuous drainage plane behind that cladding—including a properly detailed WRB integrated with flashing—is what directs incidental water down and back outside instead of allowing it to accumulate in the wall.

WRBs have evolved considerably. The important question is not simply which product is newest, but whether the selected system is compatible with the wall and installed continuously at seams, openings, penetrations, and transitions.

Asphalt-Saturated Felt and Other Sheet Materials

Asphalt-saturated felt was a longstanding drainage-plane material. Modern codes recognize multiple compliant WRB paths, and the properties of felt and synthetic sheet products vary. Selection and installation should follow the locally adopted code and the current product documentation.

Plastic Housewrap

Qualifying plastic housewrap products are designed to resist liquid water while allowing some water vapor to pass. Properly lapped, taped, flashed, and integrated with windows and doors, housewrap can form an effective WRB—and, when fully sealed as specified, some products can also serve as an air barrier. Those properties are product-specific, and poor detailing at the connections can defeat even a good product.

Integrated Sheathing

Integrated sheathing systems combine structural panels with a factory-applied water-resistive surface. The familiar green ZIP System panels are one example; Huber describes the installed panel-and-tape system as an integrated air and water-resistive barrier with a 12-perm WRB layer. It can eliminate separate housewrap only when the seams, fasteners, openings, damage, and transitions are treated exactly as the current system instructions require.

Liquid-Applied WRBs

Liquid-applied products are rolled or sprayed onto the substrate to create a continuous layer. Their vapor permeance, coat count, thickness, curing, and ability to serve as an air barrier vary by product. They can be especially useful around complex geometry, but they are not automatically superior and demand careful quality control.

Rigid Foam and Rainscreens

Some qualifying rigid-foam products can serve as a drainage plane when their joints and transitions are detailed as an approved system; rigid foam in general should not be assumed to be a WRB. DOE/PNNL guidance identifies taped XPS and faced polyisocyanurate as possible drainage-plane materials and cautions that unfaced polyisocyanurate needs a separate WRB. A rainscreen is different again: it is an added drainage and ventilation space between the WRB and cladding.

Whether you are building a new home or adding onto an existing one, ask how the builder will maintain a continuous drainage plane from the roof and wall transitions all the way to the foundation. The brand matters less than a compatible system installed correctly.