The Secondary Water Resistive Barrier
April 1, 2010
Codes have been around a long time. They are living documents and as such, change is inevitable. One would think that as the codes have evolved, it would be more clear and concise, and help designers, contractors and others with understanding materials and applications. This should be particularly true of products that have been around for decades. Unfortunately, the opposite is true: the code language for water resistive barriers is more confusing, misleading and convoluted than ever.
If you are interested, the code section is 1404.2 of the International Building Code, but that will not help you understand a thing. Not to blame anyone or infer the code is wrong. It is simply there are so many options and schools of thought on the right product; the code has no idea where to go. Basically, asphalt-saturated felts and Kraft papers are allowed under claddings. Housewraps or plastic wraps are also allowed, if they have an International Code Council report stating compliance with section 1404.2. Liquid applied membranes are also code compliant, if they have the same ICC report for compliance. The choice is really the architect’s and approval is ultimately the building official’s.
The problem for code officials is how do you take a test meant for felt and use it to test a liquid. You can’t. All of these products keep moisture out, some are more vapor permeable than others; some integrate better with flashings; designers must decide how much water resistance, how much vapor permeance, how much cost. What has a proven history? It is not easy being an architect.
Some WRBs require special attention or a unique application and integration with flashings. Some installation instructions are overwhelming, complicated and incomplete. It is not easy being an installer, either. Since we are limited in space, and manufacturers of proprietary products are the best place to get installation for their products, let’s focus on installation of the old standbys-felt and paper.
FELTS AND PAPERS
Most plaster bureaus prefer Kraft paper over felts. This is not to say felts do not work or that they may not be appropriate in some areas.
The bonding of cement to the secondary WRB has been a concern for plaster bureaus. This is why most recommend two layers over all sheathings. This ensures a true drainage plane.
THE RAIN IN SPAIN FALLS MAINLY IN THE PLAIN
It would seem that cement plaster applied over a flat sheathing with a Kraft paper, and then rodded to a true and flat plane, would produce a cement membrane flat on both sides. Not true, the asphalt paper wrinkles when it absorbs some of the moisture from the fresh scratch coat. This in turn makes the paper wrinkle, creating shallow waves or channels in the back of the plaster. This is the drainage plane. Not as robust as a rainscreen but, when installed correctly, functional in almost all areas of the country.
The WRB is intended to handle “incidental” moisture. We often see pictures of decayed sheathing under stucco, and this could lead one to believe stucco does not work. Proving it works can be a challenge. After all, we do not tear into buildings with no problems.
Since two layers of WRB is the standard of the day, how should it be applied? There are three basic methods to achieve a double layer:
Double-Layer Method: manufacturers produce double layer or two-ply paper. This allows both layers to be applied in a single operation and is easy to integrate flashing. It is tradition and functional.
Separate Layers Method: this method is applying a single layer completely and then another single layer over the first layer. Flashings can be more complicated to integrate, slightly more in cost to install, but it is functional.
50 percent Lap Method: this method starts with a half roll (can be full) and the succeeding layer then covers half the lower layer. This system has also proven functional.
There is no testing to confirm that one method of application is superior to another. There are many opinions and preferences as to the best method. If an architect has a preferred method, he must state that method in the specifications to allow the contractor to bid that method accordingly.
Lacking clear and specific direction in the bidding or construction documents, the installer (plastering contractor) should be allowed to install the WRB as he has determined as the best method of application. Architects should allow the contractor his “means and methods” as long as they are code compliant and installed in a shingle-fashion.
Forcing a contractor to install the WRB in a method that you prefer and he is uncomfortable with is a recipe for disaster. People not on board with the plan tend to sabotage that plan, consciously or unconsciously. Whatever application method is selected or forced to be used, the party pushing and prevailing should bear the ultimate responsibility.
Contractor: If an architect or building envelope consultant has clearly stated how the WRB will be installed at your bid time, you are aware of the direction and should have bid accordingly-if you are not comfortable with that method, do not bid the work. If the architect or consultant announces that you will apply the WRB in a method you did not plan for, you should send a letter advising the parties of your concern and ask to be allowed to use your preferred method. If forced to use the method you do not like, send another letter that you are capitulating against your better judgment and industry guidelines and keep it on file.
Architect/Consultant: If you have strong feelings about how the WRB should be applied, state it clearly in your bid documents.
Weather-proofing a building for cement plaster is not brain surgery. Good products, common sense and a qualified contractor are worth every penny. W&C