Nobody would worry about moisture in wood, if it would not be for shrinking and warping or the danger of mold development. Both problems have made the moisture content in wood an important factor.
The first step in the prevention of moisture problems is knowing the moisture content of wood. But, it is actually not enough to know the moisture content, the acceptable, no-problem moisture range has to be considered to determine if a piece of wood has the right moisture content or not.
The once beautiful floor has been damaged by moisture. Not only wood floors are moisture sensitive!
Definition of Moisture Content
The moisture in wood is calculated as a percent of weight loss compared to the oven dry weight. The formula is very simple (wet weight minus dry weight divided by the dry weight x100).
Wet weight is the weight of the sample piece before oven drying.
Dry weight is the weight of the sample piece after it has been dried in an oven.
That procedure is used to calibrate a moisture meter. If this is the method by which you want to determine the moisture of a piece of wood, make sure, that your sample piece has the same moisture content throughout the entire piece, otherwise the result is a number between the highest moisture content and the lowest moisture content in your sample piece. Also, when drying small sample pieces an accurate scale is needed. Using a moisture meter is easier.
Determine Moisture Content with a Moisture Meter
Moisture meters use 2 methods to measure the moisture content in hardwood and softwood, dry and wet wood.
Pin moisture meters for wood measure the electrical resistance. The electrical resistance changes if there is more or less water in the wood.
Pinless, non-invasive or scan moisture meters for wood use electro-magnetic wave technology, which measures the density of materials. Density changes if there is more or less water in the wood.
In order to measure the moisture in wood accurately, all pin and pinless moisture meters have to be corrected for the wood species. For pin moisture meters the wood temperature also matters and corrections should be applied if the wood temperature is above 85oF and below 55oF.
What Affects the Moisture Content of Wood
Wood is a hygroscopic material, meaning it will absorb and loose moisture. Even if wood has been dry at one time, does not mean it is dry now. The surrounding relative humidity is the deciding factor, if wood will absorb and loose moisture or if wood is stable. Stable conditions for wood are listed in the EMC chart, where for every relative humidity and every temperature a stable wood moisture content is listed. Once the wood moisture content of the wood and the relative humidity of the air is not in accordance with the values from the chart, wood will change towards the EMC value.
Examples:
1) Lumber at 7.7% will not change its moisture content if kept at a relative humidity of 40% and a temperature of 70°F.
2) Conditions in a warehouse are 60% relative humidity at 50°F. If dry wood is left in the warehouse for an extended period of time, the wood will pick up moisture until 11.2% wood moisture is reached, regardless of wood species and initial moisture content.
Problem-free Moisture Ranges for Wood
Depending on the application the permissible or problem-free moisture range differs. Higher moisture values are allowed for construction lumber in buildings than for wood used for floors, cabinets, furnishings and moldings.
In order to be used for construction projects, lumber must be dried to a moisture content of at least 19%. However, the moisture content for woodworking, flooring, cabinetry and furnishings for indoor use should be between 6-9%.
These two moisture ranges are guidelines. Mold can grow at lower moisture contents and wood in humid climate zones could be stable above 9%. Moldings inside buildings can be installed without problems below a moisture content of 14%.
In any case, if you want to avoid moisture problems, the first step is to measure the moisture content of wood and the humidity of the surrounding air and see if those values fall within the permissible ranges according to the EMC chart.
Often customers ask, what should the Red Oak moisture content be? The answer is not dependent on whether you work with Red Oak , Hickory or Douglas Fir. The question should be rephrased: Is the wood used for fire wood, construction lumber or woodworking. Where is the location, high desert or humid coastal range?
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