L12. Moisture in Wood
Introduction
Wood is only a useful building material if it is strong, durable, and dimensionally stable (not expanding or shrinking). In Lesson 11 we discussed the process of making lumber - from the tree growing in the forest to the shelf in the store. One important step in this process that we only briefly mentioned was the drying of the wood.
Growing wood is full of water (remember sap?) and needs to be dried before it can be used for construction. This is because water content in wood affects its dimensional stability (how much it shrinks or expands), and its longevity (how long it will last before decaying), and subsequently its strength. If we're building a deck, we want to make sure it doesn't rot away or fall down.
In this lesson we will help you:
- Understand why wood is dried prior to use
- Understand the terms fiber saturation point and equilibrium moisture content
- Understand shrinking and swelling and why it matters
- Understand why moisture content of wood matters in construction projects and what considerations regarding wood moisture you need to make for projects such as building a deck
Let's begin with a short video from Penn State on Moisture Content in Wood. The lesson text that follows supports and explains this video.
Water in Wood
Wood is a hygroscopic material, meaning that it absorbs water. As we learned, plants have xylem and phloem tissues (xylem carrying water, phloem carrying nutrients). The cells that make up the xylem are fibrous, and in woody plants, are the primary components of what we think of as “wood.” Understanding a little about the cells that make up the xylem is important in understanding wood properties and constraints in using it as a construction material.
Xylem tissue is organized in a tube-like fashion along the main axes of stems and roots (figure 1). It consists of a combination of various cell types, including: parenchyma, fibers, vessels, tracheids, and ray cells. Tracheid cells are a key player in considering the moisture content of wood. In a live tree, woody material is in contact with liquid water (drawn up from the soil), but once the tree is cut, the material will begin to dry out. It is water in the tracheid that plays a large role in issues of decay and rot, and in wood swelling or shrinking. Tracheid cells can be thought of as soda straws. When the tracheid cell is mature, the cell dies, so that all that's left is the cell wall with an empty interior. The cell wall is the plastic straw in our analogy, and the empty space inside the 'straw' (where the living cell material once was) is called the lumen (Figure 2). Tracheid cells connect to each other via different types of pits (holes in the cell walls) which conduct water between cells. In a living plant, the role of tracheids is to provide structural support and act as a water conduit.
Figure 1. Important tissues types in wood, and important cell types that make up the xylem.
Figure 2. Scanning electron micrograph of tracheids showing their “soda-straw” structure. (From Shmulsky and Jones, “Forest products and wood science,” Wiley-Blackwell).
Fiber Saturation Point
When the tree has been cut, water begins to diffuse out of plant’s cells into the surrounding air. Water has a greater affinity for cell walls than for the lumen, and so will move out of the lumen first.
The water confined in the cell wall is called “bound water”, while the water that resides in the lumen is called “free water.” When freshly cut wood is dried in air, water will leave lumen first, and move into the cell wall. Eventually, it reaches a point where there is no "free water" left, and the cell wall is holding the maximum amount of "bound water" it possibly can (Figure 4). This is called the Fiber Saturation Point (FSP). If we continue to dry the wood past the FSP, the cell wall will start to lose water.
Figure 4. Water in wood. At the Fiber Saturation Point, there is no water in the cell lumen space, but the cell walls are fully saturated. Above the FSP, the lumen is also saturated. Below the FSP, the cell wall still contains water, but is not fully saturated.
Moisture Content Calculations
To quantify the amount of water in plant materials, the term moisture content (MC) is used. A number of wood's physical properties are tied to its moisture content, so it is important to understand where these values come from.
For most wood products moisture content is calculated on a dry basis. Dry moisture content (MCd) is the ratio of weight of the water in the wood to the dry weight. As a percent, it is computed by:
In the formula, the "current weight" (frequently termed “green weight”, refers to the material’s actual weight. "Green" wood is wood that has been recently cut and hasn't had a chance to dry out yet. "Oven dry weight" refers to the weight of the same material but dried to remove all moisture. To determine the weight of the water, they take the current weight of the wood and then subtract the weight of the wood after it has been dried. The equation below is another way to express how we calculate the dry moisture content ratio.
Example: Calculating Dry Moisture Content
Plant material initially weighs 10 g. After oven-drying it weighed 6 grams.
How much water was in the original material?
Answer: 10g current weight - 6g oven dry weight = 4 g water
What is the moisture content on a dry basis (MCd)?
Answer: 4 grams water / 6 grams oven dry weight x 100 = 67%
Equilibrium Moisture Content
Once wood (and other plant) materials are harvested and lose contact with liquid water from the soil, the rate of moisture movement out of the cells will depend on the water vapor content (humidity) of the surrounding air. Moisture will move from the cells to the air first rapidly, then at decreasing rates, and will continue until equilibrium is achieved between the wood and the ambient air. This is known as the "Equilibrium Moisture Content" or EMC. If a dry wood is exposed to a humid environment (e.g. wood cabinet in shower room; wood siding in summer), it can pick up moisture from the air until it reaches a new equilibrium. Note: humidity in homes fluctuates constantly which means the cell walls will always be gaining or loosing moisture to achieve equilibrium.
The moisture content of the ambient air is measured as relative humidity (RH). Most homes have a RH of 40-50%. Possibly a bit higher in summer and a bit lower in winter. As the relative humidity increases the wood will gain moisture from the air which meaning the EMC of the wood will be higher. Conversely, if the relative humidity of the air is low, the wood will lose moisture and the EMC will be lower. Note that these changes in wood moisture content take can hours or days.
Table 1 shows the approximate equilibrium moisture content (dry basis) of wood at various levels of relative humidity. From this table, we can see that even with a high relative humidity of 99% (very humid air), wood will have an EMC of about 23%.
The table below shows that wood reaches an EMC of 9% at an RH of 50, which is the typical RH inside a home. Now think back to the video. It was stated that wood used for inside furniture is dried to 7%. This is very close to 9% which means the wood will not shrink or swell much under normal humidity conditions in the house.
Table 1. Equilibrium moisture content vs. relative humidity at 21oC
|
Relative Humidity |
Equilibrium Moisture Content (%)* |
|---|---|
|
0 |
0 |
|
25 |
5 |
|
50 |
9 |
|
75 |
14 |
|
99 |
23 |
Importance of Moisture in Building Materials
When moisture is gained or lost below FSP (meaning that water is moving into/out of the cell wall), it causes shrinking and swelling of the wood. As the cell wall loses water, it causes the hemicellulose and long chain cellulose molecules to move closer together causing an overall shrinking of the wood. As water enters the cell walls, it causes the molecules to move farther apart, making the wood swell. Figure 5 summarizes how moisture content change affects dimensional stability (i.e. swelling or shrinking that changes the dimensions of the wood).
Any moisture content changes that happen above FSP means that water is moving into/out of the lumen. This water movement does not affect dimensional changes and most wood strength properties.
Knowing the FSP value, we can predict whether or not a moisture content change would affect shrinking or swelling of the cell structures. The value of FSP in most wood occurs at a dry moisture content of about 30%. Figure 5 summarizes when your wood is going to shrink/swell, and when it's not.
Figure 5. Moisture held in a plant fiber in relation to FSP: Effects of moisture change on swelling/shrinking of the fiber (hence plant materials)
Drying Wood for Use
Wood for construction needs to be dried properly prior to being used in construction (more on kiln drying later). All of the moisture in the lumen must be removed, which means drying it below the fiber saturation point (a MCd of about 30%). In practice, it is dried to between 7 and 20% depending on use (more later on this).
Dimensional Change
A piece of wood cut to specific dimensions for lumber is called dimensional lumber. As the individual wood cells shrink and expand according to the moisture content in the cell walls, the different dimensions of the cut wood will shrink and expand differently. The wood can change sizes in all three directions: tangential, radial, and longitudinal (referring to how the wood was cut from the log - see Figure 6).
Figure 6. Orientation of dimensional change in a cut board. Look at the orientation of the tree rings in each of the boards.
For most wood the longitudinal shrinkage is negligible; the wood cells don’t get longer as they increase in moisture content, or shrink as they lose moisture. However, an increase or decrease in the water content of the trachieds’ cell walls will cause shrinking or swelling in the tangential and radial directions. Thinking back to our soda straw analogy: when moisture content decreases below FSP, the "straw" shrinks in diameter but it won't change in length.
The dimensional change is different in each orientation and is represented by a percent change. Table 2 is shows the general range of shrinkage in each direction, although this varies by the type of wood (e.g oak vs pine). Remember this shrinkage % when the wood is going from a moisture content at or above the FSP (about 30% MCd or more) to oven dry moisture (0% moisture).
Table 2. Orientation shrinkage of wood from FSP to Oven dry Moisture (0%) as a percent.
|
Direction |
% shrink from FSP to MCd=0% |
|---|---|
|
Longitudinal |
<0.1% |
|
Radial |
2-5% |
|
Tangential |
4-12% |
Example: Shrinkage
Austin's deck boards are 3/4 inches thick x 6 inches wide at MCd. =0%. They will expand when it rains because she did not paint them yet. The 6 inch measurement is in tangent direction. For this problem use tangential shrinkage of 7%. (Swelling and shrinkage are the same %) How much will the wood expand when it gets wet?
Answer: 6 inches x 7% = 6 x 0.07 = 0.42 inches
Considering Dimensional Change in Construction
As we now know, lumber must be dried before use. The amount of drying required depends on its use. Framing lumber is dried to 15-19% MCd. That is 5-10% higher than what it will eventually equilibrate to in most areas of the U.S. (Table 1a at 50% RH). This is usually not a problem because small changes in the dimensions of studs, rafters, and floor joists are not noticeable. However, for interior uses (wood cabinets, furniture, doors, etc.) it is dried to about 7% MCd so that there is minimal change in shrinking and swelling for typical ambient conditions.
Methods of Drying Lumber and Warping
Lumber can be air dried but most is dried in a kiln. Conventional kilns operate at temperatures up to 100°C or more depending on the type of wood. Drying in a kiln progresses via a series of temperature and relative humidity steps designed to dry to wood gently while it is at a higher moisture content (when there is still free water, e.g. water in the lumen). After the free water has been removed (moisture content has reached FSP), temperatures are increased and humidity reduced to remove water from the cell walls. Kiln drying can take from 10 hours to many days depending on a variety of factors that include the dimensions of the wood, the type of wood, and the end use. Think about the energy required in this step!!
Drying of the wood is important for dimensional stability. In addition, the techniques of drying are designed to limit the warping of the wood. Warp is caused by differential directional shrinkage as the wood dries from its green state. When one edge or face or end of a piece of wood shrinks more than the opposite edge or face or end, the piece warps. Wood that is not dried enough in the kiln, or is over-dried so that in humid conditions will re-enter the wood, is likely to warp. Imagine all of the problems that might come about when trying to build a structure or furniture with warped wood. It is nearly impossible. At the lumber yards customers sort through the lumber finding the wood with the least amount of warp to use on their projects. Home builders are also constantly on watch for warped lumber because of all the problems it brings. No one wants a crooked wall.
Here are some warped deck boards - see how they're bowing up on the ends? That looks like a stubbed toe waiting to happen!
Moisture & Decay
Besides dimensional change associated with moisture content, the moisture content of wood is an important determiner in how quickly it decays. Biological agents are the major causes of wood deterioration. These include fungi, insects (termites, carpenter ants, wood boaring beetles), and bacteria. Fungi are the agent most likely to cause decay in wood.
Fungi
A fungus grows in or on a suitable host (i.e. wood) by germination of a spore followed by growth of hyphae. Hyphae then go from cell to cell through pit pairs or by holes created in the cell wall. As fungal growth continues, wood material is consumed, eaten by the ravenous fungi which digest the cellulose, hemicellulose, or lignin in the plant cells. As fungal growth continues, the fungus is converting the carbon in the plant material into CO2 gas, causing the wood to lose strength and mass.
Preventing Wood Decay
Water is necessary in order for the enzymes and other metabolites produced by the fungus to break down plant cell constituents. Therefore, one of the best ways to prevent wood decay is to keep wood dry. Generally, wood decay is limited if wood is below the fiber saturation point. In most cases, buildings can be designed to avoid conditions where wood moisture content will exceed this level (i.e. no flooding or leaks for rain to come in contact with the wood). Proper building construction (e.g. roofing, ventilation, vapor barriers, window installation, caulking) can ensure that liquid water does not contact wood.
In some projects (like our deck) wood cannot be protected from the elements. For these end uses additional actions must be taken in order to protect against deterioration. In the last lesson we talked briefly about using a pressure treatment to force preservative chemicals into the wood. The act of treating wood with any type of preservative is, in essence, an attempt to toxify or poison the wood to fungus.
Another common action is to treat wood with paints or oils that inhibit fungi or create a water resistant barrier on the outside of the wood to keep moisture out. This is the reason people stain their decks - in addition to looking nice, the stain helps seal and protect the wood from moisture.
As we've discussed in previous lessons, there are some species of wood that are naturally decay resistant. Woods such as cypress, cedar, redwood, and teak naturally contain fungitoxic and hydrophobic extractives, which inhibit fungal growth and keep wood moisture content low. However, these species are often expensive, and have less availability than other species.
Summary
We know this was a rather technical lesson. We hope the math did not scare you. Next lesson is similar as we discuss the strength of wood. So why is it that we can build deck out of wood, but not skyscrapers? You will soon know.