OSB Sealer

Oriented Strand Board (OSB) has become one of the most popular plywood products available today, surpassing the production of construction-grade plywood. This increase in use is largely due to the popularity of OSB in the construction industry, specifically for the manufacture of new homes. OSB is accepted in the building code for almost all plywood applications, including floor underlayment and sheathing.

But there is still some concern in the minds of many woodworkers about using OSB. Part of that comes from the confusion between OSB and the old chipboard. While there is a lot of similarity between the appearance of the two materials, they really aren’t the same. Chipboard is made of scraps leftover from cutting wood in the lumberyard, essentially sweeping sawdust off the floor. This gives chips which are not of uniform size and are not laid up in the board in any predictable manner. On the other hand, the thin chips used for making OSB are deliberately created as thin, rectangular slices of wood. When laid up to make OSB, the grain ends up in random directions, but it is all parallel with the face of the board. As many as 50 layers of randomly oriented wood grain may be found in the cross-section of one piece of OSB.

This difference makes OSB considerably stronger than chipboard, even though the products look similar. OSB is even considered to be as strong as construction-grade plywood for similar thicknesses. But, that strength is only shear strength, not all strength figures. OSB doesn’t have the same tensile strength as plywood, although it has more tensile strength than chipboard or particle board. So take the statement that it is as strong as plywood with a grain of salt. In cases where shear strength is important, such as in making wooden I-beams, OSB is a considerably better choice than plywood. But in cases where tensile strength is more important, like covering windows for protect from a hurricane, softwood plywood is far superior.

Strength isn’t the only concern that many woodworkers have about using OSB. Many are concerned with how OSB might be affected by moisture. We’ve all seen roofs that are curled up at the edges from absorbing moisture. That’s why roof sheathing now has a drip edge installed on it. But what do you do to protect that OSB from moisture in other situations?

To start with, OSB is moist resistant, but not waterproof. Much like exterior-grade plywood, it can get wet, when using it as sheathing on a home. That will usually raise the moisture content to about 24%, which must be allowed to air dry back down to about 18% before sealing it. This is to allow the boards time to shrink back down to their original size.

The resins and wax used in the manufacture of OSB tend to make it somewhat moisture resistant. That’s not the same as waterproofing. Rather, water resistance means that water will flow off of OSB, rather than soaking in. The part of the board which is most susceptible to soaking up water is the edges, especially edges that have been cut by the woodworker or carpenter. Factory edges have some water resistance, more than freshly cut edges will.

There are some mills which produce OSB that is actually waterproofed, rather than just water resistant. This is easily identified by the boards being stamped with “waterproofed.” Even so, these boards aren’t waterproofed to the point where they should be submerged or used to make a water tank. Rather, they are waterproofed for protect from rain falling on them.

sealer, OSB joints
Sealer at the OSB joints, Alastair and Johanna Smith

Sealing OSB

For cases where OSB is more likely to be exposed to moisture, it’s a good idea to coat the OSB on the surface and edges with a waterproofing product, such as Thompson Water Seal, Flex Seal, or Liquid Rubber’s waterproofing sealant. Even latex paint will work. There are many different products available to choose from. These products are painted on with a brush or roller, just like paint. Take care when using those products which are clear or nearly clear, as it can be easy to miss spots and not realize it.

Some people ask if it is possible to seal OSB with PVA, the material that most wood glues are made from. This depends largely on what the woodworker wants to accomplish. PVA is not waterproof, but only water resistant. Therefore, applying it to a sheet of OSB will not make the OSB waterproof. Waterproof wood glues, like Titebond 3, aren’t PVA, but rather a different type of polymer, which is more resistant to water. Even that isn’t waterproof enough for using OSB to build a water tank, but it can withstand repeated exposure to water.

Before sealing anything, it’s a good idea to plan out and cut the sheets of OSB. At the same time, sealer should be applied before Since the edges are the part that is most susceptible to absorbing moist and cutting leaves an edge without any factory sealing, it is especially important to seal the edges. This should be done before assembly of the project, as assembly may hide those edges.

Good project design can help to hide those edges, providing them with some protect from moist. Nevertheless, moisture can still seep in through seams in the surface, so don’t just expect covering a seam with an overlapping piece to be sufficient to protect those edges.

There are also waterproof edge sealers specially made for sealing the edges of cut sheet goods, like OSB. I personally like using acrylic painter’s caulking, partially because I always have some on hand and partially because it is easy to work with. Applying waterproofing edge sealer or caulking will put a seal on the cut edge of the panel which will last for 25 years or more.

To apply caulking to a cut edge, run a 3/16” to ¼” bead of caulking down the center of the edge. Then using a finger, wipe the caulking into the edge grain, being sure to fill the pores in the grain. With edges that are extremely porous, like can be found on lower quality construction plywood, more caulking may be needed. The important thing is that the grain be fully sealed, leaving a coating of caulk over the entire surface. If the project is to be painted, adding a layer of paint over this caulking just adds more protect.

I’ve found that just painting the edges of most panels isn’t sufficient, as the paint soaks into the edge grain, not fully sealing it. It takes a lot of paint to actually seal the edges of OSB or plywood panels. That’s why I use caulking, rather than just applying one of the many waterproofing sealants on the market.

Making OSB Airtight

While not common, OSB can be used, as is, to make airtight structures. The material itself is fairly airtight, making an excellent air barrier. It is the seams between the panels, as well as places where the OSB panels meet up with other building materials that become the problem, as air can get through those. The solution is to use a high quality air barrier tape, such as 3M All Weather Flashing Tape to seal those seams, making a building that is essentially air tight.

For clarity, I’m not referring to air-tightness in the sense of a level 4 bio-lab, where there is a concern about chemicals or biological agents leaking out of a clean-room environment. Rather, I’m referring to the normal airtightness that is required for building a home, to prevent air leaks and keep wind from drawing the heat out of the home.

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