What Do These Moisture Meter Readings Mean?
Anyone who works with wood – woodworking professionals, wood flooring installers, even hobbyists – must know its moisture content (MC). MC of wood is defined as the ratio of weight of the water in the piece of wood to the weight of the piece of wood without water.
Knowing the MC of the wood is the first step to tell you if the wood is at the correct MC for your project. Many projects require that MC is in the range of 6 to 12 percent MC depending on the project. It may require some research to determine the correct MC for your project and local area.
Why Is Moisture Content Important?
Because wood is three dimensional, too little or too much moisture inside the wood can cause it to shrink and swell along those three dimensions. This can cause potential problems.
The amount of shrinkage or expansion varies from species to species, but generally wood shrinks or expands about 1% in size across the grain (thickness and width) when the MC changes 4%. Wood, however, barely shrinks at all along the grain (length-wise), even with large moisture changes.
While a 1% shrinkage – which occurs from a 4% MC change – seems very small, maybe only several thousandths of an inch, such a change can easily lead to gluing problems, especially cracks and joints, after the construction of furniture and cabinets, or the installation of wood flooring.
In a piece of furniture or cabinetry, 1% shrinkage or swelling can result in poor fitting drawers and doors, as well as poor fitting joints. Finish cracks and warp can also develop with 1% shrinkage.
When wood flooring is exposed to excessive moisture or to very dry conditions, the dimensions of the wood will expand or contract accordingly. When expansion or contraction changes too drastically, wood flooring will show some tell-tale and potentially problematic signs.
For instance, even a change as small as 1/32” per 2” board multiplied across and 8-foot room equals 1 ½” of gapping or swelling.
The reason for these dimensional changes rests with the nature of wood. Wood is hygroscopic. That means the cells within a piece of wood are constantly in a state of absorbing or expelling moisture in order for the wood to acclimate to its existing environment.
Know Wood’s Equilibrium Moisture Content
Once wood acclimates to its environment, it reaches what is called its Equilibrium Moisture Content or EMC. In other words, EMC occurs when the moisture inside the wood reaches a balance or equilibrium with the relative humidity (RH) and temperature of the wood’s surrounding environment. It stays that way as long as the RH and temperature of the surroundings are not changed. NOTE: The RH is the major factor affecting the EMC.
To illustrate, if the RH in the air is 40% and the temperature is 68° F, the wood will eventually attain a MC of just under 8%.
Here are some key values of RH and corresponding EMC:
- 30% RH at 68° F corresponds to 6.2% EMC
- 40% RH at 68° F corresponds to 7.7% EMC
- 50% RH at 68° F corresponds to 9.3% EMC
The first two values above are fairly typical interior EMC values for heated and air-conditioned offices and homes in most of North America. 50% RH would be at the higher end of normal interior EMC values.
In cold weather climates when a heater is running, interior conditions may be even drier than 6% EMC unless the air is humidified. On the other hand, in humid summer months, especially without air-conditioning, conditions in homes, offices, and manufacturing facilities may sometimes exceed 9% EMC.
In most of the U.S., when wood is outdoors but protected from the rain, the wood will attain an EMC of 12% when the RH is 65%. In coastal areas, including parts of Florida, Texas, Louisiana, Washington, and island climates, the EMC is higher. In these more humid locations, wood indoors can reach 12% EMC, while wood outdoors can attain 16% EMC.
Managing Wood Moisture
The best way to manage MC is by using a moisture meter. Consider it good insurance and a valuable cost-saving management tool for woodworking and flooring projects.
Industry experts, such as Dr. Gene Wengert, Professor Emeritus at the University of Wisconsin-Madison, certainly will agree with that. He claims that at least 75% of all wood manufacturing and quality problems are MC related. If you take their word for it, common sense will tell you that a moisture meter is the perfect tool for spotting potential moisture problems and taking the proper steps to avoid them.
Moisture meters are used to measure the percentage of water in a given piece of wood, and are most useful in the MC range of 5% to no more than 30%. At about 30% MC, wood is said to have reached the fiber saturation point, or FSP. Moisture meter is accuracy is greatly reduced when the wood MC is above the FSP to the point that most readings are no longer useful.
Completely dried wood has a wood MC of 0%. Wood in buildings usually has a MC of 5% to 15%. The best range of moisture for interior furniture projects is usually 6% to 8% MC, with interior softwood millwork tolerating a slightly higher MC.
Depending on the type of technology of the moisture meter (pin or pinless), either wood temperature or wood density can affect the accuracy of the measurements given by moisture meters.
Pin-type meters are more sensitive to temperature variations than pinless meters. That’s why pin meters should always come with temperature correction charts, especially if wood is not stored at room temperature. These meters are also sensitive to the chemical makeup of the wood which varies from tree to tree and is different for various species. So, it is important to correct for species. In fact, pin meters should require that you enter the species before you make moisture readings.
Pinless meters, on the other hand, are more sensitive to differences in density (or specific gravity) of different species. That’s why pinless meters require you to set the meter to the correct average specific gravity of the wood species before taking measurements.
Wagner’s pinless moisture meters allow you to set the specific gravity for different species, with a range of specific gravity setting from .20 to 1.0. This range of setting allows you to measure the MC in nearly every wood species known; temperate and tropical hardwoods and softwoods.
What’s Your Targeted Moisture Level?
For nearly all interior wood applications, and even some outdoor, wood needs to be dried to the appropriate MC for the environment to which it will ultimately be exposed. The targeted or “ideal” MC depends upon the use of the wood and the annual average RH at the location where the wood is to be used.
Dr. Wengert says it’s critical that the wood you work with be dried down to a MC within 2 percentage points of the EMC of the in-use location. The EMC of air is numerically equal to the MC that will eventually be attained by any piece of wood when stored indefinitely at a particular humidity. Temperature has no significant effect on MC or EMC.
Targeted MC varies from one region of the country to another.
|Geographical Location||Interior Use||Exterior Use|
|Optimum Wood MC||Interior RH to hold optimum MC||Optimum Wood MC
|Most of US||5-10%||25-55%||9-15%|
|Damp Southern Coastal Areas of US||8-13%||43-70%||10-15%|
|Dry Southwest US||4-9% ||17-50%||7-12%|
The RH in most homes and offices in the U.S. (except in coastal areas and exceptionally dry areas like the desert Southwest) averages 30% to 40% RH. This is 6.2% to 7.7% EMC, which means that wood in interior locations will equilibrate in this range. Therefore, lumber intended for interior use should be dried to a range approximately 6% to 8% MC and should be kept at this MC prior to and during manufacturing.
NOTE: Softwoods machine better at a little higher MC and shrink and swell less than hardwoods when the MC changes. Therefore, often the target MC for softwoods is higher.
Normal Range for Different Materials
Furniture, cabinets, other wood objects, and interior woodwork used indoors typically require a MC range from 6% to 8%, though in coastal regions or locations near large bodies of water where the local humidity is higher than normal, that range may be extended to 10%.
Construction of wood musical instruments, wood microphones, bowling pins, and other specialty items made of wood ordinarily requires a MC of 6% to 9%.
For hardwood floors, typical MC ranges from 6% to 8%. When installing hardwood floors, make sure to check the MC of both the finished hardwood and the wood subfloor. The hardwood flooring MC must then be within 2% – 4% of the subfloor moisture depending on the width of the finished hardwood.
Lumber used for framing, exterior construction, and exterior furniture normally has a MC range from 12% to 19%. Anything above 19% is susceptible to mold and decay.
Remember, wood experts say at least 75% of all failed projects occur because of moisture-related problems – the wood is either too wet or too dry.
Therefore, no matter the project or what wood you’re working with, moisture meters offer a safe, efficient, and cost-effective way to ensure your wood is dried to an optimum MC level and ready to be used.
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