Protect Your Structures, Preserve Your Reputation
We’ve said it over and over again: moisture will eventually find its way behind the wall cladding...It’s inevitable. Left unchecked, moisture in a wall cavity can lead to a range of costly consequences from rotting wood to mildew and mold. Fortunately, modern building science has developed products and test methods to ensure that any moisture that gets in also gets out before causing trouble. Here’s what we recommend:
Drainable Weather- Resistive Barriers A drainage weather-resistive barrier typically has raised “bumps” or a textured surface that creates drainage channels to carry water down and out of the wall assembly. They carry an efficiency rating which indicates how much bulk water is drained out of the wall. ASTM E2273 is the test protocol used to determine this efficiency. Currently, Oregon requires these drainable wraps to pass with at least 75% efficiency. The ICC Acceptance Criteria for Weather-Resistive Barriers (AC38) requires a weather- resistive barrier to pass ASTM E2273 with at least 90% efficiency to be a drainable wrap.
Rain Screens
A rain screen is an exterior wall detail in which the weather-facing (cladding) surface is fixed at least 6mm (just shy of ¼”) away from the weather-resistive barrier surface on the wall. That gap creates a capillary break, or drainage plane, that allows moisture to drain and evaporate before it can cause problems. The gap is key. It can be created by the use of furring strips that keep the cladding separated from the wall sheathing. There are also convenient drainage mat products that are made of a tangle of polypropylene strands that prevent cladding from compressing against the wall. In many areas like coastal regions of the southeast, local codes are requiring the use of some type of rain screen with a 6mm or greater gap. Several fiber cement cladding manufacturers like James Hardie require the use of drainable WRB’s that have at least 90% drainage efficiency in moist climates, and some advocate the use of a rain screen in the wettest areas.
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