If you’ve been around the building industry for the last twenty years or so, you’ve seen plywood being used less and Oriented Strand Board (OSB) being used more for sheathing on walls, roofs, and sub-floors. It’s widespread use has not been without problems however, and some careful considerations to its installation can ensure that it lasts and performs as desired.
The most common failure is the appearance of a warp, bow, or buckle in the sheathing used for exterior walls. As with most construction failures, there is no single “smoking gun” problem and no “silver-bullet” solution. With some understanding of the physical properties of OSB, we can determine the most likely cause.
As the name implies, Oriented Strand Board is made of flakes of wood which are oriented in an engineered pattern to provide strength and allow for expansion in a planned direction. While specs for different manufacturers will vary, in general, OSB will expand and contract 4x as much on the 4′ width of a 4’x8′ sheet than it does on the longer 8′ length. When the panels expand, if there is no room for that expansion to be absorbed, then the expansion bows out wherever it can.
Common causes of Warped OSB Sheathing
1. Installing vertical panels – While some manufacturers say it’s okay, it doesn’t really play out well in the real world. Framing crews like to install 9′ long panels in the vertical orientation for 9′ walls because the installation goes much faster. Unfortunately this method involves a nailing pattern that does not allow for the expansion into any gaps between panels. Since the nails surround the entire panel, there is no place for the expansion to go except in the space between the studs.
2. Installing super dry material – From the manufacturer, OSB panels come off the press at 2.5-3% moisture content. Most construction lumber equalizes to 9-12% which means that the material will be taking on moisture once it leaves the plant. If the time between the OSB is made and the time it is installed is too short, or if the material is stacked and stored in a way that prevents the moisture level equalization, then it might be installed dry and will equalize after installation.
The best practice is to check the material with a moisture meter and wait until the levels are closer to 9-12%. Moisture meters range in price, but the best models are between $100 and $200 and will help avoid some costly errors down the road. It would even be advisable to keep a log of material moisture readings for reference in case problems arise later.
3. Wet construction conditions – Assuming the OSB is installed at a reasonable moisture level of 9-12%, its important to maintain that moisture level without adding a lot of extra moisture during construction. Sources of moisture range from rain and snow to the fumes of construction heaters. An often overlooked source of moisture is the water entrained in the foundation which all needs to evaporate to the interior of the house. While most of these sources seem unavoidable, they are at least manageable with good site management and timing.
4. Installing without an expansion gap – Best practice is to leave a minimum of 1/8″ space between panels on the vertical and horizontal edges to allow them to expand. If the sheathing panels are butted flat together with no gap, the panels will not only buckle between the studs, but will crush along the edges causing a small ridge line.
5. Vapor diffusion – A common misconception is that moisture will automatically migrate from moist areas to dry areas. In actuality, vapor diffusion is a temperature driven phenomenon. When humid air is heated, it creates a difference in vapor pressure causing the humidity to be forced through permeable materials. As the humidity moves through an insulated wall assembly, the temperature drops (during heating season) as it approaches the exterior sheathing. If the temperature drops below the dew point, and the material is vapor permeable, then it is possible to collect condensed moisture within the sheathing. Problems can be avoided by constructing walls with more robust material profiles and managing interior moisture levels through proper mechanical ventilation.
When properly installed, OSB is a durable and versatile material. When it takes on moisture and begins to warp however, OSB changes permanently. Not only does it permanently change shape, but its structural strength and permeability change as well. Avoid issues of warping panels by installing in horizontal “running bond” patterns with gaps between the panels. Educating homeowners on the importance of monitoring humidity and using ventilation systems will also pay dividends beyond the durability of OSB.