Odds are, that if you pass a housing development of wood-framed houses, it will have OSB (oriented strand board) as exterior sheathing. When you count sub-floors, roofs and wall sheathing, OSB claims about 70% of the wood sheathing market. Both OSB and Plywood products have benefits and drawbacks; we will go over those, st, especially in relation to OSB or Plywood use for Roofs, Walls & Floors.
OSB is made of small and fairly rectangular-shaped wood chips from small resinous trees, which are bonded into a panel form with wax and synthetic resin. Plywood on the other hand, is made with thin sheets of wood that have been cut from a spinning log, then glued together with the grain laid out in alternating directions to maximise strength. OSB can be made with smaller trees while plywood requires more mature trees, big enough that a thin veneer can be cut off of a spinning tree (as seen in the video at the bottom of the page).
The history: plywood dates back about a hundred years, but it took decades before it replaced solid planks as the industry standard for closing up a house. OSB showed up in the early 60s and had a similarly slow start; it wasn't until around the year 2000 that it overtook plywood as the sheathing of choice in the North American housing market.
The current domination of OSB can be traced to one main thing - it's cheaper. Sheathing a 2,400 sq. ft. home at current prices would cost about $700 more with plywood than OSB.
Both types of panels contain phenol-formaldehyde in the adhesive, OSB also contains isocyanurate resin, but neither contains the urea-formaldehyde which is quite toxic to humans. While those names may make them sound pretty nasty, neither will off-gas and they aren't considered a major contributor to air pollution in homes anymore. Urea-formaldehydes are still found in other panels used for interior finishing, such as MDF (medium density fiberboard), which researchers have long concluded does off-gas into homes.
OSB and plywood offer similar (and sufficient) performance in terms of strength and functionality, though plywood is about 7% stronger. The main difference between the two is how they handle moisture, and oddly enough there are wildly varying conclusions - from very reputable sources - as to just how vapour permeable they are. We don't do any in-house testing so all we can do is report on the conclusions of others. With an absence of consensus among researchers, we will start off with the general conclusions that all parties agree on - plywood is more vapour permeable than OSB.
Plywood will absorb moisture quicker than OSB, but consequently it will also dry quicker. Both products will swell when wet, but plywood will return to almost its exact shape when it dries. OSB is less forgiving of being left out in the rain and will show more deformities after a wetting and drying cycle.
Beyond concerns about exposure to moisture during construction, is the moisture that can infiltrate through wall assemblies after completion. How durable a wall is depends heavily on its ability to dry, and how well it can dry depends on the materials you chose for your wall assembly, your 'recipe' if you will.
