The Complicated Role of a Water-Resistive Barrier
Choose the right product and install it well, because if you're not keeping water out, nothing else matters
Synopsis: Every house needs a dedicated and effective water-resistive barrier to let out wind-driven rain and any other moisture. The role of the material behind the siding has become of the utmost importance. This detailed article describes the purpose of a WRB and how its performance is calculated, and includes details of the six types of water resistive barriers–standard and drainable housewraps, integrated panels, self-adhering barriers, fluid-applied barriers, and rigid-foam insulation. Also included is data on nine different brands of WRB as well as frequently asked questions about WRBs asked of manufacturers.
As a teenager, I worked for a small general contracting company. In between a lot of grunt work and coffee runs, I learned to do some carpentry. On two jobs, I helped install siding. When we installed cedar shakes, the carpenters taught me to offset each seam as much as possible to keep water from getting behind the siding. When installing clapboards, we backed up all of the butt joints with flashing, which has long been best practice.
Fast forward 20 years: I’m working at Fine Homebuilding, visiting the job site of a high-performance home that was designed by a well-respected architect and is being built by a high-performance builder. On the coast, where wind-driven rain is a regular event, the crew had just finished installing the “open-gap,” or “rainscreen,” siding—that is, siding installed over furring strips with an intentional space left between the boards.
How did we get from laying a healthy bead of caulk where siding meets trim to leaving a wide open space between each course? When did we stop relying on siding to keep water out, and start installing it to let water out? Perhaps it was the mold explosion in homes at the turn of the century and the work of architects, building scientists, and educators like Steve Baczek who showed us that even the best siding installation is no match for water, and that every house needs a dedicated and effective water-resistive barrier, or WRB. “Mother nature has a perfect record,” says Baczek, “Water is the number one killer of buildings.”
The International Code Council agrees. Section R703.1.1 of the International Residential Code (IRC) calls for a waterresistive barrier behind siding and only allows exceptions for some masonry walls and wall assemblies that have been specifically tested to show resistance to wind-driven rain. Regardless of how we got here, the role of the material behind the siding has become of the utmost importance, and manufacturers have responded at warp speed. While you can find code-approved WRBs marketed for every wall assembly imaginable, there’s a lot to know to make an educated decision on how best to keep your walls dry.
Performance data is elusive
According to Yamil Moya, an engineer at the International Code Council Evaluation Service (ICC-ES), the nonprofit that evaluates building materials for the IRC, Type 1 asphalt-saturated felt meeting ASTM standard D226 is the only WRB prescribed in the code. All other products must be approved through criteria created by his organization. ICC-ES acceptance criteria 38, or AC38, is used to evaluate the durability, water resistance, vapor transmission, air leakage, and other qualities of most housewraps. Other product types have different criteria. Fluid-applied water-resistive barriers, for example, must meet ICC-ES AC212.
To meet these standards, manufacturers submit materials including product specs, test results for water resistance and permeability, and installation guidelines. All approved products have a report available online at icc-es.org. Unfortunately, the reports don’t provide test results or evaluate the products, they simple describe the applications for which the products are approved, and give limitations. “Meeting these criteria only means that they comply with minimum requirements,” said Moya, who declined to comment on the quality of individual products.
For more on water-resistive barriers:
Are Drainable Housewraps enough? — They’re a good start, but keeping your house dry and free of rot may require more than just drainage.
Making Sense of Housewraps — These plastic-based barriers help to keep wind and water at bay, but only if you choose the right product and install it correctly.
Is Your Exterior Rigid Foam Too Thin? — Energy Nerd: More builders are adding a layer of rigid foam insulation on the outside of homes. The idea makes sense, done correctly.
Self-stick WRB — Renovating a 150-year-old house presents leaky challenges.
From Fine Homebuilding #285
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