Relative Humidity and How It Influences Moisture Content

In woodworking, a project’s success depends on more than skill and design. It also depends on moisture, which you can’t see.

Imagine building a beautiful hardwood floor or a carefully designed wooden table, only to watch it warp or crack because you overlooked how relative humidity (RH) affects wood moisture content (MC).

It’s not just about looks. It’s about durability and the integrity of your work. This article explains the relationship between RH and wood MC and what every woodworker, amateur or professional, needs to know so projects look good and last.

• How Relative Humidity (RH) Affects Wood Moisture Content (MC)
• How to Avoid Moisture-Related Deformities in Your Wood Projects
• Different Types of Wood and Their Responses to RH
• Make Sure You Keep Wood at the Desired MC
• How Temperature Affects Wood MC
• Understanding Average RH, Average Wood MC, and EMC
• The MC of Kiln-Dried Wood Is Not Fixed
• FAQs

How Relative Humidity (RH) Affects Wood Moisture Content (MC)

If you work with wood, you need to understand how it interacts with the relative humidity (RH) of its environment. That’s true whether you install hardwood floors, make furniture like cabinets and tables, or use lumber for construction.

The reason is simple.

Wood is hygroscopic.

That means wood gains or loses moisture as the RH changes. If the wood is drier than its surroundings, it absorbs moisture from the air. If it’s wetter than its surroundings, it releases moisture into the air.

Eventually, after wood has been in a particular environment long enough, it reaches a point where it’s no longer gaining or losing moisture. It’s in balance with its environment. We call this balanced state the equilibrium moisture content, or EMC.

So why is this property of wood so important?

Because of what happens to wood as it absorbs and releases moisture.

It changes size.

Wood shrinks or swells as it loses or gains moisture.

As the RH rises, wood’s moisture content (MC) increases, and the wood swells. As the RH falls, wood’s MC decreases, and the wood shrinks.

And therein lies the problem, which we’ll get to in a bit.

Once wood reaches the EMC, it stops shrinking or swelling. As noted above, that happens after the wood has been in a location for a while, which could be weeks or even months in some cases.

That’s true whether the wood is lumber in the manufacturing process, an unfinished product in transit or storage, or a finished wood product.

Now let’s look more closely at what happens when wood shrinks or swells, and what it means for your projects.

How to Avoid Moisture-Related Deformities in Your Wood Projects

Nobody wants to spend time creating a valuable wood project, of any kind, only to see it ruined later by moisture-related problems.

But that can happen if you don’t understand how RH influences the moisture levels of wood and what happens to wood’s dimensions as it absorbs and releases moisture.

As a rule of thumb, a 4% change in wood MC causes wood to shrink or swell about 1% across the grain. The exact amount varies by species and grain orientation, but about 1% is a good working estimate.

In contrast, even with large changes in MC, wood shrinks or swells very little along the grain.

Now, 1% might not sound like a lot.

But even changes this small can cause serious problems, such as glue joint failures in cabinets, tables, flooring, and other projects. You might see drawers and doors that don’t fit right, loose joints, or cracked finishes.

Here’s an example of what can happen with wood flooring.

If each 2-inch-wide strip of flooring shrinks by 1/32 inch, those changes add up to about 1½ inches across an 8-foot-wide room. That’s hard to miss.

It’s a small change with a big effect.

The most important point is this: if wood isn’t at the EMC of the place where the project will be used, it will keep shrinking or swelling afterward. That’s how deformities happen.

Fortunately, this problem is easy to avoid if you know how to measure the MC of wood.

Different Types of Wood and Their Responses to RH

Not all woods respond to humidity the same way. One useful comparison is tangential shrinkage from green to oven-dry, as listed in the USDA Forest Service’s Wood Handbook. The higher the number, the more the wood tends to move as its MC changes. Here’s how some popular woods compare:

• Oak: A durable hardwood, but one that moves quite a bit with moisture changes (about 8.6% for northern red oak and 10.5% for white oak). In humid conditions oak swells, and in dry conditions it shrinks, which can lead to gaps in flooring or furniture.
• Pine: Many pines move less than oak (for example, about 6.1% for eastern white pine and 7.2% for red pine). Pine still absorbs and releases moisture, so monitor it to prevent warping, especially when humidity swings.
• Mahogany: Valued for its grain and color, true mahogany is one of the more stable woods (about 4.1%).
• Teak: Often used for outdoor furniture, teak is also relatively stable (about 5.8%), which makes it less likely to warp or crack with humidity changes.

These are general behaviors, and individual boards can vary. Proper acclimation, knowing your species, and using the right tools to measure moisture content all help ensure woodworking success.

Make Sure You Keep Wood at the Desired MC

Once wood is at its desired MC, use a quality moisture meter to make sure it doesn’t continue to absorb or release moisture.

Why?

Because it takes time to get wood back to its desired MC, and as we’ve said, you don’t want to build with wood that’s not at the proper MC.

How Temperature Affects Wood MC

Many people assume temperature plays a big role here. Its direct effect is actually small.

Wood does expand slightly when heated, but according to the Wood Handbook, at normal moisture levels the dimensional changes from moisture loss during heating are greater than the thermal expansion. Wood’s low sensitivity to temperature itself is one of its advantages over materials such as aluminum or plastic.

Temperature also has only a small effect on EMC when the RH stays the same. At 50% RH, for example, the EMC is about 9.5% at 30°F, 9.2% at 70°F, and 8.9% at 90°F.

The bigger effect is indirect: changing the temperature of air changes its RH. Heating cold winter air, for example, lowers its RH, which lowers the EMC and can dry out wood indoors. So watch the RH, which is what really drives wood MC.

Understanding Average RH, Average Wood MC, and EMC

For any given average RH, there’s an average MC that wood will eventually reach. You can see this in the following chart, which is worth memorizing if you work with wood:

RH of the in-use location	EMC of the in-use location	Corresponding MC the wood will attain at this location
19-25%	5%	5%
26-32%	6%	6%
33-39%	7%	7%
40-46%	8%	8%
47-52%	9%	9%

For example, if the RH is 30%, wood stored in that environment will eventually reach about 6% MC. If the RH is 50%, wood will eventually reach about 9% MC. This can take weeks or even months, depending on the species, the size of the wood, the grain orientation, the temperature, and whether any coating has been applied.

The MC the wood eventually reaches in a 30% or 50% RH environment is its EMC: 6% or 9%.

These two figures are worth remembering because they’re typical interior values (at normal room temperatures) for heated and air-conditioned homes and offices in most of North America.

Of course, in cold winter conditions, interiors may be drier than 6% EMC. Manufacturing facilities that have dust exhaust systems or are heated for comfort are usually drier than 6% EMC unless they’re humidified.

In humid summer months, the opposite is true. The EMC in an interior without air conditioning may exceed 9%.

For unheated structures such as sheds, warehouses, or homes under construction, the RH might be around 65%, which corresponds to an EMC of about 12%. In very humid conditions, around 80% RH, the EMC is about 16%.

Note: You might think this exchange of moisture doesn’t happen if the wood is coated. It does. Coatings slow the exchange, but coated wood still responds to RH changes. It just takes longer to reach its EMC than uncoated wood.

So what does all this mean for those who work with wood?

Wood should always be allowed to come into balance with the environment where it will finally be used.

In other words, let it reach the EMC before you start working with it, whether you’re putting down a hardwood floor, making a cabinet, or building another wood product. Otherwise, you could see problems like cracking or warping after the project is done.

The MC of Kiln-Dried Wood Is Not Fixed

Some people assume that once wood is kiln-dried to a certain MC, that MC is fixed. It isn’t. Kiln drying doesn’t make wood unresponsive to RH changes. It just dries the wood to the desired MC more quickly. If kiln-dried wood isn’t handled and stored properly, it will regain moisture.

Understanding how RH affects wood is essential if you want your projects to succeed. Once your wood is at its desired MC, use a quality moisture meter to make sure it stays that way.

Our Orion^® series of pinless wood moisture meters can help. They come with an On-Demand Calibrator that lets you check and recalibrate the meter on site, so you don’t have to send it back to the manufacturer. Just select the calibration mode, place the meter on the calibrator, and let it complete the process. A NIST-traceable version of the calibrator is also available.

Orion meters also include IntelliSense™ technology, which minimizes the effect of surface moisture on readings.

FAQs

How do seasons impact wood moisture content?

A: Seasons play a major role in wood’s moisture content. In many areas, wood tends to absorb moisture during humid spring and summer months and swell.

In drier winter months, especially in heated buildings, wood tends to release moisture and shrink. Seasonal changes are why wood acclimation and knowing the wood’s moisture content matter before you start a project.

Can reclaimed wood have different moisture properties?

A: Yes. Reclaimed wood, often from old structures, has been exposed to its previous environment for a long time, so its moisture content reflects where it came from. A seaside barn may yield wood with a different moisture content than a building in a dry, inland area. Always measure and acclimate reclaimed wood before use.

How does altitude or geographical location affect wood moisture?

A: Location matters because it determines the RH and temperature the wood lives in. Dry climates, including many high-altitude areas, lead to lower wood moisture content, while coastal regions and areas with frequent rain tend to lead to higher moisture content. Consider the local environment and let the wood acclimate to it.

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Last updated on October 8th, 2026