Relative Humidity and Moisture Content Explained

Picture this: you’re building your dream bookshelf. The pieces fit perfectly in your living room, and you’ve handpicked the finest wood. A few weeks later, the doors are sticking, or worse, your wood floor is buckling. What’s the culprit? In a word, moisture.

Understanding how moisture behaves in materials like wood and concrete, and in the air itself, is more complex than measuring a nail’s length or a plank’s width.

In this article we’ll cover the following topics:

• What is Relative Humidity?
• Relative Humidity: What It Isn’t
• Wood Moisture Content
• How Relative Humidity and Moisture Content Relate to Each Other

The moisture content (MC) of any hygroscopic material is a moving target. Sometimes the changes go unnoticed, like the bookshelf that still seems rock-solid. Other times they cause obvious problems, like sticking doors, buckled floorboards, and splitting wood.

So how do these changes happen? The key is understanding relative humidity (RH) and how it interacts with moisture content.

What is Relative Humidity?

Relative humidity (RH) is the ratio of the amount of water vapor in the air to the maximum amount the air could hold at the same temperature (the saturation point), expressed as a percentage.

In everyday terms, RH tells you how close the air is to being saturated with water vapor at its current temperature. Because warm air can hold more water vapor than cold air, the same amount of moisture produces a lower RH in warm air than in cold air.

The water vapor in the air is the main source of moisture for cut lumber and finished wood products. (The other is liquid water, from a leaking appliance, flooding, standing water, or another source.) When it comes to wood, MC and RH are closely connected.

Moisture always moves toward a balance, or equilibrium, between the wood and the air around it.

Relative Humidity: What It Isn’t

In the building and construction industry, it helps to keep a few basics about RH in mind. With wood, RH usually refers to the ambient (environmental) condition: the water vapor in the air around a stack of lumber or an unfinished floor. The moisture inside the wood is its MC.

If you see the term “RH testing,” it usually refers to a test method for concrete that measures the relative humidity inside a concrete slab to show how much moisture remains in it.

Wood Moisture Content

For practical purposes, wood moisture content is the weight of the water in a piece of wood expressed as a percentage of its oven-dry weight (the weight of the wood after oven drying a sample at a specific temperature for a specific time).

Let’s back up a step. When a living tree is cut down, its wood is considered “green.” It holds all the moisture of the living tree. If the wood is left to air-dry, that moisture gradually escapes into the surrounding air and isn’t replaced by water from the roots.

This can be a slow process, so lumber mills use large kilns to remove much of the excess moisture in a controlled environment. That way, the wood can be milled to reliable dimensions for building, woodworking, and wood products like flooring, with a minimum of damage such as warping, twisting, or splitting. Removing excess moisture also makes the wood more stable and, for most strength properties, stronger.

However, drying wood (either air-drying or kiln-drying) doesn’t lock in its moisture level. Although we tend to think of wood as solid, it’s actually a porous material made of long cells that carried water and nutrients from the living tree’s roots to its branches and leaves.

After the wood is cut and dried, that cell structure remains and still exchanges moisture with the air. That’s why wood is called a hygroscopic material: it absorbs and releases moisture.

How Relative Humidity and Moisture Content Relate to Each Other

Wood naturally moves toward a moisture balance with its environment. For any combination of RH and temperature, there’s a matching equilibrium moisture content (EMC) that the wood will eventually reach. The two numbers aren’t the same: at about 70°F, for example, air at 50% RH brings wood to an EMC of about 9%.

If the wood’s MC is below the EMC for the surrounding air, the wood absorbs water vapor until it reaches that EMC.

If the surrounding air is drier, so the wood’s MC is above the EMC, moisture moves from the wood into the air. How fast and how far that exchange goes can mean the difference between a serious flaw and a small change in dimensions. Signs of a problem can include a cracked floorboard or one that shrinks after installation and leaves a gap.

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So how can you ensure harmony between RH and MC?

Allow time for gradual acclimation when wood is moved to a new job site or location, particularly with exotic woods or wood from a very different climate. If you have your own shop kiln, set it for moderate changes over a longer schedule.

Monitor the changes with an accurate wood moisture meter.

Wood moisture meters from Wagner Meters provide easy, accurate, nondestructive measurements that let you quickly determine the MC of wood.

When RH is high and temperatures are cool, a thin layer of condensation can form on the wood’s surface. Wagner’s IntelliSense™ technology minimizes the effect of surface moisture on the reading, so you get a better picture of the wood’s internal MC. If you see standing or visible water on the wood, wipe it off before you take a reading.

Moisture meters, including multi-material meters used for building inspection, can also help you find hidden moisture-related problems in wood floors or other materials, where a change in MC could mean water is getting into the wood.

When the wood’s MC is in balance with the RH around it, your wood will perform at its best and keep its strength and beauty for years. Wagner Meters’ wood moisture meters give you a simple, effective way to tell when wood has reached equilibrium with its environment.

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Larry Loffer

Larry Loffer is a senior technician at Wagner Meters, where he has over 30 years of experience in wood moisture measurement. With a degree in Computer Systems, Larry is involved in both hardware and software development of wood moisture measurement solutions.

Last updated on October 21st, 2026