Sometimes choosing the building materials that suit us is a difficult decision. In the case of plywood ‘vs OSB, it’s a no brainer.
The home is a system, exterior claddings, framing and insulation, and interior claddings that all interact with each other on some level. The most important variables playing into this equation are: Air movement into and out of the building envelope (factoring heavily into heat loss), vapor transmission to and from the building envelope, and strength.
Air movement- how drafty your home will be- is the same for either plywood or OSB, theoretically speaking. The most important thing to remember is to properly nail the plywood on the outside walls and roof. That means extra nails at all edges, and adjusting the depth setting on the framing gun from super deep setting for framing, down to relatively shallow for shooting ply or OSB. Otherwise, your nails will blow most of the way through it and you’ve lost strength.
Speaking of strength, there is a difference between OSB and plywood. I’m no engineer, so let me relay my personal experience. I was fresh out of high school in the early 90′s and learning about framing houses on the coasts of Beverly and Marblehead, Massachusetts. We often had to build to withstand hurricane force winds, incorporating shear walls to resist lateral wind pressure. When we built the walls laying down, we had one of two choices for bracing it against the wind (no complaints about Bob Seger stuck in my head right now). We could use plywood and lay the sheets across the studs and stagger the joints every four feet in each successive course of real plywood and the stength of the plywood would brace the wall; or, we could cut diagonal bracing into the wall studs from top plate to bottom plate before sheathing with cheap-o OSB. Hopefully this information gives some perspective as to strength.
Next is vapor transmission. This is slightly more involved. Wood can absorb moisture just fine and, when allowed to properly dry out, can repeat this cycle indefinitely. It becomes a problem when the wood doesn’t have the opportunity to dry out. Therefore, the drying process is critical.
Your home is built almost entirely out of building components that hold moisture. The amount of moisture that they hold depends on: The types of materials used, the amount of humidity created by inhabitants (about 70% of a home’s humidity is from occupants cooking, showering, watering plants, dog dishes, perspiring, etc.), the relative humidity in the atmosphere, and of course the amount of air flow in and out of the home.
To allow the home to dry out, we must allow for slow vapor transmission through the building materials. This is why we should never use a vapor barrier on the warm side of a wall system. We don’t want to block all vapor diffusion. This would cause the vapor to hit that sheet plastic “vapor barrier” and condense, with no where to go. After condensing, the next step would be mold.
What we need to incorporate into walls and attic floors are vapor retarders that simply slow down vapor diffusion. We must allow the building and all of it’s components to dry out. So, we don’t use sheet plastic on the inside of the walls. Fine, but what does all this have to do with plywood versus OSB? Well, I was laying down a little building science theory, necessary for understanding the importance when choosing a wall or roof sheathing.
If we want to allow the building to breathe, which is the better choice? OSB is made up of wood wafers and glue, then treated with a film of wax to buy some time for exposure to the elements during construction. OSB has a perm rating (the means of measuring how much vapor will pass through a material) that remains very low and constant. When the relative humidity rises, it continues to allow the same miniscule amount of vapor to pass through it.
Plywood, on the other hand, has a decent perm rating and is able to ride a very nice curve. It is made of real, unpulverized wood layers that are glued together in opposing grain directions with an exterior glue. Because it is real wood, when the relative humidity rises, it’s ability to allow vapor to pass through it rises in kind. It allows the home to dry nicely under normal conditions.
Next, let’s talk about mold. Mold is a fungus. Fungi need food. Wood can be food, especially when sopping wet and beginning to break down. Additionally, when wood is pulverized, a sugar like byproduct is created. This is sugar like to mold. Oh wait. OSB is made up of pulverized wood and glue. The more and more that wood is pulverized down, the more sugary wonderful it is to mold. That’s why mold will very easily grow on the paper facing of drywall in a moist bathroom. OSB is far more susceptible to mold than plywood.
Let’s round this out by talking about the all important bottom line. OSB for wall sheathing can cost less than half of what plywood is going for. Let’s say we’re building a decent sized two story addition requiring 70 sheets of 1/2″ sheathing for walls and roof. At current rates, it would cost $525 for OSB and $1,330 for plywood. Now knowing this building science, who would choose to save the $805 and go with OSB?
Let’s consider two more things: Interior subfloor application over floor framing, and the introduction of red type (roofs) and green type (walls) super waterproof OSB sheathing.
First, the easy one. I always use OSB for interior subfloor applications. For this, we are inside of the building envelope and vapor is not struggling to pass through it. It also is cheaper and flatter for floors, making it a great choice and a green choice.
Finally, let’s discuss the introduction of super waterproof OSB. This is becoming incredibly popular for builders. I will never use it. First, you’re still not allowing the home to dry out properly via vapor diffusion. Second, it doesn’t adhere to the most fundamental of all building principles: Every single building component should be overlapped by the one directly above it in elevation. This utilizes the physics of gravity to keep the home dry and has been successful for centuries.
Then how does this other system work? First, the OSB is coated with some special magic potion to keep water out. Fine, wonderful, remembering that this also means that it keeps vapor in. What about the joints between sheets? Well, this is the kicker. It means that the carpenters must use a special- and expensive- proprietary tape gun to apply the special tape to the seams. That’s where we no longer overlap materials. You see, with plywood or regular OSB sheathing, we apply a rainscreen over the sheathing, such as tar paper or Tyvek, etc.. We overlap this rainscreen, maintaining the most fundamental principle of building. Conversely, the tape that is applied to the special OSB relies entirely on adhering over the surface of the plywood, a non glossy surface.
If you hang out at the lumber yard long enough, you’ll hear other guys asking what to do when their finicky tape gun no longer works properly. They dread shelling out the big bucks for a new one. Yes, these applicators don’t always apply the tape to create a watertight seal. And what if the sheathing has a film of sawdust on it’s face? I don’t know, you tell me. If you pay attention to addition jobsites, builders that are using this new super OSB with the taped seams on a roof with a finished living space underneath are still tarping the roof until they get the shingles on it. How much confidence do they seem to have in the product they are using on the roof? When roof plywood, on the other hand, is properly covered with ice and water shield and roof underlayment paper such as synthetic tar paper, the building will stay dry during construction every time.
Why would the lumber industry offer a product like this? Because they want to sell to the ever growing demographic of builders that don’t understand how to properly and permanently water proof a building. The lumber industry is dumbing down the products. Yes, we should all write our representatives in congress.
In conclusion, plywood made of real wood is by far the best thing for your exterior walls and roof. It only needs elementary attention to waterproofing details, and will then allow your home to breathe properly. OSB is the best choice for interior flooring substrate.
This material was largely a result of an education from Joseph Lstiburek, Ph.D., P.Eng., ASHRAE Fellow. He has been widely renowned as an international building science expert for decades. If you want to learn more about building science or cure insomnia I’m just kidding, some of the writings on the site are actually very entertaining), visit his website at http://www.buildingscience.com.
By John Bradshaw