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Building for a NW Climate

October 9, 2003

Consider your options for exterior cladding

  • Water leaks and mold have many rethinking how to stay dry
    Northwest Wall & Ceiling Bureau

     exterior barrier systems
    Photo courtesy of NWCB
    Various exterior barrier systems have been designed to lock out moisture. Several considerations should play into decisions regarding which system to install, including location, fire rating, long-term maintenance, and exposure to wind-driven rain.

    Our temperate rainforest climate, with its ample rainfall and high humidity much of the year, creates special challenges for the Northwest construction industry.

    Keeping water out of buildings has become all the more important as we have learned about molds that thrive on moist construction materials. Mold seems to be the current hot-button issue, with the possibility of contractors, suppliers and manufacturers being sued over mold years after completing a construction project.

    Building scientists are testing several common assemblies to see how they perform in our climate with regard to water penetration from the outside and vapor drive from the inside.

    Poor design, lack of details, improper construction, the new energy code, poor interior ventilation are all possible culprits.

    The cause is most likely some combination of these items, and total agreement about the cause within the building science community will probably never happen. What is agreed upon is that we all must work together to provide dry, long-lasting buildings.

    Top concerns

    The first concern contractors face is the possibility that materials on the inside will get wet during construction. The most common mistakes are improperly stocked materials, materials left exposed to the weather, poor sequencing of trades, and failure to make the building weather-tight prior to installing weather-sensitive materials.

    These wet materials, if closed up within a wall system, create an environment that is perfect for mold growth: warm, dark and damp.

    There are a lot of new materials on the market that are “mold resistant.” These are a good idea, but should not be considered a substitute for good construction practices.

    The first and foremost rule should be to “keep it dry.” Not only does this restrict mold growth, it will also promote a better working environment, which translates to better-quality workmanship.

    Most industrial hygienists agree that mold will not grow in an environment where the humidity is kept below 70 percent.

    Desiccant dehumidification heat source systems are now very popular on many Seattle projects because they provide uniform heat and humidity control.

    Many contractors use the system with field-installed monitors that record the conditions at hourly intervals, which provides a record that can be used to defend contractors if future litigation claims were to claim construction moisture as the cause of a mold problem.

    The same record-keeping procedure could be used on exteriors. Flashings are often hidden under the cladding, and I recommend having photographs taken just prior to installing the out-cladding material. This will provide future buyers and their inspectors the confidence that the underlying flashings are installed properly.

    The mold scare has not been limited to the interior of the building. Water leaks and mold have many in the industry rethinking the exterior envelope.

    The options for exterior wall claddings can be summed up into three basic categories based on how they function to handle moisture:

    • Barrier system claddings are wall systems designed to be a water-tight, single line of defense against moisture.

    • Concealed barrier system claddings are the most popular and common siding systems used. Typically they have a building paper or another secondary moisture barrier installed behind the cladding to collect moisture that may get in.

    • Rainscreen systems are exterior claddings that incorporate a designed airspace between the sheathing and the exterior cladding to allow water to flow faster to an exit point. The airspace is created by vertical furring strips (boards), a drainage mat or some other means to hold the cladding off the wall framing/sheathing and create a gap.

    The rainscreen debate

    If you are asking yourself “What’s a rainscreen?,” then get ready — you will probably be hearing the term lot more frequently. Quite simply, the idea is to allow easier and faster water drainage and some airflow.

    A rainscreen assembly can be simple or complicated, depending on the design. Initially it was believed the gap (airspace) must be 0.75 inches to allow water drainage and airflow. There has been a lot of debate in Canada as to whether you need a full 0.75-inch gap to drain moisture effectively, and the debate continues today.

    The other contested issue is the drying potential of a rainscreen wall assembly with a 0.75-inch air cavity. Rainscreens definitely will allow moisture to flow faster, and no one would argue that point.

    The drying benefit, particularly in our part of the country — where humidity is high for much of the year — is questionable, and tests conducted here and in Canada have not been conclusive on the drying benefit.

    No automatic options

    The point is that there are options, and one system is not the automatic answer for all buildings. Fire rating, long-term maintenance, constructability, location of the building, height of the building, exposure to wind-driven rain and amount of overhang should all play an important role in the decision process for selecting the proper cladding system.

    For example, a typical two-story home with overhangs located in Maple Valley would not need more than a concealed barrier system, while a five-story building with no overhangs along the coast in Ocean Shores might be best served with some form of rainscreen.

    Another overlooked possibility is to have a rainscreen only on walls that are exposed to severe weather. Other considerations regarding which cladding system to select might depend on a review of several factors with a qualified exterior envelope consultant.

    The final critical item often overlooked is the choice of a qualified exterior contractor. After all, the contractor is the one putting it all together, and the best engineering in the world will not help if it is put together wrong.

    Would you rather have the parts of a Mercedes put together by an unqualified garage mechanic or a base Chevrolet put together by a team of qualified General Motors mechanics? In other words, a building owner might want to think about the best place to allocate his money.

    Regardless of the exterior wall material, proper flashings and details are the key to keeping the moisture out. Good architectural details, good products, good tradespeople and good supervision will lead to dry and long-lasting buildings.

    Mark Fowler is an architectural consultant at the Northwest Wall & Ceiling Bureau, a non-profit construction trade organization. The NWCB is planning a seminar on exterior wall claddings, including rainscreen systems. Check its Web site, www.nwcb.org, for details.

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