How to Prevent Moisture Damage to OSB

Oriented strand board (OSB) is a popular engineered wood product in North America.¹ This may be because OSB offers greater moisture resistance than building materials like plywood—and at a lower cost.²

This quality, in addition to OSB’s durability, makes it a great choice for all kinds of projects. It’s been used as a subfloor, for sheathing in construction, and even as furniture.

Though OSB is more moisture resistant than other wood products, it can still be damaged by moisture. Whether you’re a general contractor, flooring installer, or woodworker, moisture damage in OSB panels could lead to costly repairs and crushed business reputations.

But you don’t have to let OSB moisture damage sneak up on you. Learn how to stop it as we look at:

• How moisture affects OSB
• What moisture levels are acceptable
• How to test OSB moisture levels
• What to do about excess moisture

Let’s start by understanding how OSB interacts with moisture.

How does moisture affect OSB?

OSB, like any wood-based material, is affected by moisture because wood is hygroscopic (a material that absorbs and releases water). So, OSB will also absorb or release moisture until it has acclimated to its environment and reached the equilibrium moisture content.

However, due to the way it’s made, OSB is notably less affected by moisture than solid wood products or other engineered wood products like particle board.

The manufacturing process involves oven-drying wooden strands and mixing them with wax and resins.³ According to the Engineered Wood Association, these strands are then placed in cross-oriented layers.⁴ From there, after being exposed to great amounts of heat and pressure, these mats become sheets of OSB.⁵

The resin, or adhesives used to make OSB, are usually water-resistant, allowing the OSB to resist more moisture.⁶ But just as it’s more difficult for OSB to absorb water, it’s more difficult for it to release it as well.⁷

This means OSB has a greater likelihood than plywood of facing serious moisture damage when it does absorb moisture.

Excess moisture could cause the OSB to warp or mold. Its compressed wood strands make it more susceptible to swelling.⁸ And the fact that it holds onto moisture means it has a higher chance of developing rot and decay.⁹

Thus, poor moisture conditions could cause serious problems in OSB building projects, including:

• Warping, which could ruin any OSB furniture build
• Excess swelling of an OSB subfloor or substrate, which could cause the flooring above it to buckle
• Decaying OSB, which could pose an especially serious and hazardous problem if it occurs in structural panels, joists, or building envelopes

These costly issues are all the more reason to understand what moisture levels are acceptable for OSB.

What moisture levels are acceptable for OSB?

An acceptable moisture level for OSB is 16–19% or less.¹⁰

In general, though, always follow the manufacturer’s guidelines. Manufacturers make OSB differently, so some brands might be more or less susceptible to moisture than others and have different ranges of acceptable levels.

Some kinds of OSB may even be specifically designed for certain projects, their compositions making them more or less water resistant, as one industry standard, the European EN 300, points out.

This standard classifies these types of OSB into four different grades¹¹:

1. OSB/1:
   

   – This is a general-purpose OSB. It’s a good option for interior builds like furniture.

   – It’s more sensitive to moisture, which is why its moisture levels should not exceed 12%.

2. OSB/2:
   

   – This can be used as a structural material, although it isn’t usually used in construction. It’s sensitive to moisture and should be used indoors.

   – It should not exceed 12% moisture content.

3. OSB/3:
   

   – This can be used as a structural material.

   – It was created for humid environments, making it more moisture resistant. Its moisture content should not exceed 18–20%.

4. OSB/4:
   

   – This was designed to be an outdoor structural material. It can be used for exterior walls.

   – Its moisture content can get up to 21%.

Once you know the ideal moisture content for your product, you’ll need a way to know whether it has reached that number. That’s where moisture testing comes in.

How to test for moisture in OSB

Test the moisture levels of OSB with a moisture meter. If you’re using a pinless moisture meter like those by Bessemeter, follow these two simple steps:

1. Calibrate for OSB by inputting the species setting value—0.62 or whatever is needed for the type of OSB being used.
2. Place the moisture meter’s sensor pad on top of the wood you’re trying to measure.

And just like that, you’ll get your moisture reading!

Pinless moisture meters make taking moisture measurements quick and easy.

Unlike pin meters that require you to drive pins into the board every time you take a reading, pinless meters only need to be laid on top of the board’s surface to get a measurement. You’ll be able to take as many moisture readings as you want, all without damaging your OSB with pinholes.

This way, you’ll know your OSB’s moisture levels and whether the material is ready for your project. No guesswork involved!

But what do you do if your OSB has excess moisture?

What to do about excess moisture in OSB

If your OSB boards have excessive moisture levels, try the following steps:

• Stack your boards horizontally and place wooden spacers between them to give them the space they need to dry out.
• Use a fan or dehumidifier to help them dry faster.
• Test them with a moisture meter to determine when the boards are dry enough to use.

As we mentioned earlier, the very things that make OSB moisture resistant cause it to hold the moisture it does absorb, making it more difficult to dry.

The compressed wood strands also make it swell more dramatically than plywood.¹² In many cases, this swelling is irreversible.¹³

If you notice any swelling, attempt to dry it with a fan. If the OSB doesn’t respond after a couple days, it may be time to replace it. Other signs that it’s time to replace it include mold and delamination.

Delamination, the process of the wooden strands of the OSB detaching from one another, is hazardous for another reason. When the OSB breaks apart, it weakens the material and leads to structural instability.

But better than dealing with excess moisture is preventing that moisture in the first place.

You can reduce opportunities of the OSB getting saturated by:¹⁴

• Stacking the boards at the jobsite and allowing them to acclimate to the relative humidity conditions of their environment
• Keeping them from touching the damp ground
• Putting coverings over the boards to protect them from rain

Test OSB moisture content and avoid moisture issues

OSB may be moisture resistant, but it’s not moisture proof. Checking its moisture content will ensure it has reached the appropriate levels so you don’t have to worry about issues down the road.

Give yourself that assurance and peace of mind for your next project. Why not invest in a quality moisture meter to do the job?

1. Gromicko, Nick, “OSB vs. Plywood,” International Association of Certified Home Inspectors. (↑)
2. Wallender, Lee, “OSB vs. Plywood: Best Subfloor Material Compared,” The Spruce, July 14, 2022. (↑)
3. “Wood Products,” Weyerhaeuser. (↑)
4. “Oriented Strand Board (OSB),” The Engineered Wood Association. (↑)
5. “Wood Products,” Weyerhaeuser. (↑)
6. Ibid. (↑)
7. “How Moisture Impacts OSB Board,” Theos Timber Limited, September 10, 2018. (↑)
8. Gromicko, “OSB vs. Plywood.” (↑)
9. Ibid. (↑)
10. “Guidelines for Moisture Content on Common Substrates,” Carlisle Coatings & Waterproofing. (↑)
11. “OSB and Moisture—How Much Moisture Do OSB Boards Tolerate?” Swiss Krono Group. (↑)
12. Gromicko, “OSB vs. Plywood.” (↑)
13. Ibid. (↑)
14. “OSB and Moisture—How Much Moisture Do OSB Boards Tolerate?” (↑)