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Understanding How Moisture Can Impact Your Installation

Consumers and installers alike will choose natural wood, cladding and siding products for a number of different reasons including aesthetics, price and workability. However, natural wood products, especially those of various pine species, are less stable when compared to engineered products. They are more susceptible to deformation, expansion and contraction resulting from moisture and temperature fluctuations. These variables, if not managed properly, can lead to primer and paint bonding performance problems with the substrate’s surface itself. This was observed and studied by OSI® building science expert, Paul Majka, primarily in regions of the Northwest and Northcentral United States.

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Soon after installation, movement from expansion, contraction and other forces placed normal stresses on the sealant bead.  The sealant would pull at the primer it was bonded to and cause it to release from the primed natural wood substrate.  The primer’s adhesion to the wood’s surface was failing, but the sealant’s adhesion to the primer was performing as intended.  To understand this observation better, the issue required further examination.  The process that began on the jobsite was later replicated and studied in greater detail back home at OSI’s application center just outside of Cleveland, OH.  The results are outlined below and brought to life in this short 3-minute video.

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Contributing Factors

The largest contributing factor was elevated moisture content in the pine trim.  This prevented a strong bond from forming between the primer and the surface of the natural wood when it was produced.  Ideally this is avoided by drying the wood to an acceptable moisture content during the manufacturing process before it is primed. 

It’s always good to check the moisture level of these products upon arrival, and if necessary, allow the natural-wood, primed trim, cladding and siding to dry further before installation.  On the jobsite, OSI recommends covering the product and keeping it off the ground to protect it and to prevent further absorption of moisture.  Allow it to vent so that the moisture level can be brought down to acceptable building standards (6% to 8% for interior and 9% to 14% for exterior wood products).  As the wood continues to dry, this should increase the bond strength between the primer and natural wood surface.

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Other Best Practices

Back prime your natural-wood trim and cladding.  This will help reduce overall moisture absorption by protecting the side of the product that typically sets against the Weather Resistive Barrier (WRB).  Moisture often travels down the WRB and can be drawn into unfinished wood substrates – making them more susceptible to deformation and movement over time as weather changes. 

Always brush the surface to remove dust, dirt, ice, or other debris that may affect the sealant’s ability to bond to building material surfaces.  Some sealants, including OSI QUAD® Max, will adhere very well to wet surfaces at wide temperature ranges, but ensuring the surface is clean before application is always a good practice to exercise during siding, trim and window or door installations. 

Know your substrate well.  Trim, cladding and siding products can be made of engineered wood, fiber cement, aluminum, steel and other materials.  Some are more stable that others and require different levels of preparation before installation.  Research and get familiar with the products your company typically uses and always seek out expert advice from the manufacturer or other building science experts. 

Moisture Meter Recommendations

OSI recommends you conduct your own additional research before making a final selection.  We are not responsible, and make no guarantees, for the performance outcomes or longevity of the moisture meter you purchase.

  1. Delmhorst F2000 – Buy Online
  2. Flir MR60 – Buy Online
  3. Klein ET140 Pinless (Affordable Option) - Buy Online

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