The Goldilocks rule of wood moisture

moisture2When a board neither gives nor takes moisture from the surrounding air, it is said to be at its equilibrium moisture content. The problem is that EMC is a fantasy. It doesn’t exist or, at best, it exists only for moments because the humidity and temperature of the air around us are constantly changing.

What a woodworker needs to know is that the boards he or she is using are close to the average humidity in the place where they will live. That way, the furniture or casework that he builds won’t warp, check, split, expand, contract or do other things that cause joints to fail, doors to stick, floors to buckle and a host of other problems related to wood movement.

There is no exact and perfect amount of moisture because, in the average woodshop and also in the average home where the cabinets will be installed, humidity changes with the seasons, weather systems and even from day to night. But we can come up with a decent average (see Table 1). We can estimate roughly where stability will occur most of the time and shoot for that.

Moisture can depend on your shop’s location

Woodworkers already know that solid wood needs to be monitored for moisture. Unfortunately, most of us simply don’t bother. It’s never been a big problem, so why waste the time, right?

Well, one reason is technology — but not in moisture meters. The structures in which we’re building cabinets, and also those where we’re installing them, are changing. Updates in insulation and sealing products, the popularity of radiant as opposed to traditional forced air heat and new construction materials are changing the ways that we heat and cool buildings.

What is moisture content?

Natural boards (as opposed to processed sheet goods) can change dramatically in both size and shape as they respond to changes in humidity. Furniture and accents such as turned and carved products are even more susceptible to moisture than cabinetry. And it’s not so much the presence of moisture as it is a lack of moisture that’s changing the way we do things. The tighter and more energy-efficient our buildings become, the less ambient moisture migrates into them. If a new home doesn’t have a central humidifier (and they are still rare in most parts of the country), the air can dry down to uncomfortable levels for people and wood products.

In its simplest definition, moisture in wood is the weight of the water in a board compared to the weight of the wood in a board. The weight of the water is expressed as a percentage of the weight that the wood would be if it were perfectly dry.

In freshly harvested wood, most of the moisture is “free,” in the form of sap sitting inside cell voids. Some of it is also “bound,” which means that it lies within the cell walls. A loose rule of thumb is that about one quarter of the water is bound and three quarters is free. As a board dries, essentially all of the free water leaves first, along with a very small amount of the bound water. When the free water is gone and only the bound water remains, the board is considered to have reached its fiber saturation point. The board will not noticeably change in size or shape in the first stage of the process as free water leaves, but once drying begins to reduce the volume of bound moisture, then the board will begin to shrink.

A board that has reached its saturation point will not exceed that moisture content again as long as it is not submerged in water. That is, a board that has been reduced to 28 percent moisture content (the average saturation point where all of the free water is gone) will no longer absorb humidity from the air. Airborne moisture will settle in the fibers of the cell walls, but not in the voids. So, after a board has been seasoned, a woodshop need only be concerned with moisture levels below approximately 28 percent.

That’s still a heck of a range.

Wood in the shop

Some furniture shops and most turners purchase or harvest green wood, but woodshops building casework generally buy lumber that has already been kiln-dried. The vast majority of cabinet lumber is hardwood, but there are some applications where softwood is also brought in kiln-dried. The initial moisture content in green softwoods can be quite a bit higher than it is in hardwoods. Softwoods tend to grow faster and they often have wider grain (larger cells), so they tend to have more sap flowing. That’s not always true, but lodge pole pine does grow faster than, say, black walnut.

Construction-grade softwoods are often kiln-dried down to about 18 percent moisture, especially in Western states where they grow on the lower slopes of the Rockies. The kiln process takes less than a week. As the ambient humidity is low at higher elevations, the logs are stored outside in relatively dry air before milling and the boards are stored in open sheds or outdoors in the dry air after their brief time in the kiln. They continue to dry until they reach something akin to the ambient humidity (in Wyoming, for example, that’s about 5 percent).

Hardwoods, on the other hand, are most often harvested in places such as northern Minnesota or in the Appalachians, where humidity is higher. After milling, hardwoods are generally dried down to between 4 and 11 percent in kilns, which takes about a month in most situations (thicker cuts and some species require more time).

One reason they are dried longer is that most hardwoods are a bit more temperamental than softwoods: it takes less moisture to make them misbehave. Another is that cabinet shops need to work to closer specifications than framing carpenters.

The other issue is joinery. Softwood framing lumber is connected with butt joints and inter-fibrous friction fasteners (nails). Hardwood joints are most often precisely fitted, glued and clamped. If they are mechanically secured, that’s usually done with screws, which give less than nails.

It’s got moves

Ambient humidity, in the simplest terms, describes the amount of water in the air around us. Relative humidity, on the other hand, is a more complex measurement and is perhaps easiest to understand in terms of how if affects people. On a warm day, if the humidity is low, then the weather is bearable. At the exact same temperature, if the humidity is high, then we become quite uncomfortable.

Wood doesn’t care a whole lot about relative humidity. It’s mostly concerned with absolute humidity — the actual water content in the air and not how it feels to us.

Of course, wood isn’t completely impervious to variations in relative humidity. In general, it gains or loses about 1 percent moisture content with every 5 percent change in relative humidity.

So, once it has been kiln-dried, it still gains and loses moisture, but it’s far less sensitive to rapid changes than we are.

For example, if the average temperature inside buildings in a given market is about 70 degrees and the average annual humidity in the geographical region is roughly 30 percent, then lumber that has been dried down to 6.2 percent moisture content is in the ballpark. By using lumber that gives this reading on a moisture meter, we’ll avoid a lot of potential problems related to wood movement.

Pin meters

moisture1There are two primary types of meters used in the woodworking industry. Invasive meters have pins on them that are pushed into the wood to take a reading. These usually come with a number of different pin lengths to reach the wood’s center.

The way it works is rather simple. It’s based on the fact that water conducts electricity and dry lumber doesn’t. So the area between the two pins is asked to act as part of a circuit (much like the filament in an incandescent light bulb). The meter actually measures electrical resistance and converts the ability of the wood and water to conduct current between the two pins into a number that represents how much water there is and how much wood.

Invasive meters also allow a woodworker to take readings at the surface and in the center of the board and compare the two. This is especially useful during kiln operations, as it is one way to track the drying process.

Pinless meters

The second option is a pinless meter, which uses electromagnetic wave technology. The meter sends out electrical waves at a certain electromagnetic frequency that creates an electromagnetic field in the area under the sensor pad. The meter then produces a moisture content correlated to the signal it reads back.

There are some shortcomings with these meters. For example, if there is a wet spot on the surface of a board (perhaps where stickering sat or where a worker laid a snowy glove for a minute and it melted), they might pick up on the presence of that excess moisture and report it as an average reading. The reason is that pinless meters scan a large area and average out the readings.

The biggest advantage to pinless meters is, of course, that they leave no holes.  They can estimate moisture in three dimensions (one can dial up desired depths), but it is difficult to use them to measure the difference between surface and interior moisture levels.

Ideal readings

The readings on meters are scaled. That is, some meters read a wide range of moisture, while others read only within a small range. Some meters have a button that allows the woodworker to switch between different ranges. For example, somebody at a sawmill or loading a kiln, who is working with green, wet lumber will need a meter that measures high levels of moisture (maybe 50 to 120 percent). On the other hand, somebody who is checking to see if kiln-dried boards are ready to use only needs a meter with a very small range (perhaps 5 to 28 percent).

moisture3Moisture can be more than 100 percent when the amount of water in the wood outweighs the actual wood. And that magical 28 percent is about where boards reach saturation point: after kiln drying, they can’t absorb moisture above that level unless they are physically submerged in water.

For almost all kilns and hardwoods, drying down to between 6 and 8 percent is going to be the best choice. However, if a woodshop is located in an exceptionally wet or dry area, it’s a good idea to keep control boards on hand. These are simply samples of the most commonly used species, cuts and sizes that the shop uses. By hanging them in the middle of the shop (rather than stacking them against a cold exterior wall or right in front of a furnace), one can take a moisture reading every now and then and determine the actual equilibrium level. Stock that has just arrived from a mill or warehouse or has just been shipped in from a distance can then be allowed to acclimate to this level. That is, it can be given enough time to absorb or lose moisture so that it settles at a moisture content that is close to the level in the control boards. Equilibrium is defined as that moisture content where a board is neither gaining nor losing moisture.

Meters and specific gravity

Some moisture meters compensate for changes in the specific gravity of different boards by allowing the woodworker to dial up different species of wood before a reading is taken.

moisture4Specific gravity measures wood’s density as compared to the density of water. If a species of wood is as dense as water (that is, the same volume of water and wood weigh exactly the same), then the wood is considered to have a specific gravity of 1.0. The trick here is to measure the wood when it’s completely dry — which, of course, never happens. And no two samples of the same species are absolutely identical. So, the industry has developed educated guesses as to the average specific gravity of various species. It does that by weighing samples twice. The first measurement is taken when the wood has just been cut down during full sap flow (the time of year when the most sap is flowing) and then it’s measured again when the sample has just come out of an oven and is as light and free of moisture as it can possibly be. Then, the two readings are averaged and the result is called base or basic specific gravity. Some labs use only one reading taken when the species reaches 12 percent moisture content.

The reason specific gravity is important when using a moisture meter is that the meter expresses the ratio of the weight of the water in a board to the weight of the wood in that board. As all boards don’t weigh the same (that is, they have different specific gravities), then one must compensate for those differences or the results are not comparing apples to apples. Most of the better meters allow a woodworker to dial up a species by name (or a range of species) or at least enter a specific gravity rating for the species being measured. Others provide printed tables that allow the woodworker to make manual calculations and adjust the results accordingly.

Buying a meter

moisture5As with virtually any product, you get what you pay for. Better meters are durable and not as susceptible to common errors. For example, the pins on some inexpensive meters will actually lodge in the wood and physically break off. Many of the better meters have a cap that is placed over the pins between use and it actually recalibrates the meter by taking a reading through the cap material and resetting the base every time. Some meters allow the user to press a button that keeps the reading displayed for as long as it’s needed, rather than disappearing as soon as one’s grip relaxes. Some meters use a series of small LED lights to give a range reading, while others have a digital LCD display large enough for old eyes to read. Others offer audible results — a tone or sound that helps a lot when a screen is hard to see in bright sunshine or a dark warehouse.

Meters with different pin lengths allow the user to make readings at different depths, and those extra pins need to be stored with the meter and easily found. Some meters are large and bulky, while others fit in a shirt pocket. Some come with cables that allow the reading to be taken in places that the meter won’t fit or perhaps where a remote reading is better (such as in a kiln). And some come with a decent warranty; so don’t forget to check the fine print.

This article originally appeared in the February 2014 issue.

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